DK9900007

Ris0-R-1O29(EN)

Mechanical Decontamination Testsin Areas Affected by the ChernobylAccident

J. Roed, K.G. Andersson, A.N. Barkovsky, C.L. Fogh,A.S. Mishine, S.K. Olsen, A.V. Ponamarjov, H. Prip,V.P. Ramzaev, B.F. Vorobiev

3 0 - 0 5

Ris0 National Laboratory, Roskilde, DenmarkFederal Radiological Centre, St. Petersburg, RussianAugust 1998

Abstract Decontamination was carried out around three houses in Novo Bo-bovichi, Russia, in the summer of 1997. It was demonstrated that significant re-ductions in the dose rate both indoor (DRF = 0.27) and outdoor (DRF = 0.17)can be achieved when a careful cleaning is undertaken. This report describesthe decontamination work carried out and the results obtained. The roof of oneof the houses was replaced with a new roof. This reduced the Chernobyl relateddose rate by 10 % at the ground floor and by 27 % at the first floor. The soilaround the houses was removed by a bobcat, while carefully monitoring theground for residual contamination with handheld dose meters. By monitoringthe decline in the dose rate during the different stages of the work the dose re-ducing effect of each action has been estimated. This report also describes atest of a skim-and-burial plough developed especially for treatment of contami-nated land. In the appendices of the report the measurement data is available forfurther analysis.

ISBN 87-550-2361-4ISSN 0106-2840

Information Service Department, Ris0, 1998

Contents

1 Introduction 7

1.1 The Chernobyl Accident 71.2 The Contamination of Novozybkov district 81.3 Experience from Previous Decontamination Efforts 101.4 Objectives for the Expedition in 1997 12

2 Units and Devices 13

2.1 Radiation Units 132.2 Measurement of Dose Rates 142.3 Other Contamination Assessment Methods 16

3 Guta Muravinka 18

3.1 Description of the Site 183.2 Mapping of the 137Cs Pollution in the Area 22Depth Distribution of m C s in the Soil. 24Summary of Soil Contamination 28Roof Contamination 283.3 Decontamination Test at the Grassed Area 293.4 Decontamination Around the Houses 313.5 Results of the Decontamination Work 36Results of the Reuter Stokes Ion Chamber Dose Rate Measurements 36El-1101 Dose Rate Measurements 37Unscattered 137Cs Photons from Different Directions in House 1 413.6 Cost Analysis of the Results 42

4 Skim and Burial Ploughing 44

4.1 The Radiological Purpose 444.2 Plough Design and Construction 454.3 Field Trials and Discussion 46

5 Asphalt Scraper Experiment 51

5.1 Asphalted Roads in Novo Bobovichi 515.2 Contamination of Asphalt Surfaces in General 515.3 Experiments in Novo Bobovichi 52

6 Monitoring of the 1995 Site 56

The Decontaminated Test Area Near Novo Bobovichi 56The Triple Digging Test Area 58

7 Contamination of the Forested Area 60

7.1 Sampling and Analysis 607.2 Results and Discussion 62

8 Conclusion 64

8.1 Equipment and Methods 658.2 Decontamination Results 658.3 Future Work 66

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9 Acknowledgement 67

10 References 67

Appendix A. Soil Samples 70Appendix B. Dose Rate Measurements 90Appendix C. Miscellaneous Measurements 95

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Preface

Large areas in the south-west corner of the Bryansk Region were contaminatedby the Chernobyl accident. The level of contamination exceeds 1.5 MBq m"2 inseveral villages and these have to some extent become deserted. There is a needfor decontamination, both to prevent further depopulation and to stimulate thereturn of the population to the affected areas.

In 1995, a joint Russian-Danish field exercise sponsored by the DanishEmergency Management Agency under the Ministry of the Interior decontami-nated the area around three houses in Novo Bobovichi, 25 km north-north-westof Novozybkov. The work was carried out with hand tools, shovels and wheel-barrows, and monitored carefully with handheld dosimeters. Indoor dose ratereductions of 64 % and outdoor dose rate reductions of 78 % were achieved.

A second decontamination project was initiated in 1996, and in August 1997a second field exercise was launched. Here the focus was on the application ofheavy machinery in the decontamination work. An expedition from Ris0 Na-tional Laboratory went to the Novozybkov area in August 1997 with a bobcat (amini bulldozer designed for work in gardens), an asphalt scraper and a skim andburial plough. An excavator and a tractor with a wagon were rented locally to-gether with manual labour. The work was carried out in close collaborationbetween the Danish team and a team of five scientists from the Federal Radio-logical Center at the Institute of Radiation Hygiene in St. Petersburg.

It is our hope and belief that the results presented here will form the basis foran increased interest in the possibilities provided by urban decontamination,especially in the light of the change in the Russian policy towards stopping themigration from the contaminated areas and instead remediate and resettle theareas.

This project was made possible by the funding provided by The EmergencyManagement Agency under the Danish Ministry of the Interior and all the par-ticipants would like to express their gratitude towards the Agency for the sup-port, which has resulted in the most promising decontamination work in Russiaso far.

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1 Introduction

1.1 The Chernobyl AccidentAs a consequence of the accident at the Chernobyl nuclear power plant in April1986, large areas of primarily three republics of the former Soviet Union (Be-larus, Russia and Ukraine) received high levels of surface contamination. Inmagnitude, the challenge in terms of measuring, monitoring and assessing theconsequences of the accident are likely to have exceeded all previous efforts todeal with anthropogenic disasters. Over the first three years, various protectiveactions and restrictions were implemented in the affected areas. Among the ac-tions were relocation, decontamination efforts in some of the most heavilycontaminated settlements and control of locally produced foodstuffs. Thesecountermeasures were aimed at reducing external and, particularly, internaldose rates from deposited 137Cs in the areas. At a conference in Kiev in May1988, the major short-term human, economic and environmental impacts of theaccident were summarised as follows: 31 deaths, the evacuation of more than100,000 persons from the 30 km zone surrounding the power plant, and the re-moval of a large number of livestock (several thousand), as well as a severecontamination of very large areas in cities, rural environments and forests. Thiscontamination would inevitably have a long-termed effect, since two majordose contributors, 137Cs and '"Sr, both have radiological half-lives in the rangeof 30 years. From the limited applicable experience available from previouscontamination events it was already known that the natural weathering proc-esses would only very slowly reduce the radiation levels.

Over the following few years, an increasing amount of attention was paid tothe long-term consequences, also for persons staying outside the early evacua-tion zone. Although some of the longer-termed effects could not be avoided atthis point, e.g., thyroid cancers developing years after the early exposure to ra-dioactive iodine in milk, it was clear that more guidance of the form of long-term radiation protection criteria would be required, to supplement, optimiseand revise the relocation policies implemented over the first three years. At thispoint in 1989, the so-called 'safe-living concept' was introduced by the SovietNational Committee on Radiological Protection, defining an upper limit ofequivalent dose rate of 350 mSv over a life-time, below which action in termsof countermeasures was deemed unnecessary.

A WHO founded group of experts soon after reached the conclusion that thisupper limit was rather conservative and it was estimated that its implementationwould imply the relocation of a further 100,000 persons. However, relocationwas encouraged and over a period of some five years resulted in a degree ofdesertion of large areas. According to Hubert et al. (1996), a total of 260,000people have been relocated. Only in recent years has the official policychanged, following the concept initiated by a group of experts of the USSRAcademy of Sciences in 1991. Here, an increased interest was expressed fordose reducing measures to be implemented in the contaminated areas. The psy-chological factors in terms of stress, fear and anxiety were highlighted and weresaid to be in-line with typical post-accident syndrome effects, although specialproblems had arisen from misunderstanding and misinterpretation of the infor-mation given. It was stressed that an effort, both in terms of information and ofdose reduction, would have to be made to end the mass relocations. After theseparation of the different Soviet Republics in 1991, the Russian Federation

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introduced its own recommendations (Belyaev et al., 1996). In the 1993 rec-ommendations the need was again stressed for '...effective measures for reha-bilitation of the contaminated areas and restoration of normal life and economicactivity'. The recommendation was to prepare a programme of rehabilitation tostop the degradation of residential, industrial and agricultural areas and stimu-late the return of previously relocated persons.

To stimulate the rehabilitation process in the contaminated areas the RussianFederation has introduced a monthly payment to the population to compensatefor the radiological risk of living or working in contaminated areas. The level ofcontamination by 137Cs in open areas forms the basis for the magnitude of thepayments. In the Novozybkov city area, where the contamination level is gener-ally around 550 kBq/m , the monthly compensation to the locals in 1995 corre-sponded to 40 US$.

Also decontamination and reconstruction operations in settlements have nowbeen resumed. So far, some old houses have been demolished, some newhouses have been built, contaminated roads have been covered with new as-phalt or clean gravel, which shields against the radiation and at the same timeprevents resuspension in air of radioactive material. Further, new rain waterdrainage systems have been constructed.

The relocations have been stopped almost completely, since they are not cost-effective, and the remaining inhabitants mostly have a strong wish to stay in thelocal areas.

1.2 The Contamination of the Novozybkov districtThe news of the accident on April 26th 1986 at the Chernobyl nuclear powerplant reached the population of the Novozybkov area of the Bryansk region inRussia on the 3rd of May (IAEA, 1991). Although some simple protectivemeasures were soon after introduced in for instance kindergartens, it was notuntil the 9th of May that the authorities instructed the population to stay in-doors, keep their windows closed and wash themselves thoroughly. However, atthis point, the air concentrations of many of the most important radioisotopeshad already decreased by several orders of magnitude (Kryshev, 1996), and theinhalation dose could thus not be reduced significantly by these countermea-sures. Since the Novozybkov area received prolonged heavy rain soon after theChernobyl accident, the air was, however, effectively cleaned by wash-out ofthe contaminants, and inhalation doses were not high in this area. The drydeposition processes are estimated to be responsible for only about 1 % of the137Cs contamination of the area (Roed et al., 1998). The wet deposition was,however, quite substantial in the Novozybkov area (as indicated by the red ar-eas in plate 1, where the contamination level exceeds 1.5 MBq/m2). Wet depo-sition, in general, leads to much greater deposition of pollutants per unit of timethan does dry deposition. In the Bryansk region as a whole, 15 settlements witha total of about 22,000 people were contaminated with caesium of levels ex-ceeding 1.5 MBq/m2 (IAEA, 1991).

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Plate 1.1 Initial l37Cs contamination levels in the Novozybkov area after theChernobyl accident. Iso-lines are shown for contamination levels of 1 Ci/km2, 5Ci/km2, 15 Ci/km2 and 40 Ci/km2 (corresponding to 37, 185, 555 and 1500kBq/m2).

Some three weeks after the accident had occurred, the town was supplied withpowdered milk, which was free of 131I. Potassium iodide tablets were at thesame time distributed to children in the area, but as the introduction of thiscountermeasure was obviously too late, it was stopped again about a week later.Obviously, this sort of inconsistency increased the anxiety of the locals. In thetime prior to this, however, particularly the children in the area received signifi-cant thyroid doses from the intake of 131I. The thyroid doses to more than 17,000inhabitants were estimated through a large-scaled measurement programme overthe following weeks. About 7,000 of these were children. It was found thatabout 200 had received doses to the thyroid exceeding 0.75 Gy. According toICRP publication 60 (1990), a dose of 0.75 Gy to the thyroid gives a fatal cancerrisk of 6 10"4. However, it must be stressed that risk coefficients vary greatlyamong different authors of recent papers (Likhtarev et al., 1994). From the earlyphase, milk was imported from a radiologically clean area about 120 km away,and local food products checked regularly for the content of radioactive matter.

1.3 Experience from Previous Decontamination Ef-fortsInhabited areas of the Bryansk region were subjected to decontamination duringthe summer of 1989. The decontamination work was carried out by units of theSoviet army, and covered 93 settlements with a total population of about 90,000(Anisimova et al., 1994). At the time that the decontamination work was initi-ated the distribution of the contamination was described as rather heterogene-ous. The mechanical impact of human activity on the contaminated surfaceswas reported to be insignificant in some almost deserted areas, whereas it washighly significant in for instance working areas.

Essentially, the army units relatively consistently carried out only two proce-dures:

• A layer of topsoil was removed, which was supposed to include most of thecontamination.

• A layer of clean sand or gravel was applied afterwards to shield againstresidual contamination in the soil.

Decontamination generally only took place in yards of private houses, aroundpublic buildings and along roads of a total length of about 190 km.

The St. Petersburg Institute of Radiation Hygiene carried out the dosimetricassessment of the effect of the work. The dose reducing effect was found to bedisappointingly low. Decreases in dose rate by generally a factor of 1.1 to 1.5were recorded. A similarly low efficiency was found to be the result when thesame procedures were carried out in the Belarussian settlement Kirov. Here, thedose rate was reported to have been reduced by some 8 %.

The costs of the efforts in the Briansk region were in 1989 found to amount to15,359 rubles per averted man-Sv, and since an averted man-Sv was then giventhe value of 5,000 rubles, the operation was clearly not cost-effective. There-fore, decontamination was not considered to be a realistic option in the follow-ing few years.

Very little is described about how the actions were carried out, but it isknown that not all landowners allowed intervention on their ground. Therefore,coherent treated areas may not have been very large. As previously reported(Roed et al., 1996 a), the difference in dose rate reduction between treating onlya 10 m by 10 m area and treating an extremely large area of land may in somecases be as much as a factor of 5. It is also evident that the planning of the op-eration and assessment of the local radiological conditions prior to treatmentwas insufficient. Any potentially effective countermeasure may fail to signifi-cantly reduce the dose rate if it is carried out regardless of the local contamina-tion distribution. In some situations, the dose rate may even increase (Roed &Andersson, 1996). The thickness of the removed soil layer may not have beensufficient, and judging from radiological maps of the effect, the work was notcarried out consistently, even over small garden areas. Further, it is unlikelythat special care was taken to treat 'hot spot' areas, such as the soil immediatelyadjacent to buildings, soil under roof-gutters, etc. It has been established (Roedet al., 1996 a), that contamination on roofs may contribute significantly to thedose rate, but practically no efforts were made to treat these.

Four years later, an international effort supported by the Commission of theEuropean Communities, was made to test a number of dose-reducing proce-dures, which were thought to be particularly promising for inhabited areas. Thein situ tests, which took place in Russia and in the Ukraine in 1993-1994, indi-cated that it was indeed possible to reduce the dose rate significantly, although

10 Ris0-R-1O29(EN)

many years had already at that time passed since the Chernobyl accident, and itwas certain that much of the contamination was strongly bound and much lessaccessible by the procedures than had previously been the case. Separate meth-ods were tested and evaluated in a catalogue, which was published as a Ris0report (Roed et al., 1995). However, the test areas were rather small and thedose-reducing effect of carrying out a complete clean-up strategy was neverinvestigated.

Many different aspects need to be considered in the formation of a strategy. Aprocedure may be well suited for some types of environments, but may for spe-cific reasons (e.g., soil texture, contamination distribution, time-period, season,etc.) be totally inapplicable in other environments. Further, the effect of carry-ing out a strategy can not be evaluated by merely adding up the effects oftreating the individual surfaces. It might well be that in some cases, a decon-tamination procedure for one type of surface translocates the contaminationfrom that surface onto a different area. In some cases, this may even lead to anincrease in dose rate, and it is therefore important to carry out the different pro-cedures of a strategy in the right sequential order.

Therefore, an expedition sponsored by the Danish Emergency ManagementAgency was launched in the autumn of 1995 to the contaminated Russian set-tlement Novo Bobovichi in the Briansk region, to which Chernobyl debris haddescended in rain. The main objective was to prove that it was still possible byrelatively simple and inexpensive means to carry out an effective dose reduc-tion strategy in settlements contaminated by the Chernobyl accident.

The main contributor to radiation levels inside the wooden houses in the testarea was found to be the contaminated soil but the roofs also made a significantcontribution, whereas radiation from walls was comparatively insignificant.The many large pine trees in the area had shed their needles several times oversince the accident and their contribution to radiation levels was very much lessthan that of the contamination in the soil. Contamination on roads was verysmall, and this was not surprising since earlier studies had shown that the levelson such surfaces usually decrease by at least one order of magnitude in the dec-ade following wet deposition due to traffic and weathering. Clearly, decon-tamination of soil and roofs offered the greatest potential for reducing radiationlevels in the settlement.

For logistical reasons, it would have been better to treat the roofs before thesoil since there was a risk that contamination displaced from the roofs couldland on the soil. However, for this demonstration it was decided to treat the soilfirst since this would facilitate a more accurate measurement of what can beachieved through roof-cleaning alone. The decontamination scheme was there-fore as follows:

1. Removal of the topmost 5-10 cm of soil from an area of 20m by 20maround three wooden houses: only hand-tools (e.g. spades, shovels, wheel-barrows, etc.) were used in order to demonstrate what can be achieved bysimple means.

2. Cleaning of the asbestos roofs of the three houses by first removing theloose litter (e.g. leaves and pine needles) and secondly by using a speciallyconstructed scrubbing device.

3. Application of clean gravel to the decontaminated land to attenuate residualradiation.

This approach to decontamination enabled a calculation of the fraction bywhich the initial dose rate level was reduced by each of the decontaminationsteps. It was also possible to identify the likely sources of the residual contami-

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nation. The overall mean reduction in dose rate achieved as a result of the totaloperation was as much as 64%.

1.4 Objectives for the Expedition in 1997The present report describes the results of a second field investigation cam-paign supported by the Danish Emergency Management Agency, in the latesummer of 1997. Like in 1995, scientists from Ris0 and from the St. PetersburgInstitute of Radiation Hygiene participated jointly in the experiments. Thisyear, most of the assessments were made in the Guta Muravinka area of theBryansk region.

One of the objectives for this year's work was to introduce in a clean-up strat-egy larger machines, which could carry out some of the tasks, that had in 1995been accomplished by hand. For instance, the removal of soil was in 1995 car-ried out using spades and shovels. This could probably be accomplished at amuch greater speed using a bobcat with a scraper. Due to the unevenness of thesoil surface, the question remains as to whether this equipment would enable ahomogeneous removal of a thin top layer. It might also prove difficult to ma-noeuvre around buildings and trees, and the dose rate contribution from thecontaminated soil nearest to the house is often of great importance.

Essentially the same procedure that was in 1995 performed by triple diggingwas in 1997 accomplished using a specially constructed skim-and-burialplough. Hereby, the contaminated top soil layer is buried deep in the verticalprofile, thus providing a good shielding against the radiation. Whereas it is es-timated that one man can triple dig 2 m2 of soil area per hour, the test showedthat one plough can treat some 3,000 m2 of land in an hour.

Some of the locations in which the dose rate measurements were carried outat Novo Bobovichi in 1995 were re-assessed in 1997. This would show if anyre-contamination or other movement of the contamination had occurred.

One of the surprises from the 1995 measurements in Novo Bobovichi was thehigh level of contamination on roofs. The 1997 measurements at GutaMuravinka would show if this result could be verified, as a more general fea-ture.

Also included in the 1997 measurement programme was an assessment of thedose rate in various locations at 1st floor level inside a building. Not only thetotal dose rate was observed, but also the decrease in dose rate by treatment ofeach surface. This type of measurements have to our knowledge never previ-ously been made, although modelling has indicated the great significance ofdoses received indoors on various floors, due to the large fraction of time spenthere.

In 1995 it was concluded, that a major contributor to dose rate after decon-tamination was the contaminated adjacent forest. Therefore, the forest contami-nation was further investigated in 1997, where dose rates were measured andsamples brought to the laboratory for analysis. Forest decontamination has re-cently attracted great attention in Belarus and the Ukraine, since preliminaryplans have been introduced for safe application of the removed contaminatedbio-mass as fuel for specially designed power plants.

12 Ris0-R-1O29(EN)

2 Units and Devices

2.1 Radiation UnitsThe results of the work carried out are explained by essentially three units:

Doses were measured in units of Sv (Sievert, 1 Sv = 1 J kg"1), which is the SIunit for equivalent dose and is defined as the mean energy imparted by ionisingradiation to matter of a given mass, multiplied by a factor indicating the bio-logical effectiveness of the radiation. This latter factor is 1 for gamma radia-tion. Similarly, the term dose rate refers to the increment of the equivalent doseper unit of time, and is generally expressed in units of Sv h"\ In some cases, theexposure rate has been measured in stead of the dose rate. Exposure is definedas the quotient by air mass of the absolute value of the total charge of ions ofone sign produced in air when all electrons liberated by photons in air of thegiven mass have been stopped in air. The SI unit for exposure is C kg"1. Oftenthe unit Rontgen, R, is used instead (1R = 2.58 10"4 C kg"1 in air). The exposurerate is simply the increment of exposure per unit of time, here denoted as R h"1.Although the relationship between the dose rate and the exposure rate is at lowenergies (less than 100 keV) greatly dependent on the tissue type (bone, mus-cle, fat, etc.), the major part of the doses received from 137Cs radiation in thefield is sufficiently high-energetic to justify the application of the general rela-tionship of 100 R/h exposure rate corresponding to 1 Sv/h dose rate.

The dose reduction factor refers to the relationship between the dose rate af-ter the introduction of a dose reductive measure and that, which was recordedinitially. The dose rates applied here are not including the 'background1 contri-butions from natural terrestrial radiation and cosmic radiation. Thus, the for-mula for the dose reduction factor can be expressed as:

^ ^ ^ dose rate after decontamination - background dose rateDRF = -

dose rate before decontamination - background dose rate

The decontamination factor DF refers to the relationship between the contami-nant concentration on a surface prior to decontamination and the concentrationafter the surface has been treated:

surface concentration before decontaminationJJr =

surface concentration after decontamination

Using this factor, the effect of an effort to decontaminate a specific contami-nated surface (such as a grassed field, a house roof, a road paving, etc.) can bedescribed. The two surface concentrations, through which the DF is defined,have in this report predominantly been assessed by gamma spectrometric analy-sis using germanium or sodium iodide detectors. For in situ applications in ageometrically complex inhabited area, it is often necessary to collimate the de-tector in order to view defined areas of horizontal or vertical surfaces. Collima-tion of the detector was achieved with lead shielding blocks assembled on anadjustable steel table. Another option for determination of the DF is, wherepossible, to take samples of the treated and untreated surface for gamma spec-trometric analysis in the laboratory.

Ris0-R-1O29(EN) 13

2.2 Measurement of Dose RatesMeasurement of dose rate in the investigated areas was accomplished usingvarious different devices. One of these is a Reuter Stokes ion-chamber (modelRSS-112). The active volume of this device is 7.9 litres in magnitude and isfilled with ultra-high purity argon at a pressure of 25 atmospheres. Radiationincident to the chamber produces ion-pairs, which are swept to the electrodesdue to an electrical potential. The resulting current can be related directly to theexposure rate in air, which is then again related to a dose rate. The sensitivityresponse energy function is relatively flat between ca. 100 keV and ca. 10MeV, but the chamber inevitably has a higher sensitivity towards scattered (lowenergy) radiation. The dose can be assumed to be a linear function of the de-tector response in the area up to 1 mSv/h. If the chamber is moved from oneposition to another, the corresponding time constant of the change of the signalhas been found to be about 5 seconds. At the dose rate levels for, which the de-vice was applied in the contaminated areas around Novo Bobovichi and GutaMuravinka, the standard deviation of the result of a 5 seconds measurementwas found to be about 1 %, whereas the uncertainty including systematic errorsintroduced through, for instance, the calibration and the atmospheric pressurewas found to amount to about 5 %. Since a measurement with a low standarddeviation only took 5 seconds to produce, a series of measurements over at leasta minute was generally recorded to determine the deviations of the individualmeasurements. If this standard deviation exceeded 1%, it was concluded thatthe signal was not stable and the procedure was repeated.

As much of the 137Cs has penetrated several centimetres into the ground, thedose rate in air will depend on the attenuation of the photons in the ground. Theattenuation will depend on the varying moisture content in the ground. InFigure 2.1 the dose rate measured at a reference point is presented. It can beseen that the dose rate increases by about 5 % from the start of the measure-ments in Guta Muravinka to the end. It rained the night before the work started,but the following weeks the weather was dry. Throughout the first week thetopsoil dried out and the dose rate increased. After about one week with dryweather the dose rate remained stable.

Ambient reference doserate1200

1190

J51 H80

IIH70

1160

1150

1140

r4V

M

=¥[

-* - Point (20,00) -

trg—

6-Aug 8-Aug 10-Aug 12-Aug 14-Aug 16-Aug 18-Aug 20-Aug 22-Aug

Date

Figure 2.1. Total dose rate measured in Guta Muravinka at location (20, 00).

14 Ris0-R-1O29(EN)

Also an easily portable instrument called EL 1101 was applied for dose rateassessment. The EL 1101 was produced by the Belarussian Company Atomtehand is based on a Nal crystal with a diameter of 16 mm and a length of 25 mm.This instrument measures the energy spectrum of the y-photons in the energyrange between 0.04 and 3 MeV. This spectrum is converted to a dose ratereading by a special algorithm. The El-1101 was used for the measurement ofexposure rates and dose rates.

In Figure 2.2 a comparison between El-1101 and the Reuter Stokes ion cham-ber is shown. The measurements were made in different locations in Novo Bo-bovichi in August 1997. It can be seen that there is a good linear agreementbetween the two data sets.

.5

160

140

120

100

80

60

40

20

0

•

50 100

Reuter Stokes reading, |jR/h

150

Figure 2.2. Exposure rates measured with an EL 1101 device plotted againstexposure rates measured at the same locations using a Reuter Stokes RSS-112ion chamber.

Another device that was less frequently applied to measure dose rates is theDRG-001. This instrument was constructed at the Institute of Radiation Hy-giene in St. Petersburg and is based on Geiger-Miiller tubes. The device, whichwas calibrated with the principal isotope in these investigations (137Cs) actuallymeasures the exposure rate.

A hand-held device, also developed at the Institute of Radiation Hygiene,with the name of DRG-01-T1, was applied more frequently. Also this instru-ment was based on Geiger-Muller tubes.

By calibration measurements in the Finnish Gulf the combined values of thenoise and cosmic radiation response were found for the instruments:

• forDRG-01-Tl 8.5• for DRG-001 4.5 pRh '• for El-1101 1.5pRh"

As can be seen, there is a good correlation between the results of the two de-vices, with the measured dose rates ranging from 20 to 130 uRem/hour = 0.2 -1.3|iSv/h.

The natural background was determined from measurements in 1995 to be 60nSv h"1 in Novo Bobovichi (Roed et. al, 1996a). Here, the 'cosmic' dose ratecontribution due to primarily high energy muons, photons and electrons, was

Ris0-R-1O29(EN) 15

estimated to be 35 nSv/h. This corresponds well with the exposure rate figureof 3.59 |jR/h, which is stated in the RSS-112 operation manual for sea levelaltitude.

The terrestrial background contribution to dose rate was in 1995 estimated tobe 25 nSv/h. This estimate was based on measurements of the soil contents ofthe naturally occurring radionuclides 40K, 226Ra and 232Th, made both in situ andin the laboratory for soil samples, with the normally valid assumption that theisotopes are homogeneously distributed over the relevant vertical and horizon-tal distances. Measurements made in 1997 on soil samples from GutaMuravinka showed a similar content of these three radionuclides and the 'natu-ral' dose rate contribution in Guta Muravinka can therefore be assumed to besimilar to that found in Novo Bobovichi.

2.3 Other Contamination Assessment MethodsAn other technique that was applied to investigate the distribution of the con-tamination and the effect of dose-reducing countermeasures was sampling forfurther analysis in the laboratory.

From spectrometric measurements at the surface, an equivalent 'averagedepth' of the contamination in soil may be estimated. However, the vertical dis-tribution was previously erroneously considered to be adequately described byan exponential function of the depth. Recent investigations have shown that theChernobyl radiocaesium depth distribution in soil is often better described by aLorenz function. The exact vertical distribution of the contaminants in the soil,both before and after the introduction of a dose-reducing countermeasure, canbe obtained only through soil sampling.

It is essential to know the distribution of the contamination in soil, both verti-cally and horizontally, before attempts are made to reduce the dose. If the depthdistribution is not known, for instance, a too thin soil layer may be removed andthe dose-reductive effect may be much less than what could have beenachieved. This is one explanation for the low efficiency of the decontaminationwork that was first carried out by the authorities in the areas affected by theChernobyl accident. Indeed, the result may be a rise in dose rate. On the otherhand, if a too thick layer of soil is removed, it may severely affect the soil fer-tility and it will generate large volumes of waste.

The soil sampling was accomplished by driving polyethylene tubes measuring81.0 mm in diameter and 4.5 mm in wall thickness into the ground. Tubesmeasuring about 30 cm in length were applied to investigate the vertical 137Csdistribution in the soil prior to any dose-reductive treatment, whereas longertubes measuring more than 50 cm were applied to determine the vertical distri-bution after digging or ploughing, which are procedures that affect the con-taminant distribution throughout the uppermost half metre soil layer. When thefull length of the tube had penetrated the ground, it was withdrawn togetherwith a soil core. The withdrawal was performed with special care to preventloose material from falling out. Therefore, the soil surrounding the outer sur-face of the tube was first excavated. The sample cores, which were still in plas-tic tubes, were then wrapped in a thin plastic film to prevent them from dryingout and falling apart.

When the sample cores had reached the laboratory, 200 ml of distilled waterwas added to each tube, which was subsequently transferred to a deep freezer(at -20°C), where it was left for two days.

The tubes containing the samples were then sliced with a diamond saw. Theblade of the saw is 2.57 mm thick. Consequently, by this slicing procedure, acorresponding layer of the soil core was lost by each sectioning. A spacing de-

16 Ris0-R-1O29(EN)

vice on the diamond saw made it possible to accurately adjust the thickness ofthe slices. The saw blade usually has to be cooled with water, but at this pointthe samples contained sufficient amounts of water to grease/cool during theslicing process. As the samples were deep frozen the slices kept their parallel-side circular disc shape and even stones in the samples were cut through.

The reproducibility of this 'precision engineered' technique has been evalu-ated through an examination of a series of sectioned core samples, whichshowed that the variations in thickness have a standard deviation of about 1 %.

Since the samples stayed in the plastic tube during the slicing process, thesliced tube sections formed rings around the sample slices. These rings sup-ported the samples during the different analysis sequences (drying, weighingand y-measurement). Further, a removal of the whole soil core from the tubewould have introduced an unnecessary cross-contamination hazard.

The samples were then dried in 3 days at 60°C. After drying, the density ofthe samples was established through a weighing, where the mass of the plasticring was subtracted. Stones and grass roots were not removed. This enabled adetermination of the bulk soil density and the attenuation as a function of depth.The density of the sample was also used to allow for the attenuation of gamma-rays through the soil slice while measuring the gamma-activity.

The photopeak count rate of ' 37Cs (661.6 keV) in each sample was measuredin a lead shielded gamma spectrometer which includes a 15.6 % efficiencyGe(Li) detector. In this calibrated detector system the sample was placed di-rectly on top of the upward facing detector end cap.

Finally, the photopeak net count rate was converted to a contamination level(Bq cm"3 of l37Cs) in the sample, taking into account differences in slice thick-ness and densities relative to the calibration standard.

In Appendix A, the vertical distribution of the contamination is shown bothbefore and after dose-reductive countermeasures had been carried out. Themean depth in centimetres of each slice of the cylinder is given, allowing forthe thickness of material removed by the saw blade. More importantly, depthsare given in units of accumulated mass of soil per area above the given depth (gcm'2). This figure is indicative of the attenuation that the soil provides againstthe radiation at the different depths, as it incorporates the soil density. It is evi-dent that the soil density will be small in the top layers, which contain muchorganic material and has larger pores. The content of radioactivity is expressedin units of Bq per gramme of soil sample mass. This figure is a good measure ofthe amount of contamination associated with each amount of soil in the verticalprofile.

In other cases, soil samples were taken using a specially constructed excava-tion device made out of steel. The samples that were produced in this waymeasured 20 cm in length, and 5 cm in diameter. These samples were sectionedin the field at 5 cm + 10 % intervals, using a knife. The slices were placed incylindrical metal containers and the content of I37Cs was measured in a leadshielded spectrometer based on a NaI(Tl) detector (measuring 3 inches in bothlength and diameter) and a portable 480 channel analyser. The measurementtime for the samples in the field was in all cases 120 seconds.

In some cases, another detector system was applied in the field - the CORADdeveloped by RECOM Ltd., Moscow, which is based on a lead collimated cy-lindrical Nal(Tl) detector crystal with both diameter and length of 50 mm oper-ated with a photo multiplier. This system, which measures the contaminationlevels rather than dose rates, was here primarily applied to investigate the effectof decontamination trials on impermeable surfaces, particularly to scrape off athin contaminated surface layer of asphalt. However, the system can also,through analysis of the detector response in different energy windows, be ap-

Ris0-R-1O29(EN) 17

plied to estimate the average depth of a contamination distribution in for in-stance soil. A detailed description of this feature is given by Roed et al.(1996a), together with an account of how the calibration procedure is accom-plished.

3 Guta MuravinkaThe main effort of the 1997 expedition to Russia was the decontamination ofthe recreational zone in Guta Muravinka. The area was closed to the public af-ter the Chernobyl accident, as it is situated in an area that has received a 137Cscontamination of about 1.5 MBq m~2. Prior to this investigation, no attemptshad been made to reduce the dose rate in the area. Three years ago the area wasagain opened to the public, and there is now a steady stream of visitors, espe-cially in the week-ends. Some come for the day and use the smaller cabins, oth-ers stay overnight in the houses. The average dose rate outdoors is about 1 |iSvh"1.

Two areas were selected for decontamination. A grassed area by the 'beach'and a larger area around the houses. These two areas represented the locationswhere people spend most time. The cleaning of the grassed area was a rela-tively simple task and it was done to get some experience in working with thebobcat. The cleaning around the houses was a greater task, which also gavemore useful information about the obtainable effect of application of decon-tamination measures in urban areas.

The main innovation compared to earlier decontamination exercises was theuse of heavy machinery for the soil removal. Here a bobcat was used. The bob-cat is a 'mini-bulldozer' suitable for work in gardens and around houses. Anexcavator was used to dig pits for some of the waste and tractors with trailerswere used to transport a part of the waste to remote depositories.

The work progressed according to the following schedule:

1. Mapping of the dimensions of the site.2. Mapping of the 137Cs pollution.3. Test cleaning in the green area.4. Removal of the soil5. Measurements of the effect6. Renewal of one roof7. Final mapping of the contamination situation

The combined Russian-Danish team worked along this schedule for 3 weeks.

3.1 Description of the SiteThe recreational area is located in a picturesque part of the east bank of theriver Iput. It is a very attractive place, especially in good weather. The water inthe river is clean and transparent, and the sandy shore makes it an excellent sitefor bathing. The shape of the riverbank area is shown in Figure 3.1. After asteep rise from the shoreline, the landscape becomes plane and forested. On thisplane surface the recreational resort had been founded. A map of the part of thearea that was selected for cleaning is shown in Figure 3.2. It includes fiveidentical houses, in this study referred to as Houses 1 through 5. The houses

18 Ris0-R-1O29(EN)

were Wi stories high, as can be seen in the sketch of the floors in Figure 3.3,and were intended for people staying for more than one day in the area. A seriesof smaller houses located to the left of the shown area were intended for one-day visitors. Below the area shown in Figure 3.2 there were a large and a smallkitchen building for cooking.

The shown area measured 60 by 150 meters, as indicated in the scales at thebottom and left sides of Figure 3.2. The entire area was marked with a 10 by 10metres grid by setting a yellow wooden stick for each ten metres. In this way allmeasurement locations could be easily positioned by measuring the distance tothe nearest sticks. All points could thus be referenced by a (x, y) co-ordinate,where x was the position along the bottom axis and y was the position along theleft axis in Figure 3.2.

Prior to the Chernobyl accident this area was full of people on vacation in thehouses or one day 'beach'-visits. The area is very easy to reach from Novo-zybkov - the trip takes a mere 15 minutes by train, and the train calls directly atthe entrance of the recreational area. Now the number of visitors has declinedconsiderably, as the recreational area lies in a zone where the contaminationlevel exceeds 1.5 MBq m~2, the Russian limit for areas that should be evacu-ated.

Houses 1 through 5 are identical, constructed as standard houses. Most likely,they were built out of prefabricated Finnish materials, since the size of thehouses is very uniform. The main materials are wood and asbestos sheets.These houses were selected for decontamination as they had a good size, with atotal ground area of 120 m2, making them representable for typical familyhouses both in Russia and in western countries. The fact that they had firstfloors made them particularly interesting, as no measurement data are availableon the dose composition and decontamination effect at first floor level of abuilding. Whereas first floors are unusual in Russian settlements they are verywidespread in most western countries and the experience gained would be veryuseful for verification of modelling results.

Point 00,60

s -0)E_r tn _CD

CD

CD

ora0)

-a

5 -10 -15 30 35 .30Distance from the(0O,XX) line, meters

Figure 3.1. The river bank prof He from the recreational site to the Iput river.

Ris0-R-1O29(EN) 19

Plate 3.1. View of the houses before the decontamination. From the right to theleft houses 1, 2 and 3 can be seen.

Plate 3.2. The first exercise with the Bobcat cleaning a grassed area close tothe river bank.

Figure 3.2. The recreational area seen from above

H=TP

13310

JO96

1 ^ -""106

.783

217 119 86 119 2177SB

340

?—I

402

18101

722

308 89 119 119 90

758

1700

1265

f215~

| 0

•1244

1135

lo

Figure 3.3. Diagram of houses 1 through 5.

3.2 Mapping of the 137Cs Pollution in the AreaAs shown in Figure 3.2 the area was mapped on a 10 by 10 metre grid. Thedose rate was measured in all grid points with the Reuter Stokes ion chamber.Further, the dose rate was measured in 5 locations at a height of 1 metre abovethe ground level floor and in three locations 1 metre above the first floor insidethe 5 houses. Measurements were also made around houses 1 and 2 and at theriverbank. In Table 3.1 a summary of the dose rate measurements is given forthe 112 grid intersection points distributed along 7 lines with a constant x-co-ordinate. It can be seen that the dose rate is relatively constant in the area. Thelowest values were generally obtained close to the river, as the river representsa radiologically 'clean' area. At greater distances from the river the average

22 Ris0-R-1O29(EN)

dose rate was found to be 1.1 ±0.] |̂ Sv h"1. Inside the houses the radiation lev-els were very similar. At the ground floor level the dose rate was 509 ± 25 nSvh'1, and at the first floor level it was 528 + 8 nSv h~'. This increase in dose ratewith height was also observed in the houses in Novo Bobovichi in 1995 (Roedet al. 1996 a). Three reasons can be given for this:

• the soil surfaces are seen at a less flat angle, reducing the attenuation in theground.

• The roof has a lower shielding effect than the walls.• The roof contamination itself gives a higher contribution at first floor level.

In the centre of the houses the dose rate was only 340 ± 11 nSv h"! - or 30 %lower than the average value of all the measurements made in the 5 houses (inthe 4 corners and the centre).

Table 3.1. Review table of dose rates in Guta Muravinka measured with theRenter Stokes ion-chamber.

Type of location

Line 0, near riverLine 10, central green areaLine 20, along roadLine 30, along housesLine 40, between housesLine 50, in forest 9 m from housesLine 60, in forestGround floor house 1Ground floor house 2Ground floor house 3Ground floor house 4Ground floor house 5Average of 5 housesCentre of houses2nd floor house 12nd floor house 22nd floor house 3

Numberof points

1616161613161655555

5x55333

Dose rate[nSv h"!]

896107411151022121310861103484502501503552509340536529520

s.d.[nSv h"1]

7662471532004980109107102881302511616067

The contamination in the area was also measured with the CORAD device de-scribed in chapter two. Every other grid point was measured. The CORADmeasurements showed some variation in the contamination inside the area,between 1.1 and 1.4 MBq m'2. In contrast, a more uniform contaminant distri-bution was found in the forest, -1.34 MBq m"2 (see Table 3.2). The CORADmeasurements confirmed the results of the soil sampling (see below), namelythat the contamination was situated in upper 2 - 3 cm of the soil.

Ris0-R-1O29(EN) 23

LineOLine 1Line 2Line 3Line 4Line 5Line 6

6565656

Table 3.2. CORAD measurements of the surface contamination in the area.

Number of Surface contamination Standard deviationmeasurementlocations [MBq rrf2] [%]

1.07 171.14 91.36 151.06 261.36 241.32 171.33 38

Depth Distribution of I37Cs in the Soil.

The depth distribution of 137Cs was measured by analysis of a series of soilcores in a provisional detector system on the site. Detailed knowledge on thedepth distribution of the 137Cs contamination is necessary for the planning ofdecontamination work, as the thickness of the soil layer that needs to be re-moved is dependent on the penetration of the contaminant. For this purpose 63soil cores were sampled on the territory of the recreational area. On the basis ofthe analysis of the soil samples, it was possible to evaluate the spatial variationin the depth distribution of I37Cs and from this estimate the quantity of con-taminated soil that should be removed and disposed of. Besides helping in theplanning of the work, both these soil samples and additional soil samples ob-tained for analysis in the laboratory were intended for a continued investigationof the migration of radioactive substances in soil.

A special sampler, consisting of a tube inserted into a cylindrical casing, wasused to sample soil cores. To facilitate the penetration into the ground, the bot-tom end of the tube was sharpened. The sampler was driven 20 cm into theground, and subsequently excavated using a shovel, and the tube was dis-mounted. A cylindrical sample, 20 cm long and with a cross sectional area of21 cm2, was obtained and cut into 5 cm thick slices, beginning from the deepestlayer, in order to avoid cross-contamination from the more active top part.Thus, four samples were obtained, representing 0-5, 5-10, 10-15 and 15-20 cmdepth. The volume of one soil sample was about 100 cm3.

In order to characterise typical areas soil was sampled from 9 different typesof sites, each with homogeneous morphology. In order to have a statisticallyacceptable amount of information 5 to 10 soil samples were collected from eachsite. The characteristics of these sites are presented in Table 3.3. The tenth sitein Novo Bobovichi was the place at which testing of the skim and burial ploughwas to take place.

24 Ris0-R-1O29(EN)

Table 3.3. Description and location of the areas of soil sampling

Type of site Visual characteristic of Co-ordinatesthe surface

Strip of grass betweenhouses and the road.

The track of the road

Roadside

Middle of the road

Forest

Area between houses

Trampled-down plotsbetween houses

Area between pathways

Flower-bed

Very similar to virgin x=27 lineland

the Left trackthe Right track

x =22 and x=24

Left roadside. Same al- Line 21 ;YYtitude as the road

The grassed area be- Line23;YYtween tracks of the road

Natural forest between The area beyond line 45; YY in the di-recreational area and rection of the X-axis.railway

Area between houses, Between lines 30;YY and 40;YYcovered by grass

Areas of no vegetation. Between lines 30;YY and 40;YY

Plot of virgin land,similar to the forest.Even mushrooms growthere

Between lines 05;YY and 20;YY

Small hill with a diame- In the map the beds are designated byter of about 7 metres in white ellipses. The centres are located onthe virgin land area be- linel 6;YYtween roads. Before theChernobyl accident thiswas a flower-bed.

The 137Cs measurements were carried out using a spectrometer, which was set-up in one of the houses. A portable 480-channel analyser and a Nal(Tl) detector(80 by 80 mm) were placed in lead shielding with 50-100 mm wall thickness.The shielding was effected with 23 lead bricks 5*10*20 cm. A holder made outof a tin coffee can and an aluminium wire ensures that the sample is in the sameposition, on the detector axis, for each measurement. The measurement time forall samples was 120 seconds. Hereby, the sensitivity of the determination of the137Cs soil surface contamination became 20 kBq m"2 with a 50% uncertainty ofmeasurement. Due to lack of standard sources of volume activity in the recrea-tional area, calibration of the spectrometer was made in relative units.

When the full-energy peak of I37Cs is measured in this lead-shielded Nal(Tl)counter, it is necessary to allow for the contributions from natural gammaemitters to the count rate in the B7Cs energy window. This particularly con-cerns contributions from the photo peak of 226Ra at 609 keV and from scatteredradiation emitted by ^K. For this purpose, it was assumed that the vertical dis-tribution of I37Cs was exponential:

where Co is the activity concentration of 137Cs at the surface, Bq kg"1;C(x) is the activity concentration of 137Cs at depth x, Bq kg"1;a is the reciprocal relaxation length of the soil, cm"1.

Ris0-R-1O29(EN) 25

The mean a = 0.58 cm" was determined after statistical analysis of all themeasured samples from the recreational area.

B-5cm

5-10cm

1B-I5cn

15-2Bcn

137CS activity concentration in soil.

Guta-Muravinka. 1997

•137,Figure 3.4. Average relative depth distribution of Cs on the territory of thetest site. Fractions of total activity, based on the analysis of 86 soil samples.

137/-The results of measurements of the ' Cs activity concentration in the soil sam-ples are shown in the Table below. Here, the averages of the measurements inareas of different types are shown.

Table 3.4. Cs depth distribution for each type of sample area.

Typeof site

Strip of grass betweenhouses and the road.

The dirt road

Roadside

Middle of the road

Forest

Virgin land between houses

Trampled down plots aroundthe porch

Virgin soil between path-ways

Flower-bed

Surface

0-5 cm

1.5±0.4

0.4+0.15

2.5+0.4

1.2+0.3

1.7±0.4

1.8+0.5

1.3±0.5

1.3±0.5

1.4±0.4

137Cs contamination of ground [MBq m'2]

5-10 cm

0.0210.014

0.0110.01

0.10+0.02

0.04+0.02

0.1110.06

0.06+0.03

0.1410.26

0.06+0.05

0.0510.01

10-15 cm

-0.001+0.005

-0.00310.009

0.01510.015

-0.00710.007

0.01510.017

0.00710.010

0.008+0.006

0.010+0.006

0.007+0.01

15-20 cm Total

-0.014+0.006 1.5+0.4

-0.014+0.008 0.4+0.16

-0.00110.004 2.610.4

-0.01410.004 1.210.32

0.02010.02 1.810.4

-0.00110.004 1.910.6

0.00710.02 1.410.7

0.01410.003 1.4+0.5

0.00410.01 1.5+0.4

As shown in the table, although the absolute level of caesium contaminationmay vary (0.4 MBq m"2 on a road and 2.5 by the road side), the relative 137Csdepth distribution varies little over the whole area.

The average value of the reciprocal relaxation length, a, over the whole testarea in Guta Muravinka, assuming an exponential depth distribution, was foundto be cc=0.58 cm"1. In Novo Bobovichi the average reciprocal relaxation lengthwas found to be cc=0.24 cm"1 (averaged over all samples) and oc=O.49 cm"1 ex-cluding samples taken in areas where the soil was may have been disturbed af-ter the Chernobyl accident. Compared with published data (Golikov et al, 1991)

26 Ris0-R-1O29(EN)

the value of a=0.49-0.58 cm"1 corresponds to Cs depth distribution for 1988,whereas 0=0.24 cm"1 is closer to the present time value.

Another interesting point is that the caesium depth distribution under thedrain of a roof can also be adequately described as exponentially decreasing.Here, the reciprocal relaxation length was found to be a=0.28 cm"1, which doesnot differ significantly from what is generally observed, although the level of137Cs at that point exceeds the average value in the area by a factor of approxi-mately 20.

The observed shape of the vertical contamination profiles and the similaritiesof the different profiles despite widely varying absolute levels of contaminationis believed to have a simple explanation. Immediately after the deposition fromthe Chernobyl accident had occurred, 137Cs ions dissolved in rainwater pene-trated into ground and generated the initial exponential depth distribution of theactivity. The 137Cs ions were fixed, mainly by micaceous mineral particles inthe soil, so strongly that it can be assumed that no further migration of 137Cs insimple form has occurred. The micaceous particles that are most efficient infixing the radiocaesium are the illites. Compared with other soil substances,these are very small particles, which are normally mixed in a 'random-like'stacking together with for instance organic matter. The organic matter will de-generate with time, releasing the caesium-containing illite particles, which maybe transferred to deeper layers with the water flow through natural cracks in theground.

In previous investigations of soil contamination, the diameter of the samplecore was greater (about 100 cm2). The samples selected from these rather largeareas contained several large pores or cracks, through which surface water runs.Using this method which averages the contamination levels out over a largearea, less variation was recorded within a lawn, compared with the smaller di-ameter cores, that were sampled for these investigations, which may by coinci-dence contain several large pores or no macro-pores at all. Therefore, the ab-solute contamination levels will vary greatly between small-diameter samples.Due to frost during winters or water flow, new cracks will appear in the ground,through which the small particles (and thereby the radiocaesium) will migrateto greater depth. Hereby, the reciprocal relaxation length slowly decreases.

It might have been expected that since the pores in the soil under a roof drainwould be very well developed, due to the water flow, and would have a greatercapacity to retain the contaminated water than in other parts of a lawn, the ra-diocaesium would have penetrated deeper under the roof drain. After all, theabsolute level of contamination under the roof drain is about 20 times as highas that of the rest of the area. However, the soil is a very efficient filter, whichremoves the contaminants from the water flow, and there have not been re-corded great differences between the shapes of the contaminant depth profilesunder a roof drain and averaged over the whole area.

After returning from Guta Muravinka a number soil cores were analysed inthe Laboratory. These results are presented in Appendix A. The 10 profilesfrom the open areas in the settlement showed an average contamination of 1.25± 0.36 MBq m"2. This in good agreement with the CORAD results, but some-what smaller then the average results obtained from the in situ soil samplemeasurements. The average attenuation depth was 2.6 ± 1.3 g cm"3. This is ingood agreement with the in situ observations that the I37Cs contamination wassituated very near the soil surface.

Ris0-R-1O29(EN) 27

Summary of Soil Contamination

• As was found from the soil sample analysis, the 137Cs contamination ofvisually different sites within the recreational area is practically identical.Very nearly all the contamination is concentrated in the top 5-centimeterlayer.

• One exception is a road, where the level of contamination is, due to weath-ering, only 30 % of the average value for the recreational area and anotherexception is a road gutter, where the contamination is twice the averagelevel in the area. This is in-line with the observations reported in Chapter 5,which show that the contamination on a road is usually attached to roaddust particles that are now very strongly bound to the road and can rathereasily be translocated by mechanical impact.

• The contamination in flowerbeds is a bit lower (80 %) than the averagecontamination. This could be explained as follows: during deposition inrain, the water ran down from the higher positioned beds to the surround-ings. Practically all the activity is concentrated in the top 5-centimeter soillayer.

• The surface contamination of the ground below the drain of the roofs is 20times higher than the average value over the recreational area.

• the sensitivity of the portable spectrometer, was sufficient both for planningthe decontamination exercise and for scientific applications.

• Caesium migration in the soil is so slow that buried activity will remainfixed in the soil and penetration into the ground water will be insignificant.

Roof Contamination

The roof contamination was measured in two ways: with the portable SKIF 3gamma analyser based on a collimated Nal crystal and with the El-1117 f$-detector.

The SKIF-3 gamma-analyser was used for gamma measurements of 16 as-bestos sheets, taken from House 1. The whole sheets were measured. For cali-bration of the instrument parts of 5 sheets were granulated and analysed in thelaboratory. The surface activity was calculated on the basis of obtained results.The sheets with known I37Cs activity were measured with a collimated SKIF-3.Results of the roof measurements are presented in Table 3.5.

The average contamination of the asbestos sheets with 137Cs was 144 kBq m~2

(12 % of the soil surface contamination level). It should be mentioned that sig-nificant differences between individual sheets were observed. For reference,contamination levels of asbestos sheet contamination from Zaborie and NovoBobovichi are also shown in Table 3.5.

Beta measurements of roof contamination were done after the soil decon-tamination. 14 sheets from House 1 and 12 sheets from House 2 were analysed.The east side of the roof of House 1 was more contaminated than was the westside. For House 2 the opposite relationship was found. The average level of thecontamination of the roof of House 1 was found with the beta counter to be 59kBq m"2, which was 2.4 times lower, than the values obtained by gamma meas-urements. A laboratory experiment was made with separate sheets, to determinethis discrepancy between y and (3 measurements. The slight penetration of 137Csinto the asbestos-cement matrix is believed to be the cause for the twice as highgamma as beta results. This shows the weakness of p-measurements for esti-mation of contamination of roofs. Results of the p-measurements are presentedin Appendix C.

28 Ris0-R-1O29(EN)

Table 3.5. Measurements of the n7Cs roof contamination of House 1 in GutaMuravinka. 16 old sheets were counted after the replacement of the roof. Themeasurements were made with the collimated Nal detector, SKIF. Averages arecompared with measurement results from Zaborie and Novo Bobovichi.

N of sheet

12345678910111213141516

AverageZabor 1N.Bob 2N.Bob 3Zabor 5Zabor 6

Detector signalCount/sec

0.730.630.750.80.670.520.840.410.840.790.650.771.020.60.70.33

0.69±0.170.730.390.160.330.19

Surface contamination[kBq m"2]

15213015616813910817585

17616613616021412514667

144±3615469366838

3.3 Decontamination Test at the Grassed AreaAn area close to the riverbank, where many visitors were sunbathing, was se-lected for decontamination. It was the wish of the owner of the place that suchan area should be cleaned, as well as an area around the houses. The riverbankarea provided an excellent opportunity to test the Bobcat before beginning thework around the houses. The test area was a 10 by 10 m plot at the riverbankbetween co-ordinates y=140 and y=150 and x=10 and X=20 in Figure 3.2. InPlate 3.2 the Bobcat can be seen working at this plot.

The crucial experience to be gained from this work was the determination ofthe thickness of the soil layer that should be removed and a routine in manoeu-vring with the bobcat so that exactly this amount was scraped off. For abouttwo hours a man with a handheld dosimeter measured the surface dose rate af-ter each scraping and reported the results to the bobcat operator. Also, while thebobcat was scraping, a man reported back to the operator on the exact perform-ance of the scraper. After this exercise the team was reassured that it would bepossible to scrape-off a surface soil layer of a well-controlled thickness, aroundthe houses.

The removed soil was placed in a trailer and in a pile north of the cleanedarea. After the area had been scraped the bobcat was used to make a hole asindicated in Figure 3.5. This hole was filled with the contaminated topsoil. Thesand from the excavation was placed in a pile south of the hole and used tocover the hole and an area with clean sand in the end. In this way the original

Ris0-R-1O29(EN) 29

contours of the landscape were maintained. A similar method was used for thewaste from the decontamination around the houses, but here an excavator wasused to dig the holes, and this speeded up the work process considerably.

The decontamination at the riverbank took about 6 hours, and 100 m2 weretreated. This corresponds to about 16 m2 h"', including a slow start where thedriver of the bobcat was practising. The bobcat was also used to dig the hole forthe waste in the centre. A task it is not very well suited for.

10mcut A

20 m

140m 150m

liiiil cleaned area

cut A

clean sand

top soil

Figure 3.5, Diagram of the decontamination at the riverbank. The lower partshows how the removed topsoil was placed in a hole and covered with cleansand.

The dose rate was measured over the area both before and after decontamina-tion along the line y=145. The result is shown in Figure 3.6. A background doserate contribution of about 60 nSv/h should be subtracted to determine the effectof the work carried out. As can be seen, over the treated area, the dose rate wasreduced by some 60 %. This figure would certainly have been greater if theprocedure had been carried out over a larger area, as a large contribution to theresidual dose rate in the treated area comes from scattered radiation from con-tamination at rather large distances from the treated spot.

30 Ris0-R-1O29(EN)

Effect of decontamination

1400

1200

400 •

200 •

Before decontamination

- After decontamination

10 12 14 16 18 20 22 24

Distance [meters]

26 28 30 32

Figure 3.6. Dose rate profile over the green area at the 'beach'. The 'Distance'is the x co-ordinate along the y = 145 metres line.

Also 4 soil cores were obtained: two before (GM01 and GM02) and two afterthe decontamination work (GM03 and GM03). These profiles can be seen inAppendix A. The two first profiles confirm the previous result that 95 % of theactivity was located in the top 5 cm of the soil. The profiles taken after the de-contamination show that about 90 % of the contamination had been removedand that the top 5 to 7 centimetres consist of clean sand. The GM04 profile wastaken over the buried waste, and at 30 cm depth some activity can be seen. Thismay be the start of the 'waste zone'.

In conclusion, the soil profiles analysed in the laboratory confirmed the firstimpression, that the exercise at the 'beach' plot was successful in removing 5 to7 centimetres of the topsoii containing more than 90 % of the contamination.

3.4 Decontamination Around the HousesAfter the decontamination at the riverbank the work was started around thehouses. It was decided to begin the work around house 1 and move down to-wards house 5. The objective was to clean around at least three houses so thatthe house in the middle would have 'clean' houses to both sides and thus amaximum reduction in the radiation level.

The work was carried out through the following stages:

1. The bobcat was used to move the topsoii into piles.2. A team with dose meters and shovels checked the area and removed 'hot

spots', especially around trees and other difficult accessible places.3. An excavator moved into the cleaned area and dug a number of pits for the

waste.4. The bobcat was used to move the waste into the pits.5. The bobcat distributed the clean sand from the pits over the cleaned area,

restoring the previous level of the surface.

The first scraping with the bobcat moved the topsoii into piles about 10 metersapart. The narrow width of the scraper enables the bobcat to move freely evenin relatively densely overgrown areas. In the beginning, smaller bushes were

Ris0-R-1O29(EN) 31

removed manually, but as it was established that these could easily be removedby the bobcat, this was stopped and after that, the bobcat was applied withoutpre-treatment. Another problem was the roots from larger trees. Typically thebobcat could not work nearer than 0.5 meters to large trees. Here the soil had tobe removed by hand.

Whenever an area had been treated, either by the bobcat or by hand, it waschecked with handheld dosimeters and additional soil was removed if neces-sary. The procedure was followed that if the dosimeters read more than 300 nSvh"1 then an extra effort was required. Levels lower than 300 nSv h"1 were diffi-cult to obtain without treating large areas, due to the large amounts of scatteredradiation.

As the bobcat is not suited for digging pits an excavator was used for thistask. The excavator worked for about 4 hours over two days in order to dig 8pits. Some soil was also removed on wagons towed by a tractor. Some 7 - 10 %of the waste was removed in this way. The main advantage of the pits was thatwaste buried within the area was not regarded as radioactive waste and there-fore it was exempted from the strict rules applying for radioactive waste. Also,the clean sand from the pits was useful to cover possible remaining activity onthe ground and restoring the previous level of the ground. In Plate 3.3 the exca-vator can be seen digging the pit between houses 2 and 3 around co-ordinate(45, 65). To the left the clean sand is dug up into a pile and to the right thecontaminated soil is lying ready to be pushed into the pit. In the background thebobcat is filling another pit with contaminated soil. The excavator took about30 minutes to dig a hole for 8 - 10 m3 of waste.

In Figure 3.7 the decontaminated area is shown together with the position ofthe pits. As can be seen, the soil has been removed around three houses. In gen-eral, decontamination out to 10 metres from each house was desired. To thenorth (the left in Figure 3.7) the dirt road was used as a limit. The road wasfound to have considerably lower levels of contamination than had the sur-roundings (about 30 %, see Table 3.4) and it was for this reason decided not totreat the road. So in this direction the area was cleaned to distances of 8 metersfrom the houses. To the east of house 3 the soil was only removed between thehouse and the asphalted road to the toilets, due to time restraints. So in this di-rection the soil was only removed out to a distance of 2 metres from the house.

In total, the topsoil was removed from about 2000 m2. This took about 35hours with the bobcat, including filling the waste into pits and covering themwith clean sand. Scraping of the topsoil was accomplished at a work rate ofabout 75 -100 m2 h"1, depending on the number of obstacles in the area.

In Plate 3.4 the bobcat can be seen, applying clean sand from the pits to thedecontaminated area. The application of sand took about 12 hours for the 2000m2, giving a work rate of 160 m2 h"1. In Plate 3.4 the bobcat is distributing sandat the south-east corner of house 1. Although the shape of the profile was notsignificantly different from that recorded in other places, the absolute amountof contamination which had penetrated deeply into the ground was some 20times greater at the south corners of the houses, where the contaminated rain-water from the roof drains ended, than it was in the rest of the area. It wastherefore necessary to dig deeper here. Soil samples showed contamination lev-els of up to 12 MBq m'2 (see GM09 and GM11 in appendix A) and CORADmeasurements showed an average contamination level of 6 MBq m"2 at thesouth corners of the houses.

In Plate 3.5 the bobcat can be seen working behind house 1 around positionnumber (55, 35). The topsoil has been removed from the area around house 1and clean sand has been added. The bobcat is now distributing sand over the pitdug south of house 1. The picture gives an impression of the area after the de-contamination.

32 Ris0-R-1O29(EN)

At the end of the work the roof of house 1 was replaced with a new cleanroof. Various methods for decontamination of roofs had already been examinedduring the previous work in Novo Bobovichi in 1995 (Roed et aL, 1996 a) andby others (e.g., Andersson, 1991; Sandalls, 1987; Gj0rup et al, 1985).

Measurements with handheld p-detectors had indicated that the roof carried asubstantial amount of activity and the previous work in Novo Bobovichi hadshown a significant influence from the activity deposited on the roof. Still therewas a discussion as to whether there would also be a significant influence inhouses as large as those in Guta Muravinka. But after measurements with acollimated Nal detector it became clear that the contamination on the roofs wasa significant contributor to the total dose rate, especially in the first floor. Thiswas confirmed by the measurements of the reduction in the dose rate after thesoil removal, which showed that the dose rate in the first floor was reduced byonly 32 %. After this, it was decided that the roof also needed to be replaced asit had a significant influence on the dose rate.

Ris0-R-1O29(EN) 33

Figure 3.7. Plan of the recreational area with indications of the different typesof surface. Grid co-ordinates are given in units of metres.

Plate 3.3. Excavator digging holes for waste burial in the forest. The sand isplaced in a pile on the cleaned ground and the topsoil lies ready on the otherside of the hole.

Plate 3.4. Application of clean sand at the corner of house number 1.

Plate 3.5 The Bobcat in the forest behind house 1. An area in the forest iscleaned to make room for an extra pit around (54, 32).

3.5 Results of the Decontamination WorkAs described in section 3.2 the dose rate was measured at all grid points asshown in Figure 3.2 and at a number of locations inside the houses. During thework the reduction in the dose rate was measured after each stage of the work,as follows:

1. After the removal of the soil.2. After the distribution of clean sand.3. After the removal of the roof of house number 1.

Results of the Reuter Stokes Ion Chamber Dose Rate Measurements

During the decontamination work the dose rate was measured with the ReuterStokes ion chamber inside houses 1 and 2 and along a line between houses 1and 2. In Figure 3.8 the centreline data is presented for the four situations: ini-tial value, the value after soil removal, the value after the sand has been appliedand the value after the renewal of the roof of house 1. It can be seen that thedose rate changes very rapidly from the cleaned area to the contaminated area.In the centre of the cleaned area the dose rate is almost constant, as the re-maining contamination and sky-shine dominates the dose rate here (as opposedto the border areas where direct radiation from the un-treated areas is the domi-nating source). Spectroscopy measurements inside the cleaned area confirmedthat scattered radiation was at that point dominant in the spectrum.

Based on the average dose rate in the central area, at a distance of 34 to 38metres in Figure 3.8, the dose reductions were calculated, and these are pre-sented in Figure 3.9. It was found that the Chernobyl-related part of the doserate (i.e. the dose rate excluding natural background and cosmic radiation con-tributions) was reduced by some 83 %. The removal of the soil gave the largestreduction, 79 %, whereas addition of sand and removal of the roof each gave 2%. It is interesting to note that the application of clean sand only gave a further1.8 % reduction. In 1995 in Novo Bobovichi, the application of clean sand gavea 7 % further dose rate reduction relative to the initial level. The thickness ofthe clean sand layer was about the same in these two situations and this differ-ence clearly indicates that the bobcat was much more efficient in achieving acomplete removal of the radiocaesium, than were the manual methods used in1995. It is also interesting to note that the removal of the roof actually gave ameasurable reduction in the dose rate level of 2.4 %. This effect was just forone roof. If a similar contribution is assumed from the roof of house number 2,it actually contributes about 15 % of the remaining dose rate in the area be-tween the two houses.

In Figure 3.10 the dose reductions at ground floor level in house number 1 areshown, and Figure 3.11 shows the effect at first floor level in the same house.

36 Ris0-R-1O29(EN)

Dose rate between house 1 and 2

1.400

1.200 •

~ 1,000 •

800 "

600 -

400 •

200

Initial value

Soil removed

Sand added

Roof replaced

10 15 20 25 30 35 40

Distance [meters)

45 50 55 60

Figure 3.8. Dose rate along the centreline between houses 1 and 2 during dif-ferent stages of the work.

El-1101 Dose Rate Measurements

The dose rate was measured diagonally across House 1 and House 2 with theEI-1101 fixed on a tripod 1 meter above ground. The positions of the measuringlocations are shown in Figure 3.12. The measurements in the corners were onlymade at the ground floor level. The rest of the measurements were made atground floor and first floor level.

The curves of DR. inside houses were asymmetrical. There was a strong influ-ence of the 'hot spots' under the roof drainage (as marked in Figure 3.12),which can be seen at the ground floor even at the distance of 3.8 m from thecentre (Figure 3.13, Figure 3.14 and Figure 3.15). The average dose rate at thefirst floor level was significantly higher than that at the ground floor level; theinfluence of 'hot' corners by the drains could also be seen here (Figure 3.16).Removal of the topsoil around the houses led to a significant decrease in thedose rate (Figure 3.14, Figure 3.15 and Figure 3.16). After decontamination thedose rate profile became more flat. The highest reductions were observed at theground floor. At the ground floor level the Chernobyl component of the doserate was reduced by 58 % when the topsoil was scraped off. At the first floorlevel the reduction was only 35%. The application of clean sand over theground did not have a measurable effect on the dose rate. The roof removalshowed that 9% of the Chernobyl contribution to the dose rate at the groundfloor level and 27 % of the dose rate at the first floor level were caused by ra-diocaesium in the asbestos roofing sheets. In this particular case, a contamina-tion level of 144 kBq m'2 137Cs on the roof caused an exposure rate of 3.5 jiR/hat the ground floor and 13.6 (J.R/h at the first floor level (see Table B2 in ap-pendix B).

The contribution of the roof contamination to the total value of the outdoordose rate was investigated on the line, which crossed the middle of House lafrom west to east (see Table B3). The distance between individual measure-ments was 1 m. The dose rate profile across the area shows the clear influenceof radiocaesium roof contamination on the dose formation at distances of up to10 m from the outer walls of the house. A contamination level of 144 kBq m'2

of 137Cs on the roof gave an exposure rate of approximately 3 (J.R h'1 outdoors ata distance of 5 m from the outer wall of House 1.

Ris0-R-1O29(EN) 37

Dose reduction composition, outdoors

Rr:iminn^ (Ins;: rale

J7.Z).,

Figure 3.9. Reduction of the Chernobyl related dose rate contribution outdoorsbetween houses 1 and 2, obtained from the dose rate data presented in Figure3.8.

Result of decantairiratianGround floor of house 1

Remaining dose rate30.7%

Roof replaced11.3P/o

Sanda&fed

Figure 3.10. The effect on the Chernobyl-related dose rate of the various stepsof the decontamination work on the ground floor of house 1.

Effect of decantaniratiaii1st floor of house 1

Remaiiing dose rate

Sand aided1.5%

Figure 3.11. The effect on the Chernobyl-related dose rate of the various stepsof the decontamination work on the first floor of house 1.

38 Ris0-R-1O29(EN)

North-west corner(NWC)

Drainage

oSouth-west corner(SWC)

North-east corner (NEC)

Drainage

South-east corner (SEC)

Positions of detector for dose rate measurements

Figure 3.12. Positions of the measurement locations on the ground floor ofHouse 1 and House 2

-SAGNE;gund

-*—SKXWQground

• SACfirst

first- 4 - 2 0 2

Dstance from center [m]

Figure 3.13. Dose rate profile at ground and first floor level inside house onemeasured with EL-1101.. South-west corner to north-east corner and south-east corner to north-west corner.

Ris0-R-1O29(EN) 39

250 -

200 -

150 --

Ig 100 +

5 0 -•

Under drainage

-10

DR before, pR/h

DR after, |jR/h

Wall

-5 0 5

Distance from center of house [m]

10

Figure 3.14. Chernobyl component of dose rate inside (ground floor) and out-side House 2 before and after intervention, east-west corner direction meas-ured with El-1101.

Ground floor

Before

Soilremoving

Si 4 0 -

oQ

- 4 - 2 0 2 4

Distance from center of the house [m]

•A--- Pouring sand

-o—Roofremoving

-*— New roofput on

Figure 3.15. Dose rate at ground level in house 1 at different stages of inter-vention, Chernobyl component, measured with El-1101.

40 Ris0-R-1O29(EN)

First floor

i

60

50

40- -

30--

oQ 20

10--

South-west corner North-east corner- Initial

— • — S o i lremoved

- - -A- . - Sand added

-Roofremoved

-Roofrenewed

- 6 - 4 - 2 0 2 4 6

Distance from center of the house [m]

Figure 3.16. Dose rate in House 1 at different stages of intervention, Chernobylcomponent, first floor, measured with El-1101.

Unscattered Cs Photons from Different Directions in House 1

A portable gamma-analyser, SKIF-3 (Sinco, Russia), based on a 25*25 mm so-dium iodide detector crystal, was used for a series of angular in situ gammaspectrometric measurements. The detector was placed in a lead collimator on atripod (minimum lead wall thickness: 5 cm). The construction of the tripod andlead shielding made it possible to point the collimator opening (with a 45 de-grees spatial angle) in any direction and thereby to assess the angular distribu-tion of the gamma flux.

Only the unscattered part of the I37Cs gamma radiation was measured. InTable 3.6 the results of these measurements, made before and after the totaldose reduction strategy was carried out, are presented. If it is assumed that the 6measurements pointing in different directions adequately represent the contri-butions from all angles, the results of the directional measurements express thefractions of the total unscattered radiation (shown as percentages in Table 3.6).The collimated sodium iodide detector measurements show that the roof con-tamination was a significant contributor to the unscattered part of the gammaflux, and thereby to the dose rate. This conclusion was confirmed by the doserate reductions obtained through replacement of the roof.

Ris0-R-1O29(EN) 41

Table 3.6. Angular distribution of the flux of gamma photons with energy 661.6keV in the middle of house I before and after the dose reduction strategy wascarried out. The measurement results are also expressed as percentages of thetotal (sum of measurements) before action. The uncertainty of each measure-ment was about 15 %.

Direction

rooffloorsouth wallwest wallnorth walleast wall (Sum

( forest)( gate)(road )House 2 )

Peak area,

Before action

0.3800.590.800.230.532.53

counts/sec

After action

0.0500.240.220.170.200.88

Fraction oftotal

[/oj

15%0%

2 3 %3 2 %

9%2 1 %

100%

Fraction ofinitial total

ro/1

2%0%9%9%7 %8%

3 5 %

The results from Table 3.6 are also visualised in Figure 3.17.

Angle distribution of gamma flux

east wall ( House 2

north wall (road )

floor

south wal,1 Before

1 After

west wall { gate )

Figure 3.17. Unscattered gamma flux from different directions in house 1,ground floor, before and after dose reductive action.

3.6 Cost Analysis of the ResultsAs hired local workers carried out most of the actual decontamination work, itis possible to estimate the costs of an implementation of the strategy in Russia.The following example refers to a single house with a 1000 m2 garden. If morehouses were treated in the same area the work could have a greater effect, dueto the removal of contamination at large distances, and a lower cost per house.In 1997 skilled workers were paid 20 $ per day, whereas unskilled workerswere paid 10 $ per day.

First a survey of the area should be made in order to estimate the depth distri-bution of the contamination, the influence of the roof, etc. This can be done intwo days by a skilled technician. One day would be required for measuring andsampling, and one day for analysis.

42 Ris0-R-1O29(EN)

A new Bobcat (mini-bulldozer) would cost about 30,000 $. If it is assumedthat this is written off over 6 years and that it is used for 25 weeks per year theweekly cost is 200 $. An additional ca. 100 $ per week would be required forgasoline (diesel) and maintenance, giving a total cost of the Bobcat of 300 $ forone week's work (5 days). The bobcat driver must be a skilled worker, andwould earn 20 $ per day, corresponding to 100 $ per week. Two workers withshovels should assist the driver in areas that are difficult to access. This wouldcost another 100 $ over a week.

Three options may be considered for the roof: no action, cleaning the roof orreplacing it. Cleaning the roof would require that two men work for three dayswith high pressure water hosing equipment. The costs for changing the roof canbe based on the actual costs in 1997 (334 $ for new sheets and 80 $ for labour).

The averted dose is calculated from an assumption of the occupancy pattern,as follows: 10 % of the time spent in the garden, 20 % down stairs, 30 % up-stairs (sleeping, etc.) and 40 % outside the area (possibly working). Cleaningthe roof would reduce the dose rate by half as much as would be achieved byreplacing the roof (Roed et al. 1996 a). In the collective dose calculation it isassumed that 6 people live the house over the next 50 years.

A summary list of the costs and the calculated averted doses is shown inTable 3.7.

Table 3.7. Cost and averted doses when a single house is decontaminated. Theaverted dose for the first year refers to individual doses, whereas the avertedcollective dose is based on the assumption that six people live in the house.

Item

Initial surveyBobcatDriverExcavatorManual assistanceRoof

Sum

Averted dose 1st year

Averted collective doseCost per averted Sv

Untreated roof

40$300$100$50$

100$0$

590$

1.8 mSv

0.30 manSv

1967 $ manSv"1

Cleaning roof

40$300$100$50$

100$90$

680$

2.0 mSv

0.34 manSv

2015$manSv"1

Replacing roof

4030010050

100414

1004

2.2 mS

0.37 manS2713$manSv

The IAEA (1983) recommend a minimum value of 3000 $ manSv"1, while IRCP(1989) recommend a value of 20,000 $ for most developed countries. The costof the dose reductions obtained through this work are well below these valuesand it seems that the methods would be useful in the areas affected by theChernobyl accident, also seen from a pure cost benefit view.

Another consideration that could be made is the comparison between the costof decontamination of a house and the cost of Chernobyl compensation for theresidents. Four adult people in a house in the most contaminated zones receiveabout 160 $ per month in Chernobyl compensation. By for instance reducingthis amount by 25 % for people living in decontaminated houses, the Russiangovernment would have a profit after 1 or 2 years.

While the option including replacement of the roof has the highest cost peraverted Sv it should be noted that this option has the additional benefit of abrand new roof.

Ris0-R-1O29(EN) 43

It must be stressed that the figures presented in Table 3.7 are connected withlarge uncertainties. Wages are at the moment low in Russia and they must beexpected to rise over the coming years, making a decontamination more expen-sive.

Even larger uncertainties are associated with the dose calculations. The dosereductions obtained will vary from house to house, and the collective dose willvary with the number of people living in the house and their behaviour patterns.

4 Skim and Burial Ploughing

4.1 The Radiological PurposeNumerous different countermeasures were considered and applied to reduceexternal as well as internal doses due to contamination of large open (rural)land areas after the Chernobyl accident. In the early phase after the accident noclear contingency strategy had been formulated, and as a consequence, the in-troduced operations (listed below) were of a hit or miss nature.

• harvesting and disposal of surface-contaminated crops (leaving the landrelatively clean).

• spraying plastic or rubber-based fixatives on soil and dust to suppress resus-pension.

• application of lime and inorganic fertilisers to soils to reduce root uptake ofcertain radioelements.

• removal of surface soil, and thereby removal of most of the contaminationfrom the land.

• normal mouldboard ploughing (to 20-30 cm depth) to suppress resuspen-sion, dilute the contamination and disperse it from the surface of the ground.

Some of these countermeasures were ineffective and others had adverse sideeffects. For instance, there was no means of predicting in advance the optimumamounts of lime and fertilisers to be added to achieve a known reduction insoil-to-plant transfer of the relevant radioelements (Desmet, 1991). The processwas therefore not as cost-effective as it might have been.

The application of fixatives in the contaminated land areas has been limitedto suppression of resuspension. Fixatives have, however, also been consideredfor decontamination operations in smaller land areas (Andersson & Roed, 1994a)-

Scraping off a 10 cm layer of surface soil from an area of 1 km2 by bulldozers

generated about 50,000 metric tonnes of radioactive waste for disposal. Inshallow virgin soils of the former Soviet Union this type of operation wouldoften remove the entire fertile soil layer. Melin et al. (1991) reported difficul-ties in scraping off thinner layers with a bulldozer.

An external dose reduction factor of about 15 was obtained by ploughing to adepth of 30 cm in a 86Rb contaminated field (gamma photon energy 1.078MeV) with an ordinary mouldboard plough (Roed et al., 1996 b).

44 Ris0-R-1O29(EN)

One advantage of ploughing procedures as large scale dose reducing meas-ures is that they can be carried out by local agricultural workers. The localgroundwater conditions should, however, be considered prior to any ploughingprocedure to ensure that the contamination is not brought too close to thegroundwater level. In the Chernobyl case, a fine mist of water has often beenapplied immediately before ploughing, in order to reduce resuspension.

In terms of cost and availability, normal ploughing is an attractive counter-measure, but it is of limited effectiveness since the normal mouldboard ploughoperates down to a depth of only about 25 cm and much of the contaminationremains accessible to plant roots. Further ploughing may not improve the situa-tion since much of the buried contamination would then be returned to the sur-face.

By deep-ploughing, to 45 cm instead, contaminants such as radiocaesium areplaced beyond the root-zone of many crops and will remain in a thin layer deepin the soil profile without further intervention, due to the strong soil fixation(Andersson & Roed, 1994 b).

Experiments by Menzel et al. (1968) demonstrated an effectiveness of morethan 90 % in reducing radiostrontium uptake by various subsequently growncrops by deep-ploughing. The effectiveness was substantially increased by ad-dition of sodium carbonate to the soil. Since sodium carbonate is a root inhibi-tor rather than a chemical, which affects strontium availability, the resultsshould also apply to other radioisotopes.

The reduction of the external radiation in the area will also be larger than byconventional ploughing. Also, the radioactive matter will have been placed suf-ficiently deep in the soil profile that subsequent ploughing does not redistributeit.

The main disadvantage of deep- ploughing is that poorer quality soil isbrought to the surface.

In order to overcome the problems associated with both normal ploughingand deep-ploughing, a new type of plough has been developed jointly by Ris0National Laboratory and Bovlund Agricultural Engineers Denmark. Thisplough ideally skims off the topmost layer of soil (about 5 cm) and buries it at adepth of some 40-50 cm without inverting the intermediate layer. Hence thename 'skim and burial plough'. The removal of only about a 5 cm layer of top-soil is a minimising of the effect on the fertility of the land.

Overall, the skim and burial plough greatly reduces radiation levels at theground surface, the resuspension hazard is eliminated, most of the contamina-tion is made inaccessible to plant roots and the effect on soil quality is rela-tively small, but the procedure should not be applied in very shallow virginsoils. The plough would only have little beneficial effect in areas, which havebeen cultivated after the contamination took place.

4.2 Plough Design and ConstructionExamination of various types of common ploughs and attachments showed thatnothing was readily available which could be easily modified to achieve thedesired objective. A machine which lifts a 50 cm layer of soil to a height ofabout 60 cm while simultaneously placing the topmost 5 cm layer in the bottomof the trench was considered but this was rejected on account of its excessivepower requirements making it unsuitable for use with normal-sized tractors. Itwas therefore decided to design and build an entirely new type of plough andthis has been done.

A skim coulter first places the upper 5 cm layer of soil in a trench made bythe main ploughshare. In one movement, the main ploughshare then digs a new

Ris0-R-1O29(EN) 45

the top layer from the next furrow in the new trench. In this way, the 5-50 cmsoil layer is lifted only about 10-15 cm and the power requirements are mini-mised. However, the possibility exists that the 5-50 cm layer will be partiallyinverted and that not all of the contaminated top layer will be placed in the bot-tom of the trench.

Initially, a 1/10 scale model of the skim and burial plough was constructed andtested. After various adjustments and modifications, a full-sized version wasconstructed. As a result of further attempts to improve performance subsequentto testing, various modifications were made to the original design.

Based on the much modified full-sized model, a prototype full-scale ploughwas constructed and tested on a soil consisting of a 30 cm layer of light soiloverlaying a sandy soil. This field was specially selected for a rigorous test ofthe performance of the plough on a light soil. The tests showed that it was nec-essary to build a higher and broader shield in order to prevent the sandy soilfrom falling into the furrow before the top layer had been placed in the bottomof the furrow.

Further testing and modifications resulted in a plough considered to be worthyof extended and intensive field trials. The plough is shown in Figure 4-1.

Figure 4.1. The Skim and Burial Plough showing the positions of the mainploughshare (I) and the skim coulter (2).

4.3 Field Trials and DiscussionThe plough was tested in a field near Novo Bobovichi in August 1997. The testfield was one of only few pastures in the area, which had not been cultivatedsince the Chernobyl accident occurred. The area which was skim-and-burialploughed measured 20 m by 50 m. The soil type was characterised as a sandyloam horizon grading into sand below about 10 cm depth. The plough, whichweighs about 880 kg, was drawn by a normal Russian four wheel driven farmtractor. The power requirement of the plough is some 90 kW (120 hp). Theplough produced a furrow about 60 cm wide and 50 cm deep. The skim coulter,which is adjustable, was set to skim off the contaminated topmost 5 cm soil ho-rizon, but often went a few cm deeper and occasionally did not remove anything.

Ris0-R-1O29(EN)

which is adjustable, was set to skim off the contaminated topmost 5 cm soilhorizon, but often went a few cm deeper and occasionally did not remove any-thing.

The soil measurement data for samples NB21-24 (in Appendix A) show thevertical distribution profile of the B7Cs contamination in the test field prior toploughing. All these profiles have sharp l37Cs peaks, which supports the hy-pothesis that the area is pasture land. Distribution profiles NB21 and NB23show that by far the most of the radiocaesium is located at a depth of only a fewcentimetres in the soil. Contrary to this, distribution profiles NB22 and, par-ticularly, NB24 have shapes which could be indicative of a much deeper mi-gration. However, it became apparent after the soil had been ploughed that apart of the test area had been in use after the Chernobyl accident to store largeamounts of lime, which had possibly been applied to farmland to reduce theradiostrontium uptake by plants. The small increase in the contamination in thevery topmost few mm of these vertical profiles must be ascribed to eithersmearing of contamination during mechanical removal of the chalk or possiblyresuspension, although the latter process is reported to have had little impact inrecent years.

Plate 4.1 shows the ploughing process. This picture shows that the unskilledlocal operator was going so fast that the shield did not properly prevent the soilfrom falling into the furrow before the top layer had been placed in the bottomof the furrow. Further, the ploughing depth was often not properly adjusted andthis greatly affected the performance of the skim coulter. In the picture, theskim coulter goes too deep, and due to the high speed the skimmed-off layer isdislocated horizontally and not falling into the bottom of the trench. Thisstresses the importance of experience with ploughing (at a suitable velocity),and in particular, understanding of the objective to be achieved. Compared withprevious trials of the plough the latter problem was in this case complicatedsince instructions were given through a local instructor, as it would often occurin reality.

Plate 4.2 shows the furrows after a ploughing. The furrow produced by theskim coulter is seen to the right in the picture. It is seen that the furrow pro-duced by the main coulter (immediately to the left of the skim coulter furrow)has to some extent collapsed. The reason for this was that the ploughing wascarried out in the late summer, following a very warm and dry period. The soiltherefore contained very little moisture and the subsoil crumbled and fell to thebottom of the trench. Consequently, the skimmed-off layer containing the con-tamination was not buried sufficiently deep. As the soil had not been cultivatedin many years the thick top soil horizon was held together by the mature turfmat. Layers removed by the skim coulter and in some cases even layers underthat contained so much organic material that they were held together and werein some cases stacked vertically because of the high velocity (sometimes bro-ken at the middle) and only partially tilted. Therefore, several contaminationpeaks are in some cases seen at different depths in the vertical profiles afterploughing (see sample profiles NB25-28 in Appendix A. Bits of grass turf andchalk in the left part of the picture show that the skimmed-off top layer was,due to too high speed, in some cases slung far away from the furrow in which itshould have been placed.

Plate 4.3 shows the area during the ploughing process. The white colour oflime is here clearly seen in the track produced by the skim-coulter. This picturealso shows that due to the mature grass cover the large amounts of lime werenot visible before the ploughing. The thickness of the lime layer (partly mixedwith the topsoil) corresponded well with the ca. 2-4 cm increased contamina-tion depth in profiles NB22 and NB24.

Ris0-R-1O29(EN) 47

Monte Carlo calculations of gamma radiation transport have shown that in alarge area where the contamination lies about 4 cm deep ca. 50 % of the doserate is due to contamination at distances greater than 10 m. Also by MonteCarlo calculations, it was found that the dose rate from within an area of radius10 m is reduced to about 50 % if a contamination at a depth of ca. 4-5 cm istransferred to a depth of ca. 8 cm. This means that a 2-4 cm lime top layer in anarea with a radius of 10 m would be expected to reduce the dose rate at thecentre by some 25 %.

Figure 4-2 shows the measured dose rate on a horizontal line across theploughed area. The area that was ploughed is that from -10 m to 10 m from thecentre. In the area where the samples with the deepest contamination peak weretaken the initial dose rate was found to be the lowest. It is seen that the doserate level after action is as expected slightly lower in the area with the moreshallow contamination, since the skimmed-off layer is (ideally) positioned withthe turf facing down. As can be seen, the shielding effect of the lime layer inthe central area is in good agreement with the results of the Monte Carlo cal-culations, assuming that the dimensions of the lime-covered area has approxi-mately the same dimensions as the test area. The 'level' lines in Figure 4-2 indi-cate the dose rate levels in areas that were least affected by the lime. Fromthese two lines together with the natural background line (this was measured in1995 in the area and was assumed to be practically constant), the dose reduc-tion factor, R could be found as:

R=(D'a - D'b) / CD's - D'b),

where D'; and D'a are the dose rates 1 m above the surface, respectively beforeand after skim-and-burial ploughing, whereas D'b is the dose rate contributionfrom the background radiation.

In this case R was found to be 2.2. The rather limited effect compared withpreviously recorded results is partly due to the rather small size of the treatedarea, since much of the dose rate is caused by contamination at great distances.If very large areas were treated it has been found by the Monte Carlo method(Roed et al., 1996 a) that this treatment would result in a dose rate reduction bya factor of 6-7. From previous trials in a Danish pasture (Roed et al., 1996 b) aswell as in a pasture in the Ukrainian part of the 30 km zone around the Cherno-byl power plant (Hubert et al., 1996) dose reduction factors as high as 15-20were in some cases reported to be expectable for very large areas, where theconditions were most favourable.

48 Ris0-R-1O29(EN)

Plate 4.1. The plough inuse in the Novo Bobovichiarea. The skim furrow ismade on the left by the skimcoulter while the maincoulter simultaneouslycreates the deeper trenchin which the top layer isburied. The plough is heregoing too deep and theperson behind it is trying toadjust the ploughing depth.

Plate 4.2. This pictureshows the furrows after aploughing. Traces of limeare clearly seen, both in thefurrow produced by theskim coulter and in thetreated land to the left,where soil from lower lay-ers covers most of the area.Note that also bits of grassturf thrown to rather largedistances are visible in theleft part of the picture.

Plate 4.3. The tractor andplough are here about tostart a new furrow. Note thewhite colour of lime in theskim furrow created by theprevious run and that bits ofturf have not been removedat the centre of the skimfurrow.

1400

(0

oQ

600 • -

400 • -

200

0

-before ploughing

- after ploughing

-30 -20 -10 0 10

Distance from centre of plot (m)

Level before action

Level after action

' Natural background20

Figure 4.2. Dose rate [nSv/h] traverse (east to west) the 20 m by 50 m skim-and-burial ploughed area before and after ploughing, as measured with aReuter Stokes ion chamber at a height of 1 m above the ground. The naturalbackground contribution is indicated by a dotted line. The lines shown for thelevels before and after treatment refer to those parts of the treated plot, whichwere least affected by a lime top layer.

The potential reducing effect of the procedure on the internal dose receivedfrom crops grown and consumed through the food-chain was not assessed in thearea. Naturally, this would greatly depend on the local farming practice andtype of crops applied (e.g., shallow or deep rooted), and also on the prepara-tion/refining of the food products prior to consumption. Finally, the local tradi-tional diet is an important factor. The internal doses might well be dominatedby the digested highly contaminated mushrooms collected in forest areas ratherthan by agricultural products as such.

An important feature concerning application of the skim-and-burial plough inagricultural practice is the limited effect on soil fertility. In some virgin landareas of the former Soviet Union, however, the fertile layer is not much thickerthan that which is skimmed off by this plough. In a virgin land area near NovoBobovichi, very near the ploughing test area, effectively the same procedurewas in 1995 carried out manually by triple digging (Roed et al, 1996 a). Al-though the treatment by skim-and-burial ploughing of such shallow soils can beproblematic, the triple digging test plot is now completely overgrown, indicat-ing that the procedure has not had a devastating effect on the fertility. Reduc-tions in plant radiocaesium uptake by as much as a factor of 10 may under fa-vourable conditions be expected (Roed et al., 1995).

The cost of a skim and burial plough is at present about 4000 ECU, which isapproximately twice the cost of an ordinary plough in Western Europe. How-ever, the annual discount will be dominated by the much more expensive,though often readily available, ordinary farming tractor that is required.

With the skim and burial plough an area of about 3000 m2 can be treated perhour, corresponding to 3 106 m2/year. The total costs per unit of area treatedwill therefore be low.

50 Ris0-R-1O29(EN)

5 Asphalt Scraper Experiment

5.1 Asphalted Roads in Novo BobovichiIn 1995 it was established that some of the asphalted areas in Novo Bobovichicarried considerable amounts of 137Cs. After completion of the decontaminationstrategy which was carried out in 1995, the contamination of the area was re-assessed. It was found that there was a marked increase in the dose rate at 1 mabove the roads leading to the houses. This is clearly seen in Figure 5.1, whichshows a series of measurements of the dose rate across the treated area. It wasmainly on the pavements and walkways around the houses that dose rates wereelevated. The paved areas used for heavy traffic had at some point after theChernobyl accident been treated with a layer of new asphalt. In any case, themechanical removal of the contamination by heavy traffic would have substan-tially reduced the dose-rate contribution from these surfaces over almost a dec-ade. Due to the close proximity to the houses the contamination on the walk-ways would give a significant dose rate contribution to people staying in thehouses. It was therefore decided to investigate the dose reducing effect oftreating an asphalted area with a commercial road scraper, and already in 1995an area in Novo Bobovichi behind the treated houses was reserved for the ex-periment, which was carried out in 1997.

5.2 Contamination of Asphalt Surfaces in GeneralThroughout the years, many investigations have been made in attempts to re-move radioactive contamination bound to dust particles of different sizes onroads. These were performed by various means including sweeping (Clark andCobbins, 1964; Sartor et al., 1974, Calvert et al., 1984, Andersson, 1991), waterhosing (Roed and Andersson, 1994; Warming , 1984) or other means (e.g.,'shot-blasting', Warming, 1987). Although only few of these methods could beapplied successfully after the Chernobyl accident, they generally demonstratedthat much of the radioactive material deposited on a road will remain attachedto a thin superficial dust layer or adsorbed directly on the surface. After depo-sition, the level of contamination will decrease depending on the amount ofmechanical impact, surface water flow, etc.

An experiment was carried out with the objective to give a relative measureof the retention of radiocaesium in the bitumen fraction of asphalt coatings. Thesurface of roofing-felt covered with stone chippings has approximately thesame roughness as an asphalt surface. A surface of 46 cm by 53 cm of this ma-terial was covered with bitumen, which had been heated to 150 °C for 4 hours.When the surface had dried, 1.09 litres of water, corresponding to 4.5 mm ofwater over the total area, containing 1061 Bq/1 of 137Cs was slowly and evenlyapplied by spraying homogeneously over a period of 45 minutes. The surfacewas placed horizontally, but at the middle of one side, wherefrom the exceedingwater was to run off, the surface was held 8 mm down. 0.99 1 of the water wasfound to run off. This corresponds to 90.8 % of the water. A measurement on aGe(Li) detector of the activity in the run off water revealed that only 7.3 % ofthe activity was intercepted by the surface. The fact that very little activity wasleft on the surface corresponds well with the visual impression that the bitumensurface repelled the water. As very little contamination was intercepted by thebitumen fraction of asphalt, the contaminants on such surfaces must be associ-ated with exposed pebbles and street dust.

Ris0-R-1O29(EN) 51

An experiment has been carried out in order to establish that the low porosityof an asphalt surface will permit virtually no downward migration through thesurface. Here, an asphalt core with a diameter of 72 mm, corresponding to aroad surface area of 0.0041 m2 had been taken from a trafficked area in Swedenwhich had been contaminated by the Chernobyl accident several years earlier.The 'hot spots' on the asphalt surface were delineated with white chalk marks.These spots were then carefully ground-off using a steel chisel and weighed.Knowing the area of sample removed and the mass, the approximate thicknesswas calculated. Following each grinding-off of high spots, the radiocaesiumcontent of the samples was measured. The first grinding removed 10.0 g, corre-sponding to 1.23 mm and took with it 78 % of the caesium contamination. Thesecond grinding removed a further 9 % in 9.4 g, while the third grounding re-moved 10 % in 15.4 g. The conclusion drawn from this experiment was there-fore that several years after deposition the upper 1 mm containing small, moreor less firmly fixed dust particles holds practically all the deposited radiocae-sium in the asphalt profile. As the method applied may have given rise to somecross-contamination of the sample, it is likely that en even greater fraction washeld in the upper 1 mm layer. This finding is in-line with the results of in situmeasurements over longer time-periods after the Chernobyl accident, whichshowed that in trafficked areas the mechanical wearing of asphalt surfaces of-ten reduced contamination levels by more than a factor of 20 over a decade.Further experiments (Andersson, 1991) have shown that the effect of sweepingroads and thereby also of natural weathering processes on road surfaces, whichhave received a radiocaesium deposition, will strongly depend on the surfacedust loading.

5.3 Experiments in Novo BobovichiThe size of the test area in Novo Bobovichi was app. 8 by 10 meters and theasphalt had remained undisturbed since the Chernobyl accident. The scraper,which was brought from Denmark, weighed about 100 kg. The grinding deviceof the scraper consisted of four cross-shaped grinders mounted at the ends oftwo perpendicular rods, rotating around a common, central axis. A shield pre-vents the loosened material from being spread over a large area.

The test area was covered by considerable amounts of organic litter. The or-ganic cover consisted of partly mouldered leaves and needles from the sur-rounding trees as well as of moss and lichens, indicating that the mechanicalimpact in the area since the Chernobyl accident had been very limited. The137Cs contamination level before decontamination was measured by in-situgamma spectrometry and using the CORAD device. Throughout the exerciseCORAD measurements were performed in the four different points indicated inFigure 5.2, and at the centre of the circular test area. In situ gamma spectromet-ric measurements were made only at the centre of the area. These measurementseries were repeated after each treatment of the asphalt area. Further, the dose-rate was measured with the Reuter Stokes ion chamber to investigate the totaldose reducing effect in the area of the treatment.

First, the organic matter was removed from an area with a diameter of 2 me-tres (see Figure 5.2), by thorough sweeping using an ordinary broom. The or-ganic material was collected and it was found to amount to 7.5 kg m"2, contain-ing 255 kBq m"2 of 137Cs contamination. The initial level of 137Cs contaminationwas found by both in situ spectrometry and by the CORAD method to be of theorder of 630 kBq m"2. The residual contamination level would therefore be ex-pected to amount to about 375 kBq m~2, which was verified by in situ measure-ments (as shown in Table 5.1). After this organic 'litter' sample had been taken,

52 Ris0-R-1O29(EN)

a larger area with a diameter of 6 metres was swept in preparation for the appli-cation of the scraper. The height of the scraping device was set so that it wouldremove a ca. 1 cm surface layer of the asphalt. The specific height of thegrinding device was set relative to the height of the wheel-pairs of the scraper,and since the asphalt surface was not plane, it was soon found that the methodremoved more than 1 cm in some places and practically nothing in others. Itwas therefore decided to give the test area a second treatment with the scraper.However, this time, the grinding depth was not pre-set, but continuously manu-ally adjusted, so that a layer of a more homogeneous thickness was removed.The slight colour changes in the treated areas gave a good indication of the ho-mogeneity. After each scraping, the loosened material was swept away with abroom. The results of these two successive scrapings are also shown in Table5.1. An analysis of the removed material from the first scraping showed thecontent of 137Cs to be 21.8 kBq kg"1, corresponding to a surface contaminationof 195 kBq m'2. Subtracting this from the ca. 380 kBq m"2 before the treatment,the residual contamination level would be about 185 kBq m'2, and this wasagain verified by the in situ measurements (as shown in Table 5.1). The secondscraping was found to remove less contamination per unit of mass (13.2 kBqkg'1), and reduced the contamination level by a further ca. 100 kBq m'2 to about85 kBq m~2. After the second scraping treatment, the effect of running over thearea with a municipal vacuum sweeping device was simulated by ordinary vac-uuming, to remove any residual loose contamination which had not been re-moved by the broom. This would particularly affect the small particles. Thevacuuming removed 2.7 kg per m2, corresponding to a surface contamination of36 kBq m"2, so that the residual contamination on the surface was at the end ofthe order of 50 kBq m~2. The average decontamination factor for the scrapingprocedure was found by the in situ measurements to be about 5. As shown inTable 5.1, the corresponding reduction in dose- rate at a height of 1 m abovethe treated surface was from 656 nSv/h to 517 nSv/h (about 20 %). As the doseprofile in Figure 5.1 indicates the dose rate is dominated by contributions froma large area.

The first of the two plates below shows the CORAD device in operation inthe test area, and the second plate shows the asphalt scraper in action.

Dosis Profile over the decontaminated area

15

Distance [m]

Figure 5.1. Dose profile across the decontaminated area in Novo Bobovichi in1995, as measured with the Reuter Stokes ion chamber. The arrows show thepositions of the roads leading to the houses.

Ris0-R-1O29(EN) 53

i— 2 metres —»Asphalt sample

— 2.5 metres —Org. litter sample

- 5.4 metres - asphalt scraped off-

6 metres - organic litter removed

Figure 5.2. Plan of the asphalt scraping test area. The four dots on the innercircle indicate points of CORAD measurements. CORAD, dose rate and in situgamma spectrometric measurements were also performed at the centre of thearea.

54 Ris0-R-1O29(EN)

Plate 5.1. The CORAD device measuring the contamination of the asphaltedarea after removal of the organic cover. The picture shows that the photo-multiplier of the Nal crystal based system is facing down. This is the onlyopening in the lead collimator surrounding the crystal. The box with the elec-tronics for data collection and treatment is seen to the left in the picture. Themeasurements are made at a height oflm above ground level.

Plate 5.2. The asphalt scraper working on the contaminated pavement. As canbe faintly seen from the picture, the road is covered with a powder of loosenedasphalt particles after the operation.

Table 5.1. Measurement results from the dose reduction exercise. Contamina-tion level measurements obtained with CORAD (number of measurements inparentheses) and gamma spectrometer and dose rates measured with the Reu-ter Stokes ion chamber.

Description

Initial valueAfter litter removalAfter 1st scrapingAfter 2nd scrapingAfter vacuumingTotal DF

CORAD[kBq/m2]

629 ± 4 (2)381 ±20 (10)192 ± 28 (6)85 ±40 (3)

• 78 ±30 (6)5.0

Reuter Stokes[nSv h"1]

826656

517

In-situ Ge[kBq/m2]

370

83

6 Monitoring of the 1995 Site

The results of the investigations in 1995 are described in detail in an earlierreport by Roed et al. (1996 a). This Chapter reports work carried out to exam-ine the changes in dose rate levels that have occurred in these locations in theperiod 1995-1997.

The Decontaminated Test Area Near Novo Bobovichi

In 1995 decontamination tests took place in a Russian settlement near the vil-lage of Novo Bobovichi (ca. 25 km north-north-east of Novozybkov). In thisarea, where deposition of contaminants occurred in rainfall, the 137Cs level wasoriginally close to the threshold value for resettlement, and the outdoor doserate was in 1995 found to exceed 1 uSv/h in parts of the test area. Overall, thearea was forested, but an area measuring about 100 m by 100 m had partiallybeen cleared of trees to make room for the settlement. The dose rate was foundto be particularly high in places in or adjacent to the large contaminated forest.

The main objective of the decontamination work in 1995 was to reduce thedose rates in and around three selected houses. In an area measuring about 20 mby 20 m around the houses, a topsoil layer of about 5-10 cm in thickness, con-taining much of the contamination, was skimmed off. Subsequently roofs of thehouses were either cleaned or completely renewed, and finally, a ca. 5 cm layerof clean sand was added to provide shielding against residual contamination inthe areas where soil had been removed. Excluding the contributions from cos-mic radiation and from naturally abundant radionuclides, the dose rate insidethe houses was found to have been reduced by up to two thirds. Although thisresult gave the clear indication that it is still possible to significantly reducedose rates in areas heavily contaminated by the Chernobyl accident, somescepticism was expressed as to whether the dose reduction would be permanentor deposition of resuspended soil from the surrounding untreated areas wouldre-contaminate the decontaminated test plot. Therefore, dose rates were re-measured in 1997 in the same positions as in 1995. Table 6.1 shows the resultsof the new measurements in comparison with those of 1995. The measurementstandard deviations of the individual results were generally less than 1 %.

56 Ris0-R-1O29(EN)

In 1995, the positions of the measurement points had been marked withwooden sticks. In 1997 most of these sticks were impossible to retrieve, and thepositions outside the houses were therefore only approximately the same as theoriginal (within one meter). However, Table 6.1 shows that the standard devia-tions of the relationship between the dose rate contributions of the contami-nants measured in 1997 and 1995 in the different types of area are rather lim-ited. It is also seen that the standard deviations do not appear to change withincreased number of measurements points nor with the size or location of thearea, suggesting that the deviations relate to the difficulties to find the exactsame positions, rather than to changes in the contamination distribution pattern.Further, it is seen that there is no significant difference between the averagerelationships (nor standard deviations) of those areas that were treated andthose that were not. This also indicates that no significant re-contamination ofthe treated areas has occurred over the two years.

From all the measurements of contamination levels of 134Cs and 137Cs in sam-ples in the area in September 1995 it was found that there was 36 times more137Cs than B4Cs, with a standard deviation of only a few percent.

The exposure rate coefficients for the two isotopes are respectively:

r(137Cs) = 0.3224 R*m2/h/Ci

r(!34Cs) = 0.8820 R*m2/h/Ci

This, in turn, implies that in September 1995, the fraction of the dose ratecaused by 134Cs could be found as:

D'rei(mCs, 1995)= 1*0.8820/(1*0.8820+36*0.3224) = 0.071,

whereas the fraction of the dose rate due to 137Cs contamination was at the sametime

D'rei(137Cs, 1995) = 36*0.3224/(1*0.8820+36*0.3224) = 0.929.

Over the 23 months that passed between the measurement series in 1995 andthat in 1997, radioactive decay was responsible for a reduction of the contribu-tion of I3?Cs to dose rate by a factor of 0.957, and for a reduction of the l34Csdose rate contribution by a factor of 0.525. Thereby, the reduction in dose ratecould be found as:

DVD'95 = 0.071 * 0.525 + 0.929 * 0.957 = 0.037 + 0.889 = 0.926,

which corresponds well with the observed reductions for the caesium isotopes(see Table 1). This means that the only measurable change in dose rate levelover the two years is that from radioactive decay. In other words: no significanttraces of any re-distribution, vertical or horizontal, were detectable in any of themeasurement plots.

The 1:36 relationship between contamination levels of 134Cs and I37Cs in 1995was verified in the following way: From calculations of natural decay it is eas-ily found that the total amount of 134Cs was immediately after the Chernobylaccident in 1986 about 23 times as great as that which was observed during thefield campaign in 1995. Similarly, the total amount of 137Cs is found to havebeen 1.2 times greater in 1986 than in 1995. As the two isotopes are chemicallyidentical and follow the same distribution pattern and further emit gamma ra-

Ris0-R-1O29(EN) 57

diation of on average almost the same energy, it follows that the dose rate rela-tionship D'(l34Cs) / D'(l37Cs) should have been

1*23*0.8820/36*1.2*0.3224 = 1.45

in 1986, which agrees very well with the value given in the report of the Inter-national Chernobyl Project (IAEA, 1991).

The International Chernobyl Project also states that the relationship betweenthe amounts of l37Cs and m Cs released by the Chernobyl accident was veryclose to 2. From this it follows that the total dose rate contribution of the tworadiocaesium isotopes had in 1997 decreased by a factor of 3 since the deposi-tion took place in 1986. However, in the early phase (first few months) after theChernobyl accident the external dose rate in the test area was dominated bycontributions of relatively short-lived isotopes of for instance iodine and ruthe-nium (Kelly, 1987).

The Triple Digging Test Area

In a 10 m by 10 m plot on the banks of the river Iput, the triple digging proce-dure was tested in 1995. By triple digging, the order of three vertical layers ofsoil is changed (initially: the top ca. 5 cm, the middle ca. 15 cm and the bottomca. 15 cm), whereby a shielding against the radioactive matter is achieved. Thetop soil layer, which contains practically all the radioactive caesium, is placedin the bottom of the vertical profile, with the turf facing down. Soil from thebottom is placed immediately on top of this, while the intermediate layer, whichmust not be inverted, is placed at the top, ensuring the optimal effect with theleast impact on the fertility of the area.

The procedure was found to be effective (dose rate reduction of the order of afactor of 10 if a large open area is treated), and the effect has a permanent char-acter unless the soil is subsequently deep-cultivated. The area was initiallyovergrown with a mixture of different grass types, ranging from rye grasses(Lolium), orchard grasses (Dactylis) and meadow grasses (Pod) to fescuegrasses (Festuca). However, when we visited the area which was triple dug in1995 it was apparent that the vegetation in the test spot was different from thatof the surrounding area. A resistant fescue grass {Festuca Rubra), which was atthe time seminiferous and was seen to have a thinner straw and a more reddish/brownish colour, had taken over (see Plate 4), presumably due to the changes infertility in this shallow type of soil.

58 Ris0-R-1O29(EN)

Table 6.1. Dose rates (nSv/h) in the test area ofNovo Bobovichi. Measurementsmade in the autumn of 1995 and 1997. Also given are the relationships betweendose rate contributions from contamination in 1997/1995 (background/cosmiccontribution of 60 nSv/h excluded). Averages and standard deviations are givenfor assessments of this relationship in different areas.

Point refe-rence number

1920252671

4244464749515253

2324394041

787980818586

Measurementarea

ForestForestForestForestForest

Treated soilTreated soilTreated soilTreated soilTreated soilTreated soilTreated soilTreated soil

untreated soiluntreated soiluntreated soiluntreated soiluntreated soil

House no. 6House no. 4House no. 3House no. 1House no. 5House no. 2

D1 (nSv/h),1997

1057110899510381055

280258228253277288208296

863755941858796

172216334420191343

D1 (nSv/h),1995

11981201123710861126

302281245284266339223310

9828141015947807

185235357439204386

D'(1997)/D'(1995) corr

0.8760.9180.7940.9530.933

0.9090.8950.9080.8611.0530.8170.9080.944

0.8710.9210.9230.9000.985

0.8960.8910.9230.9500.9100.868

Relationshipstatistics

Average:0.90Standard d.:0.06

Average:0.91Standard d.:0.06

Average:0.92Standard d.:0.04

Average:0.91Standard d.:0.03

Plate 6.1. The 10 m by 10 m plot that was triple dug in September 1995, photo-graphed in August 1997. The test area is shown in the centre of the picture. Thecolour difference indicates that a new type of vegetation (a fescue grass - Fes-tuca Rubra) now dominates in the treated area.

Ris0-R-1O29(EN)

7 Contamination of the Forested Area

Both of the settlements subjected to decontamination in 1995 and 1997 weresituated in forested areas. After the field tests in 1995 it was discussed whethercontamination present in the trees would give a substantial contribution to thedose rate or if the trees would act as a shield against irradiation from distantareas. McGee et al. (1998) reported that 7 % of the total 137Cs Chernobyl depo-sition to a Swedish spruce forest was present in the above ground part of thetrees. This amount of activity would give rise to a measurable dose rate in thehouses as the radiation from the trees will have high probability of entering thehouses through the roof, that has a lower shielding factor. But the trees will alsoreduce the dose rate in the houses as they shield against photons coming frommore distant areas.

Trees have also received an increasing attention over the latest years, as oneof the major economic penalties of the Chernobyl accident is the contaminationof large forested areas.

7.1 Sampling and AnalysisIn 1997 a sampling programme was planned, aimed at obtaining a preliminarypicture of the contamination situation in the forested areas around Novozybkov.Three areas were selected for sampling. One near each of the two settlements,Guta Muravinka and Novo Bobovichi, where decontamination work was car-ried out, and a third area close to the village of Zaborie. This village received avery high level of contamination from the Chernobyl accident - about 4 MBqm"2 on average. In Zaborie both trees planted before and after the Chernobylaccident were sampled. The 'new' trees were planted in an area that had beendeep ploughed. This area was only about 200 metres from the 'old' forestwhere a pine was also sampled. The soil was in all sampling areas sandy with athin organic horizon.

The result of soil sampling in the four locations is shown in Table 7.1. Foursoil samples were taken at each location and analysed in the laboratory. In twolocations only two soil profiles have been analysed so far.

It can be seen that the variability of the results is very high in the area inZaborie where there had been deep ploughed, whereas it is low in the threeother locations where we have undisturbed soil profiles. The locations with un-disturbed soil had very similar mean attenuation depths, about 1.7 g cm"2, withlow variability. Overall, the surface contamination was three times higher inZaborie than in the two other places and the attenuation depth was three timesgreater in the place in Zaborie that had been deep ploughed.

Table 7.1. Review of analyses of soil samples taken at the wood sample sites.

Location

Zaborie YoungZaborie OldNovo BobovichiGuta Muravinka

Number ofsamplesanalysed

4224

Mean surfacecontamination

[MBq m"2]

4.2 ± 2.22.95 ± 0.06

1.041 ±0.0041.2 ±0.3

Meandepth[mm]

48 ±1235 ±338 ±227 ±8

Meanattenuation

[gem"2]

5.4 ±1.11.8 ±0.61.7 ±0.21.6 ±0.3

60 Ris0-R-1O29(EN)

Pine, Birch and Oak trees were sampled. In total, seven trees were cut, as pre-sented in Table 7.2. As pine is the most common tree in the Russian forestsspecial focus was put on this species. A mature pine tree was sampled from alllocations as well as a younger pine tree from one site. Birch is also very com-mon and was sampled in two locations. Oak was only found in GutaMuravinka.

Sections of the trunks of the trees were sampled at a height of 1 metre aboveground. For the pine tree additional pieces of the trunk were sampled at inter-vals corresponding to 5 years' growth in Zaborie and Novo Bobovichi and with2 year growth intervals in Guta Muravinka. Branches were sampled with leavesor needles. On location, the leaves and needles were separated from the twigs.For the pine trees the needles and branches were divided according to the yearof growth, which can easily be identified on a pine tree.

After returning to the laboratory the bio-material samples were dried to con-stant weight at 80 °C, except for the trunk sections, which were dried at roomtemperature.

The bio-material samples were homogenised and measured by gamma spec-trometry in standard containers, and the results were expressed in relation to thedry mass. The piece from 1 metre's height above ground and the 5 year oldpiece from the top of the tree were cut into discs and separated according toyear rings.

Table 7.2. Trees sampled during the 1997 campaign

Location

Zaborie

Zaborie

Zaborie

Novo Bobovichi

Guta Muravinka

Guta Muravinka

Guta Muravinka

Description

Young birch

Young pine

Old pine

Old pine

Birch

Oak

Pine

Species

Betula pubescens

Pinus sylvestris

Pinus sylvestris

Pinus sylvestris

Betula pubescens

Quercus robur

Pinus sylvestris

Sample description

10 year old and 3.5 meterhigh birch tree.

10 year old

~ 45 year old 15 meter highpine split in two at 5 metres'height

21 year old pine tree. 21mhigh and 76 cm 0 at 1 mheight

Birch

13 year old Oak tree

Pine

The obtained samples varied some in both amount and density of the woodchips. The chips were filled into a plastic container with a diameter of 50 mmand a height of 56 mm. The sample was pressed in a hydraulic press accordingto weight, until the height of the sample corresponded to a uniform density of0.22 g cm"3. The detectors used for the analysis were efficiency calibrated with4 standards consisting of standard containers 10, 33, 66 and 100 % filled withan isotope mix adsorbed in a material of a suitable density. Figure 7.1 showsthe efficiency calibration for the three detectors used in the study

Ris0-R-1O29(EN) 61

0.0000

Three parameter fit

10 20 30

Filling [mm]

40 50 60

Figure 7.1. Efficiencies of the three detectors (DET44, DET76 and DET83)used for the analysis of the wood samples as a function of the filling of thesample container.

The efficiency factors obtained from the four standards were fitted with ananalytical expression for the efficiency (Eff) as follows:

where A, B and C are empirical constants and f is the filling height in the con-tainer.

7.2 Results and Discussion137,Samples have been analysed for their mean content of " Cs activity. Seven

wood samples have been divided into year rings. In Table 7.3 the specific ac-tivities in Bq per gram of dry matter are shown for the samples. The corre-sponding transfer factors, TF, were calculated by dividing the specific activitiesby the contamination level in Table 7.1. The TFs are presented in Table 7.4.

62 Ris0-R-1O29(EN)

Table 7.3. Specific activities in different parts of the sampled species. The re-sults are for mixed samples including bark for the wood and twig samples. The'New needles' column refers to needles or leaves from the current year,whereas '3 year needles' refers to needles from 1995. Similarly the 1 and 3year twig columns refer to twigs from the current year and from 1995.

Sample

Pine Za-oldPine Za-yngPine NBPine GMBirch ZaBirch GMOakGM

Root

[Bq g1]

7.71.760.40

---

-

Wood1 m.

[Bq g"!]

5.70.310.780.810.0441.111.83

Wood5y.

[Bq g1]

4.90.410.230.820.0520.83-

Newneedles[Bq g ' ]

56.54.91.179.10.252.6

12.9

3 yearneedles

[Bqg1]

12.4-

0.252.01--

-

1 yeartwigs

[Bq g1]

58.74.641.175.690.1761.133.13

3 yeartwigs

[Bq g1]

14.6-

0.381.48

-

0.672.71

Barkl m .

[Bqg' l

6.781.46

--

0.089-

9.39

137,Table 7.4. Cs transfer factors from soil to wood matter. The specific activi-ties shown in Table 7.3 were divided by the surface contamination obtainedfrom analysis of soil samples. The TF's are expressed in units ofia3 m2 kg'.

Sample

Pine Za-oldPine Za-yo.Pine NBPine GMBirch ZaBirch GMOakGM

Root

2.60.600.38

---

-

Woodlm.

1.950.110.750.680.015

-

1.53

Wood5y.

1.650.140.220.680.018

-

-

Newneedles

19.21.71.17.60.082.1

10.8

3 yearneedles

4.2-

0.241.68--

-

1 yeartwigs

19.91.571.154.70.060.942.6

3 yeartwigs

4.95-

0.371.23

-

0.562.3

Barklm .

_

---

-

-

In the tree trunks, the specific activity was generally found to be highest in thecambium parts. For pine trees the specific activity was highest for fresh needlesand twigs, followed by 2 and 3 year old needles and twigs, roots, bark and fi-nally wood. In Zaborie the specific activity in the old pine was about 10 timeshigher than that in the young pine tree. This could be indicative of two phe-nomena: one is that the surface contamination on the tree is absorbed into theold pine tree, and the other is that uptake from a ploughed soil is considerablysmaller than that from a virgin soil.

The birch trees had higher specific activities than had the pine trees, both inZaborie and in Guta Muravinka. This is in good agreement with the results pre-sented by Tikhomirov et al. (1993). However, the other deciduous tree (the oakin Guta Muravinka) had the highest specific activity in the sampled trees fromthis site.

When comparing the transfer factors for the pine trees from different sites itcan be seen that the three older trees have similar TFs, 0.7 - 2.0 10"3 m2 kg"1,whereas the young pine from Zaborie has a considerable smaller TF, 0.1 10"3 m2

kg'1. The observed TFs for pine are in good agreement with the numbers re-ported by Belli et al. (1996).

The results of the analyses of the seven samples divided into year rings arepresented in Figure 7.2. The results from analyses of sections of the old pinefrom 1997 back to 1958 show that the 137Cs is mobile in the tree trunk, as wasfound by other workers (e.g. Kohno et al. (1988) and Momoshima and Bon-

Ris0-R-1O29(EN) 63

dietti (1994)). No peak can be seen in the year ring corresponding to 1986,where the Chernobyl accident occurred. The activity is almost homogeneouslydistributed over the year rings, all the way back to the 1958 ring. The concen-tration seems to decline towards an equilibrium level in the centre of the tree.

-Young birch

-Young B.5y

- Young pine

"Old pine Lop

-Old pineZA

- Oak tree

-C»M pineSy

Growth year

Figure 7.2. I37Cs transfer factors for the analysed tree samples as a functionthe growth year.

8 Conclusion

The experimental campaign in Novo Bobovichi gave new experience to all par-ticipants regarding the applied equipment, methods, measurement devices andthe obtained decontamination results. The work has demonstrated that a signifi-cant dose reduction can be achieved in the urban area with relatively simplemethods. Compared to the results obtained in 1995, significantly higher dosereduction factors were achieved. A reason for this may be that the soil contami-nation in the test area in Guta Muravinka appeared to be confined to a thinnerlayer, wherefore a greater fraction was removed with the topsoil. In our opinionthe most important reason for the good results, both in 1995 and in 1997, wasthe careful monitoring during the dose reduction process. For instance, eachplot was re-measured for activity and subsequently treated until all 'hot spots'had been removed.

Some scepticism has been expressed as to whether resuspended contaminatedsoil from areas outside the cleaned location might rapidly lead to a re-contamination. Re-measurements of the area that was decontaminated in 1995clearly show that this is not the case.

In recent years, Russia has adopted a new administrative policy for the contami-nated areas. The massive relocation that was encouraged until recently has beenstopped, and reclamation of the contaminated land now has high priority. Therefore,remediation methods that can be applied to reduce the dose rate to people living inthe areas have received new attention. The reported work shows that it would bepossible in practically all the contaminated areas to reduce the dose rates down be-low the 1 mSv/year limit, which is recommended internationally.

64 Ris0-R-1O29(EN)

8.1 Equipment and MethodsThree machines were tested in the contaminated areas in 1997.

The bobcat proved to be an excellent tool for decontamination around houses.Large areas could be treated in reasonable time. At the same time, the manoeu-vrability of the bobcat's shovel ensured that relatively small amounts of wastewere generated.

The asphalt scraper demonstrated that smaller asphalted areas can easily becleaned. It is a relatively simple machine and easy to operate.

The skim and burial plough was tested in a field northwest of Novo Bobovi-chi. This test only gave a dose rate reduction by a factor of 2.2. However, thetest area had rather limited dimensions, and if a much larger area had beentreated the DRF would have been around 6-7. It should be stressed that the testfield conditions were far from the optimal. For instance, the ploughing was car-ried out in the late summer, following a very warm and dry period, whereforethe soil contained very little moisture and the subsoil crumbled and fell to thebottom of the trench.

Concerning measuring equipment a good correlation was found between theresults obtained with the hand-held Russian DRG-01-T instrument and with theReuter Stokes ion chamber. Another easily portable dose rate measurement de-vice made in Belarus, the El-1101, was applied frequently, and also the resultsobtained with this instrument were at both high and low dose rates found tocorrelate very well with those obtained with the Reuter Stokes ion chamber.

A portable Russian gamma-analyser, SKIF-3, was used in a specially de-signed lead collimator arrangement for series of angular in situ gamma spec-trometric measurements. This made it possible to qualitatively assess the effectof the decontamination operation on the gamma signal from different surfaces.

8.2 Decontamination ResultsThe strategy for decontamination around the houses in the village of GutaMuravinka consisted of removal of topsoil, which was replaced with radiologi-cally 'clean' sand, and removal of the roof of one of the houses. The soil re-moval accomplished with a mini-bulldozer gave a reduction of the dose ratecontribution from the contamination in the area by almost 79 % outdoors in thetreated area between two houses. The corresponding effect on the contaminantdose rate contribution inside the houses was somewhat smaller, since the con-tamination on the house is here of more importance. At ground floor level thedose rate reduction was by 57 %, whereas it was 33 % at the 1st floor level,where much of the dose rate is due to contamination at greater distances thanthe dimensions of the cleaned area. Since the soil contamination lay in a thinnerarea in Guta Muravinka than it did in Novo Bobovichi, where the 1995 meas-urements were conducted, a considerably larger fraction of the contaminationcould be removed with a thin topsoil layer. Therefore, contrary to what hadbeen observed in Novo Bobovichi, the subsequent application of a thin layer of'clean' sand had in Guta Muravinka practically no additional effect. One of thesurprising findings of the experimental work in 1995 was that the dose ratecontribution from contamination on the roofs of houses was highly significant.The roof again proved to be an important contributor to the dose rate insidehouses, especially at first floor level. Here, the dose rate was reduced by 27 %when the roof was replaced. Since only the roof of one house was replaced, aneven greater effect could be expected if the roofs of neighbouring buildings hadalso been changed. The overall result of the strategy was a reduction of the doserate outdoors in the treated area by almost a factor of 6, and indoors by gener-

Ris0-R-1O29(EN) 65

ally about a factor of 3. If larger areas were treated, as would be the case if thestrategy were to be implemented in a full-scale operation, the dose rate contri-bution from the far away areas could have been reduced considerably, and thetotal effect would have been even better.

The results of the measurements carried out both in 1995 and in 1997 clearlyindicate that a large fraction of the residual dose rate in the area after decon-tamination is caused by the contaminated forested areas in the vicinity of thetest plot. The contamination descended in rain on the test plot. It should bestressed that in areas that received a dry deposition, the relative contaminationlevel in a forest will be substantially greater, making the forest an even moreimportant dose contributor than in this case study. A preliminary investigationof the distribution of the contamination in the forest indicates that most of thecontamination in the trunk of a tree lies in the cambium. This seems to be thecase both for old trees and for young trees planted after the Chernobyl accidentoccurred in 1986. In the deeper layers of the tree trunk the contamination ap-pears to distribute rather homogeneously. However, further work should be car-ried out to establish this important point.

8.3 Future WorkDecontamination has now been tested at two sites, with manual methods andwith machines. The next step would be to carry out decontamination workaround populated houses in some of the more severely contaminated villages.This can hopefully be achieved through a continuation of this project or possi-bly through funding from the IAEA that have approval from UN of a projectinvolving decontamination work in Belarussian settlements.

Some improvements in the instrumentation would also be desirable. The EI-1101 was an improvement compared to the instruments used for monitoring thework in 1995, but testing of instruments based on (3-detectors would still beinteresting. Improvement in measuring time and precision could hopefully beobtained.

A collimated Nal based monitor could be installed on the back edge of thebobcats' shovel, monitoring the contamination of the soil after scraping. A dis-play could then be placed in front of the driver so that he could follow the re-sults of his work 'on-line'. Such a construction would make the driver moreindependent of the continuous evaluation of his work by people with handheldinstruments and would be expected to speed up the process considerably.

A more extensive analysis of the contamination in the forest and its implica-tions for dose is required to fully account for the effect that may be obtained byimplementation of a clean-up strategy in the contaminated areas. Since a largedose contribution from the forested area can be ascribed to contamination in theforest floor duff layers, it is not known whether the most important effect of thetrees as such is the shielding or the dose contribution due to their contamina-tion. Therefore it is not known if it would be beneficial from a pure dose pointof view to remove the trees. This question could be answered by Monte Carlophoton transport modelling. However, this would require a greater and moregeneral knowledge of the contaminant distribution. For instance, if small di-ameter samples were taken throughout the trunks of a great number of trees inthe specific area, these could be sectioned according to year rings, which couldbe bulked from different trees for the contamination analysis, in order to ex-amine the general contamination distribution in the area.

Investigations should be made of dose rate contributions inside houses fromindoor contamination, and more generally, to identify the important sources tothe residual dose rate in the treated areas.

66 Ris0-R-1O29(EN)

9 Acknowledgement

This work has been sponsored by the Danish Emergency Management Agency.We especially want to thank chief inspector Bj0rn Thorlaksen for his great in-terest in the project

10 References

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Andersson, K.G. (1991). Contamination and decontamination of urban areas,Ph.D. Thesis, Ris0 National Laboratory.

Andersson, K.G. & Roed, J. (1994 a). Removal of radioactive fallout from sur-face of soil and grassed surfaces using peelable coatings. J. Environ. Radio-activity vol. 22, No. 3, pp. 197-204.

Andersson, K.G. & Roed, J. (1994 b). The behaviour of Chernobyl I37Cs, 134Csand 106Ru in undisturbed soil: Implications for external radiation. J. Environ.Radioactivity 22, No. 3, pp. 183-196.

Belli, M., Rafferty, B., Shaw,G., Wirth, E., Kammere, L., Ruehm, W., Steiner,M., Delvaux, B., Maes, E., Kraytz, N., Bunzl, K., Dvornik, A.M. and N.Kuchma (1996), Dynamics of Radionuclides in Forest Environments, in Theradiological consequences of the Chernobyl accident, Minsk 1996, EUR16544 en.

Belyaev, S.T., Demin, V.F., Kutkov, V.A., Bariakhtar, V.G., Petriaev, E.P.(1996). Characteristics of the development of the radiological situation fromthe accident, intervention levels and countermeasures. In Karaoglu, A.,Desmet, G., Kelly, N. and Menzel, H.G.: The radiological consequences ofthe Chernobyl accident, EUR 16544 EN, ISBN 92-827-5248-8.

Calvert, S., Brattin, H. and Bhutra, S. (1984). Improved street sweepers forcontrolling urban inhalable particle matter, A.P.T. Inc., 4901 Morena Blvd.,San Diego, CA 97117, EPA-600/7-84-021.

Clark, D.E. and Cobbins, W.C. (1964). Removal of simulated fallout frompavements by conventional street flushers, U.S. Naval Radiological DefenseLaboratory, USNRDL-TR-797.

Desmet, G. (editor) (1991). Improvement of practical countermeasures: the ag-ricultural environment. Post-Chernobyl action, CEC report EUR 12554 EN.

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Gj0rup, H.L., Hedemann Jensen, P., Lauridsen, B., Roed, J. and Warming, L.(1985). Deposition, retention and decontamination of radioactive materialon urban surfaces - The Danish Program, Ris0, presented at the workshopon methods for assessing the off-site radiological consequences of nuclearaccidents, CEC, Luxembourg, April 1985.

Hubert, P., Annisomova, L., Antsipov, G., Ramsaev, V. and Sobotovitch, V.(editors) (1996). Strategies of decontamination, Final report APAS-COSU1991-1995: ECP4 Project. European Commission, EUR 16530 EN.

IAEA (1983). IAEA Safety Guides. Safety series no. 55, Planning for the off-site response to radiation accidents in nuclear facilities.

IAEA (1991). The International Chernobyl Project: Technical report. Report byan International Advisory Committee, ISBN 92-0-129191-4, IAEA, Vienna.

ICRP (1989). Publication 55. Optimization and decision-making in radiologicalprotection, Pergamon Press.

ICRP (1990). Publication 60. 1990 Recommendations of the InternationalCommission on Radiological Protection, Pergamon Press, ISBN 0 08041144 4.

Kelly, G.N. (1987). The importance of the urban environment for accident con-sequences, Rad. Prot. Dos. vol. 21, no. 1/3 pp. 13-20.

Kohno, M., KoizumiY., Okumura, K. and I. Mito (1988), Distribution of Envi-ronmental Cesium-137 in Tree Rings, J. Environ. Radioactivity, Vol. 8, pp.15-19.

Kryshev, I.I. (1996). Dose reconstruction for the areas of Russia affected by I31Icontamination, Rad. Prot. Dos. vol. 64, no. 1/2, pp. 93-96.

Likhtarev, LA., Gulko, G.M., Kairo, LA. Los, I.P., Henrichs, K. and Paretzke,H.G. (1994). Thyroid doses resulting from the Ukraine Chernobyl accident -part 1: Dose estimates for the population of Kiev, Health Phys. 66(2): 137-146.

McGee, E.J., Synnott, H.J., Johanson, K.J., Fawaris, B.H., Nielsen, S.P., Hor-rill, A.D., Kennedy, V.H., Barbayiannis, N., Veresoglou, D.S., Dawson,D.E., Colgan, P.A. and McGarry, A. (1998). Chernobyl fallout in a Swedishspruce forest ecosystem, accepted for publication in Journal of Environ-mental Radioactivity.

Melin, J., Backe, S., Erixon, O., Johnsson, B. and Roed, J. (1991). Saneringefter reaktorolyckan i Tjernobyl (in Swedish), SSI-rapport 91-03, StatensStralskyddsinstitut, Stockholm, Sweden, ISSN 0282-4434.

Menzel, R.G., Eck, H.V., James, P.E. and Wilkins, D.E. (1968). Reduction ofstrontium-85 uptake in field crops by deep-ploughing and sodium carbonateapplication. Agronomy Journal 60, pp. 499-502.

Momoshima, N. and E.A. Bondietti (1994), The Radial Distribution of 90Srand 137Cs in Trees, J. Environ. Radioactivity, Vol. 22, pp. 93-109.

Roed, J. and Andersson, K.G. (1994). The behaviour of Chernobyl 137Cs, 134Csand 106Ru in undisturbed soil: implications for external radiation, J. Environ.Radioactivity vol. 22, no.3.

Roed, J. and Andersson, K.G. (1996). Clean-up of urban areas in the CIS coun-tries contaminated by Chernobyl fallout. J. Environ. Radioactivity, vol.33,no.2, pp.107-116.

68 Ris0-R-1O29(EN)

Roed, J., Andersson, K.G. and Prip, H. (editors) (1995). Practical Means forDecontamination 9 Years After a Nuclear Accident, Ris0-R-828(EN), ISBN87-550-2080-1, ISSN 0106-2840.

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Ris0-R-1O29(EN) 69

Appendix A. Soil Samples

Table A.I. Review of soil samples analysed in the laboratory.

Sample ID

GM01GM02GM03GM04GM05GM06GM07GM08GM09GM10GM11GM12GM13GM14GM15GM16

GM19GM20GM21NB03NB04NB06

NBO7NB21NB22NB23NB24

NB25NB26NB27NB28NB29ZAO1ZA02ZA03ZA04ZA12

ZA13

Beach site - beforeBeach site - before

Beach site - afterBeach site - afterAround house 2 (40, 59)Around house 2 (40.4, 50.5)Around house 2 (31.8, 50.5)Around house 2 (31.8, 58.6)Center line (12, 44.5)Center line (16, 44.5)Center line (32, 44.5)Center line (36, 44.5)Center line (46, 44.5)Center line (56, 44.5)Forest AForest BCenter line - after (33, 44.5)Center line - after (35,44.5)Center line - after (37,44.5)Triple digging site ATriple digging site BForest 0.5 m North of pineForest 0.5 m North of pine

S&B experiment - before SWS&B experiment - before NES&B experiment - before NWS&B experiment - before SES&B experiment - after AS&B experiment - after BS&B experiment - after CS&B experiment - after DS&B experiment - after EYoung forestYoung forestYoung forestYoung forest

Old forestOld forest

Totaldeposition[kBq/m2]

5061,359

104104

10,6297,9621,194

4971,180

1,4791,6691,516

9291,1451,0831,645

44219

831,0851,1701,0441,038

1,5001,5511,6101,156

470200

1,1221,0651,6682,1967,0552,7514,726

2,9902,903

Mean depth

[mm]

23.331.780.3

160.184.064.641.531.425.2

26.731.251.422.022.623.837.889.369.3

102.4239.5200.1

39.5

36.458.556.326.6

102.2223.4

195.6162.0158.4176.646.564.134.746.337.0

33.0

Meanattenuation

[g/cm2]

2.843.23

11.4322.56

3.57

7.693.662.332.962.462.905.651.671.921.161.56

12.549.48

14.1425.5821.74

1.611.89

3.864.801.37

10.66

29.8026.72

20.9514.9017.174.876.924.525.092.261.35

70 Ris0-R-1O29(EN)

Cs-137 depth profile

Specific activity [Bq/g]1(1 20 3(1 4

Mean Acciiated Weight ActivityNo. depth attenuation ofsanple in sampleN Inrai Ig£rr2] [gl [Bq]

12

3

4

5

6

7

8

9

10

826

44

62

8199

118

136

155

173

1.23.7

6.3

8.9

11.414.1

16.8

19.5

22.2

24.8

Total deposition iMean depthMean attenuation

148141

148

146

147

154

155

153

149

143

103,89880

11,4

Bq/tiGnxng/tafi

8.97.5

8.5

143

151

100

12

5.4

14

2 2

g2s

tior

3(3O

ted

e\c

cum

ula

in

15

20

25

Cs-137 depth profile

Specific activity [Bq/g]0.2 0.4 O.fi 0.8 1.0 1.2

;

r)

—-—.

•

— • . -

— - • V

i

-GM 03s, Badelaml Aftcr.piis. [1 S.O : 145.0]

I _[Mean| Acc.ujated j Weight! No. f depth i attenuation |of sanjalej injsatnpjle S"™~ ''"' ~ Jl ' '"

li _ 8 L 0.9! 112! 17|"26l"

I 8] 138j 19.1J 155] _ 25[

f" 91 "_T56TT~ ^ i ^ 157J"""" 1 1 ]

~ 156!12;_ 211!

i_2Sf"230£ . __I 14] " S S ! " 35.6L "l56! " 73l__"'" :~' """"'"'" 3a3l """ ^ T ~ ~!''

"l47|16) 285! 4O9J- j a 3Q4J- 43.5J

151 73

19|_

~7li

(Mean depth ! 160

Cs-137 depth profileSpecific activity [Bq/g]

0.2 0.4 0.6 0.8 1.0

20

30

35

/*—

\

>

/

• — —

.

—

— • • — .

— — — ^ ~

—===?

—— . — ' =»—

-OM 04s, Badclund Aftcr.piis. [13.0 : 145.0]

72 Ris0-R-1O29(EN)

Mean Acc.ulatedNo depth attenuation[ 1 [nrrj [g/bnfi]

1 82 273 46~4 665 856 " 104

; 7, 124[_i 8i 143!• 9 | 163jr ioT 182J;_iii_. 202̂j I2| 222J7

! 15] 280) ,\ 16! 300|

020 612193 14 76.89.3

11.9„__

Weightof sample

tgl. .2427335080

109132146145

125113

18.6!" 135

™23l25.728.3

~3O8

^^jTotajJeDosition!Mean depth1M ean attenuation

:.:: ~M145

~~_J45*- - — • — ™ ~ ~ -

~~ 84~3.6

Activityinsanple

IBq]2530129629008889

16342" 10607

2115,__j3ij __

400l~29bT"188)

* " 2O4J

___!_

65;

"IIIl'IMClL_..ZiiP

rrm |g/fcrrfi |

g

\—ao

inti

C

£

t 3

:um

ulat

e

O

Cs-137 depth profileSpecific activity [Bq/g]

1(1(1 20(1 3(10

- G M 05s , Round house 2 , p o s . |4().(] : 5 9 . 0 ]

fif!

I Mean] Acc.ulated?\j attenuatlori

fI- yeE?\j attenuatlori |f! [nxifI""liinei"Tif 8̂ 071

if

Wj2'ght_j Activity j__of^samglej in^ampie I~ M r IBgJ f

Total depositionjMeandteptti

Ico

• -—- f - ca

Cs-137 depth profileSpecific activity [Bq/g]

20 40

5 •

10 -

15 •

20 •

30 •

35 -

40 ;

At -

:

:

_ • —

-GM 06s, Round house 2, pos. [40.4 : 50.5]

Ris0-R-1O29(EN) 73

No[]

123456789

1011121314151617

I

i i

Meandepth[mmj

827466584

103122142161180200219238'257277296316

Acculatedattenuation

tg/cm2]0 4183 86 18 5

110136163190216!241!267!294!320!346!373!39 9[

1Total deposition _j_M ean degth |Mean attenuation !

Weightof sample

lg)51

101125136135149153,151!149[14l|147|153T146!145i150i1551

_ 1 4 7 [

L

Activityin sample

IBq]993

2318986341214128574636.31!38!13!16!15!10!

46!

1.194,366 jBg/irfi j42i:3.7 |

mm !gyfcnfi !

F_o

0

a

c

att

• 0

gsu

Cs-137 depth profileSpecific activity [Bq/g]

2 15

25

30

rtt.

——̂ —

35 ' -

45

-GM 07s, Round house 2, pos. |31.8 : 50.5]

Cs-137 depth profileSpecific activity [Bq/g]

30

»—OM O8s, Round house 2, pos. [32.0 : 58.6]. 1

74 Ris0-R-1O29(EN)

Mean AcculatedNo depth attenuation

1 82 273 474 66*5 86,

093155

"&110.9

Weightof sample

18)ill

138

139

154

157

Activityin sample

[BoJ2106234344712377

! 6. - _

8o

. _ .

a1213141516

ZiZ

105' 125

144163183203222242261,

„. „,

281300320

_ 13.7) 161

ia4j__ j s i22 3̂ 164zo.x. Lor27.9! 15830.7: 1603a5! 16036.4J 161

""" 39.2!™ 15842.0J 158

ZIZZj^biZZ^iM

332314

ZZ.1§ Z37!

U 5 |1.86!1.1310.8l"1.21

—

£O«3

g

|8

Cs-137 depth profileS pecitic activity [Bq/g]

5 HI 15 2(1

5

10

15

20

25

30 ;

35

40

45 <

50

25

Mean! Acculatedj Weight j Activity ;N£j^ejpthj_attenuation| qfsampje[ins11 ! [nini|_Llg^n3§L_.J Igl I I§3l

Cs-137 depth profileSpecific activity [Bq/g]

10 20 30 4(1 50

5 -

10 -

15 ;

20 •

30 •

35 ,

41) ]

4 5 •

f•

• —

^ — — — - ~~ ™ -

- G M l ( ) s . H o u s e L i n e , p o s , (!<>.<>: 4 4 . 5 ]

Ris0-R-1O29(EN) 75

No.II

123456789

101112131415161718

Meandepth[mm]

827476685

105124143163182201220240259278298317335

Acc.nlatedattenuation

Ig/cm2)0.62.34.87.4

10.113.015.818.621.524.327.230.133.035.838.741.644.547.3

Total depositionMean depthMean attenuation

Weightof sample

fel70

130142154157165156162159165164159163163164165157159

1.668.71131

2.9

Activityin sample

IBql659

53911061

101392213132 5

4.62.2

13.91.951.230.450.490.411.22

Bq/m2mmg/cm2

Cs-137 depth profile

Specific activity [Bq/g]M) 211 Mi 4(1 50

45 <

-OM 1 Is. House Line. pox. [32.11: 44.51

No.

II123456789

101112131415161718

Meandepth(mm}

82 84 86786

1061251451641842 0 32 2 32 4 2262281301320340

Acc.ulatedattenuation

|g/cm2]0.82.54.77.3

10.012.815.618.521.324.227.130.032.935.838.841.744.647.5

Total depositionMean depthMean attenuation

Weightof sample

fel93

103145149157160161160160164165164165166169164159167

1.515.77151

5.6

Activityin sample

IBq]14481375960

1687791

563 3

1659 82 31724

4.81.411.221.121.091.19

Bq/m2mmg/cm2

<s"Euan ati

<

3a

IE3y<

Cs-137 depth profile

Specific activity [Bq/g]s in is

•

•

•

« —

, — - — •

, • — •

-GM 12s. House Une. pos. I 3 U : 44.51

76 Ris0-R-1O29(EN)

No.II

1234

56789

1011121314151617

Meandepth[mm]

827476786

1051251441641832 0 22212412 6 0280299318

Acc.ulatedattenuation

Ig/cm2|0.41.94.16.69.1

11.514.016.418.821.223.525.928.430.833.335.738.0

Total depositionMean depthMean attenuation

Weightof sample

Igl49

118135149131144136138133132135135141139139130133

928.852221.7

Activityin sample

IBql20631757

1193 044

16.35.7

10.35.95.85.2

11.45.23.62.73.02.6

Bq/m2mmg/cm2

Cs-137 depth profile

Specific activity [Bq/g]1(1 20 30 41) 511 60

a

o

a>

M

Acc

ur

u>

20 •

25 •

35 ;

*

- G M 13s, House Unc, pns, [46.0 : 44.5)

No.II

123456789

101112131415161718

Meandepthimm]

8274 76685

1051241441641842 0 32222422612803003193 3 7

Acculatedattenuation

Ig/cm2]0.41.94 .26.59.0

11.514.016.619.121.724.226.729.331.934.637.239.842.4

Total depositionMean depthMean attenuation

Weightof sample

Igl52

118136132143140145145144141141146149147149149148146

1.145.458231.9

Activityin sample

IBql30871496210

4429

19.412.410.315.020.720.214.56.0

19.75.48.98.35.5

Bq/m2mmg/cm2

Cs-137 depth profile

10

15

20

25 :

30 '

35

40

45

Specific activity [Bq/g],0 30 40 50 60 70

-GM 14s. House Lino, pos, |.«.<>:44.5]

Ris0-R-1O29(EN) 77

No.[ |

1

2

34

5

6

7

8

9

10

11

12

13

14

15

16

17

Meandepth[mm]

826

44

62

81

99

118

136

155

173

192

210

229

247

265

284

302

Acculatedattenuation

Ig/cm2]0.2

0.9

2.4

4.2

6.2

8.3

10.713.115.718.220.723.225.928.531.233.936.6

Total depositionMean depthMean attenuation

Weightof sample

Igl23

63

102

110

113

126

140

143

144

139

146

146

151

153

151

152

160

1.082.517241.2

Activityin sample

[Bq]19702023

458

131

51

26

10

5.1

7.0

5.0

3.7

21.02.8

0.240.1200.0000.096

Bq/m2mmg/cm2

Cs-137 depth profile

_ 5

2 20 j

2 25 ;

Specific activity [Bq/g]

40 611 XI) I IK) 120

1

•

» t

- G M 14s, House Line. pels. 156.0 : 44.5]

No.11

1234

567

89

101112131415161718

Meandepth[mm]

8264 4638199

1181361541731912 1 02282472652 8 43 0 23 2 0

Acculatedattenuation

Ig/cm2]1.13.56.08.6

11.313.916.619.222.024.827.630.533.336.138.941.744.447.1

Total depositionMean depthMean attenuation

Weightof sample

Igl132142148150150150148157158159162161160158159157154146

44.25989

12.5

Activityin sample

[Bql14.8

2 216.616.312.412.2

7314.7

4.63.5

0.600.400.970.39

0.1160.1360.1120.118

Bq/m2mmg/cm2

Cs-137 depth profile

5

10

15

20

25

JO

35

40

45 i

50

Specific activity [Bq/g]0.1 0.2 0.3 0.4 0.5 0.fi 0.7

f*

-• — —

—

-GM Ws,HuuscLine After. pos.(33.0:44,5]

78 Ris0-R-1O29(EN)

MeanNo. depthII

123456789

1011121314151617

Imm]8

2644628199

117136154172191209111246264283301

Acc.ulatedattenuation

[g/cm2]0.93.25.88.4

11.113.816.619.422.325.128.030.833.636.439.141.944.7

Weightof sample

Ig)119142146151154156160161163162162159159158157157157

Activityin sample

[Bq]2846

26823814612661237.42.52.62.1

1.010.550.670.730.64

I

Total depositionMean depthMean attenuation

218.948 Bq/m269 mm9.5 g/cm2

Cs-137 depth profile

Specific activity [Bq/g]I 2

V

f

11

— • _

•

— —

_ — - • -

- G M 2l)s,Hnu.st: Uiw After. ]WS.[35.U : 4-1.51

No.

n2

3

4

56

7

8

9

10

11

12

13

14

15

16

17

Meandepth(mm)

72 5

4 4

62

8 0

99

117

136

154

172

190

2 0 9

2 2 7

2 4 6

2 6 4

2 8 3

3 0 1

Ace,matedattenuation

[g/cm2J0.93.2

5.6

8.2

10.813.616.319.121.824.627.330.132.835.538.240.943.5

Total depositionMean depthMean attenuation

Weightof sample

(g!119137

142

144

156

156

158

158

156

156

156

154

156

154

150

153

151

82.509102

14.1

Activityin sample

[Bql6.36.35.65.5

79

164

5311.07.6

5.6

3.9

3.9

2.4

1.410.840.44

2.9

Bq/m2mmg/cm2

a

eg 10

Ji> is

I 20

I 2S

30

Cs-137 depth profile

Specific activity [Bq/g]

fL>-—-—-

>

« '•_ '"— - " •

— — .

i

1

- GM 21 S.HOUK Line After, pos.137.0 :44.51

Ris0-R-1O29(EN) 79

No.I)

123456789

101112131415161718192021222 324252627

Meandepth|mm|

827456482

100118] 3 7

1551741922102292472662843023213393583763944 1 3431449468486

Acc.ulatedattenuation

[g/cm2]0.72.64.76.88.9

10.812.814.817.019.020.922.824.726.528.329.931.733.836.038.340.743.145.648.150.553.055.3

Weightof sample

(gl88

1231211191131081151211201111081071071019687

117120130135135140142142139137134

Activityin sample

[Bq]3715152266

120103

3960

201215276612826928768179

4 87 57731

8.24.73.12.51.40.9

Total deposition 1.085.223 Bq/m2Mean depth 239 mmMean attenuation 25.6 g/cm2

Cs-137 depth profile

Specific activity [Bq/g]

10 12 14

a 30

•

—•-— -•

-NB 03 x Triple Digging Experiment (W).

No.I ]

1

23456789

1011121314151617181920212223242526

Meandepthimm]

72544628099

117135153171190208226244263281299318336355373391409428446465

Acc.ulatedattenuation

Ig/cm2]0 . 6

2.44.66.88.9

11.013.115.217.119.020.822.524.326.128.130.332.735.137.539.942.344.747.149.451.854.2

Weightof sample

Igl78

1211281251161241201141061039999

100111119132136137137138137137137123141138

Activityin sample

IBql9.15.8

11175236

106305392588717962954490334

768.85.24.45.53.03.23.52.22.52.8

Total depositionMean depthMean attenuation

1.169.715 Bq/m2200 mm21.7 g/cm2

Cs-137 depth profileSpecific activity [Bq/g]

2 4 6 8 10 12 14

"BD -» .

— — • —

— * ^

—IT

-NB04 Triple Digging Experiment (NE1.

80 Ris0-R-1O29(EN)

No.

1)1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

Meandepth|mm]

7

26

44

62

81

99

U S

136

154

173

191

2 1 0

2 2 8

2 4 7

2 6 6

284

3 0 3

Acc.ulatedattenuation

!g/cni2|0.1

0.4

1.4

3.0

4.9

7.0

9.1

11.313.515.717.920.222.424.426.528.730.9

Total depositionMean depthMean attenuation

Weightof sample

tei7

32

77

206

118

118

124

127

123

126

126

127

120

116

121

123

128

1.043.643401.6

Activityin sample

[Bq|314

2627784

372

136

103

94

41

30

23

9.4

4.3

4.4

4.3

2.7

1.6

2.3

Bq/m2mmg/cm2

Cs-137 depth profile

1

•a

I3

<

Specific activity [Bq/g]20 40 60 HO 100 120

20

30

35

* •

-NB06s 0.5m Nuf pine pus. 20 Novo Bubovidiie

No.

111

23

4

5

6

7

8

9

10

11

12

13

14

15

16

17

Meandepth|mm]

8

26

44

6 3

SI

100

118

137

155

173

192

2 1 0

2 2 9

2 4 8

266

2 8 5

304

Acculatedattenuation

[g/cm210.2

0.7

1.8

3.3

5.2

7.3

9.5

11.713.916.018.220.422.724.927.129.431.8

Total depositionMean depthMean attenuation

Weightof sample

[gl2 2

42

77

101

117

120

125

125

124

122

126

125

128

124

129

135

131

1.037.804361.9

Activityin sample

IBql

884

2458670

186

72

34

2 8

40

29

31

7.8

5.814.1

9.8

21

21

16.9

Bq/m2mm

g/cm2

IC

o

1

Cs-137 depth profile

Specific activity [Bq/g]

20 ,

3 30

35

-NB 07 s 0.5 m N of pine pos. 20 Novo Bobovichic

Ris0-R-1O29(EN) 81

No.II

123456789

1011121314151617

Meandepth[mml

8264 5638199

1181361541731912102282472652 8 3302

Acc.ulatedattenuation

Ig/cm2|0.10.72.24.26.48.8

11.313.816.519.121.724.326.929.532.134.737.2

Total depositionMean depthMean attenuation

Weightof sample

Igl8

66106116132145139147150149147149148147147144143

1.499.66058

3.9

Activityin sample

IBql111207

21173680

19834219621

8.36.16.85.2

12.74.94.12.1

Bq/m2mmg/cm2

Cs-137 depth profile

•B is

1 25

| 30o3 35 t

40

Specific activity [Bq/g]10 20 3(1 40

c—

•

•

•

.• — •

r=*

-NB2I S&B Experiment S\V Before Pious

No.II

123456789

1011121314151617

Meandepth|mm]

72 544628099

1171351541721902092 2 72462642 8 2301

Acculatedattenuation

Ig/cm2]0.11.23.25.47.7

10.212.815.418.020.623.225.828.431.033.636.238.8

Total depositionMean depthMean attenuation

Weightof sample

Igl17

105123120143148148148148148146146147148148149144

1.550.69856

4.8

Activityin sample

!Bq]1516 5 3

14564196

11713

7.22 5393151

4.20.840.96

7.00.450.28

Bq/m2mmg/cm2

Cs-137 depth profileSpecific activity [Bq/g]

IO

1ite

d at

ten

Acc

umul

5

10 .

15 :

2 0 •

25 ;

3 0 '•

35 \

4 0 •

4 S •

10

<

*

— * •

20 30 40 5

- ' ..——• — =»

-NB22 S&B Experinieiil NE Before Ploughing

82 Ris0-R-1O29(EN)

No.

II123456789

1011121314151617

Meandepthimm]

8264 4628 099

1171361541721912 0 92 2 72462 6 32 8 23 0 0

Acc.ulatedattenuation

|g/cm2|0.21.12.95.37.8

10.413.115.718.320.923.526.128.631.233.636.038.4

Total depositionMean depthMean attenuation

Weightof sample

ISl2678

127143148150149150149145147145147141135135138

1.609.659271.4

Activityin sample

[Bq|708

5905201

6210.911.1

6610.511.51.871.582.2

1.431.523.42.82.2

Bq/m2mmg/cm2

go

cozsc

•aS

1

Cs-137 depth profile

5

10

IS ; r

20

25

30

35

40

45

Specific activity [Bq/g]2(1 40 60 SO KHI

-NB23 S&B Experiment NW Before Ploughing

No.

ni

2

3

4

567

8

9

10

11

12

13

14

15

16

17

Meandepth[mml

8

2 6

4 4

6 2

8 0

9 9

117

135

154

172

190

209

2 2 7

246

264

2 8 3

301

Acculatedattenuation

[g/cm2]0.3

1.6

3.6

5.7

8.0

10.212.314.617.219.822.525.227.830.533.135.738.3

Total depositionMean depthMean attenuation

Weightof sample

fgi4 0

104

121

126

132

119

123

139

150

150

151

152

151

150

148

148

145

1.155.861102

10.7

Activityin sample

(Bql115

92

58

168

113

25911663

133

18.318.48.5

44

6.3

8.6

1.510.770.93

Bq/m2mmg/cm2

Cs-137 depth profile

Specific activity [Bq/g]

co\3

C

is*13

10 20 25 30

5 r10

15

20 • '

25

30

35 ;

40 •

45

-NE24 S&B Experiment SE Bcliirc Ploughing

Ris0-R-1O29(EN) 83

No.

M123456789

1011121314151617181920212 22 3242 526272 829303132

MeandepthImm]

82 644638199

1181361541731912 0 9227246264282301319338356374393411429448466484503521540558577

Acc.ulatedattenuation

Ig/cm211.03.35.88.3

10.713.215.618.120.623.025.527.930.332.735.137.740.342.845.347.749.952.354.857.460.162.765.367.970.573.275.878.3

Weightof sample

Igl121142141141140139139140142138139138136135143147145146140125127142149151147152147149147150145149

Activityin sample

IBql16.8

2 82 72 33 6

2 7 59 1

14.82181

1413 7 91791483 0

13.7382155

167177567.63.4

1.861.236.19.1

0.560.380.300.32

Total depositionMean depthMean attenuation

469.954 Bq/m2223 mm

29.8 g/cm2

Cs-137 depth profile

Specific activity [Bq/g]1 2 3 4

co•3

nua

atte

H"3g3Oo

1"o

20

30

40

50

10 "-S»;— —

-NB25 S&B Expcrimenl SE Aflcr Ploughing

No.(1

123456789

10111213141516171819202122232425262728293031

Meandepth[mml

82 64 4628199

1171351541721912 0 92 2 82 4 62 6 4283301319338356374393411429448466484503522541560

Acculatedattenuation

[g/cm2]1.03.45.98.5

11.113.716.218.821.323.826.428.931.433.936.539.041.644.146.749.351.854.356.759.261.764.166.568.871.173.275.3

Weightof sample

Igl1281431431481491471451431441441431441431441441 4 5145147147143141142140141140138135130121123118

Activityin sample

IBql3.0

178.149.210.042.257.979.7

165.553.8

6.32 .0

1.421.251.341.301.702.02.42.92.13.93.33.12.66.27.2

11.318.162.049.241.6

Total depositionMean depthMean attenuation

200.052 Bq/m2196 mm

26.7 g/cm2

Cs-137 depth profile

Specific activity [Bq/g]

I1 <S 30

20

3

3

<—*

\

. —

• > - • —

=>

r=»...

- NB26 S&B Experiment NE After Ploughing

84 Ris0-R-1O29(EN)

No.

1!123456789

101112131415161718192021222 3242 526272 8293031

Meandepth|mro)

72 54 362809 8

1161351531711902 0 82 2 62 4 42622812 9 93 1 73 3 53 5 33 7 23 9 04 0 84 2 64 4 44 6 24 8 14 9 95 1 8536554

Acc.ulatedattenuation

fe/cm2]0.93.25.68.0

10.412.815.217.5J9.722.024.326.729.231.634.136.639.141.644.246.749.351.854.457.059.662.264.867.470.172.875.5

Weightof sample

Igl116138137139138134130130126130136138140140140142144146143146145149145147147147149152155154150

Activityin sample

IBq]19.514.6

30215533261272394710

11263 1 4184141172174105

553 35751

16.62 . 5

0.581.21

5.10.930.060.870.030.200.29

Total depositionMean depthMean attenuation

1.121.982 Bq/m2162 mm

20.9 g/cm2

Cs-137 depth profileSpecific activity [Bq/g]

5 10 15

20

30

a £0 40 • |

<

>

\

\

'•

. — • 1

-NB27 S&B Experiment Center Alter Ploughing

No.11

1234567

89

101112

131415161718192 02122

232 4

Meandepth|mm|

826446 38199

1181361551731912092282462652833013 2 0

3 3 83 5 73753 9 44 1 2431

Acc.ulatedattenuation

[g/cm2]0.82.85.06.78.09.3

10.512.013.915.917.919.721.423.024.927.029.231.634.136.639.141.744.346.8

Weightof sample

Igl99

13211185706972

99112114114

988998

112125132137142144145146146147

Activityin sample

IBql237

8067

1705004 4 06 0 83 0 22.07187182

2 4 54073562 4 22 0 4

94413025

11.53.03.42.2

Total depositionMean depthMean attenuation

1.064.952 Bq/m2158 mm

14.9 g/cm2

FT6

O«3

itten

I—i3<

5

11)

15

20

25

SO

35

40

45

50

Cs-137 depth profileSpecific activity [Bq/g]

2 4 6 K 10

1

tfi

^»—• • — AS

-NB28 S&B Experiment NW Atler Ploughing

Ris0-R-1O29(EN) 85

No.

II1234567

89

10

U121314

IS1617

18

19

Meandepth[mm]

128146164

183201219

238256274293311329

3483663844 0 3421

439458

Acc.ulatedattenuation

Ig/cni2|12.714.315.817.318.920.922.925.227.529.631.734.236.739.241.844.447.049.652.2

Total depositionMean depthMean attenuation

Weightof sample

Igl90878 585

104113126

131129109134143145146146149149148145

1.668.499177

17.2

Activityin sample

IBql573

16232789

939382211

6 539

22873

27836

10.19.67.02.1

1.92.2

5.2

Bq/m2mmg/cm2

r—

S

1bJ

CO

\3ra

:tei

a•8

lat

S3

3O

o<

Cs-137 depth profileSpecific activity [Bq/g]

£ 30

-NB29 S&B Experiment SW Alier Ploughing

0.

]123456789

10111213141516171819

Meandepth[mm!

82644628199

1171361541731912092272462642 8 3301320338

Acculatedattenuation

Ig/cm210.42.14.36.68.9

11.313.816.419.021.624.226.829.532.134.837.440.042.745.3

Total depositionMean depthMean attenuation

Weightof sample

[gl56

128132127134139143148148151149146155150149149150153146

2.195.58047

4.9

Activityin sample

[Bql2536107113843333

782185964419

9.19.0152214171513

2.61.7

Bq/m2mmg/cm2

I

Cs-137 depth profile

Specific activity [Bq/g]10 21) 30 4[>

1

t

»̂=— ==•—

-ZA01 Young Y»K North

86 Ris0-R-1O29(EN)

No.

11123456789

101112131415161718

Meandepth[mml

82644638199

1181361541731912 0 9227246264282300318

Acc.ulatedattenuation

[g/cm2]0.52.1

4 .36.79.0

11.413.816.318.821.323.726.228.731.333.936.639.241.8

Total depositionMean depthMean attenuation

Weightof sample

Igl62

117138132134137140141140140140140145148150151151147

7.055.16764

6.9

Activityin sample

[Bq]137022485857

1125767012187

4202 0 3109186

6933423926

8.39.13.6

Bq/m2mmij/cm2

5

.I "I 20

<3 25• oo>.3 303

3 35

40

Cs-137 depth profile

Specific activity [Bq/gJ

-ZAII2 Yiiulif Pirn: Soolll

No.I!

1234

56

7

8

9

10

11

12

1314

15

16

17

18

Meandepth[mm|

8

2 6

44

62

81

99

117

136

154

172

191

209228246265283302321

Acc.ulatedattenuation

Ig/cm2|1.0

3.3

5.8

8.3

10.813.315.818.320.723,225.628.130.633.235.738.240.843.5

Total depositionMean depthMean attenuation

Weightof sample

Igl119

143

143

144

141

142

142

142

138

138

140

142

144

143

143

143

152

150

2.751.20135

4.5

Activityin sample

[Bql198834865720

600

100

29

17

11

9.3

9.1

3.1

2.6

1.9

1.9

2.2

2.0

1.9

2.0

Bq/m2mmg/cm2

Cs-137 depth profile

Specific activity [Bq/g]111 20 30 41) 50

H 25 | r

B -™ '<

E 35

45

ZA03 Young Birch North

Ris0-R-1O29(EN) 87

No.1)

123456789

101112131415161718

MeandepthImm|

826446381

100118137155173192210228247265284302320

Acc.ulatedattenuation

[g/cm2]0.72.54.87.29.7

12.314.917.520.222.925.728.431.133.736.339.041.644.2

Total depositionMean depthMean attenuation

Weightof sample

Igl81

125137143144146147152153157154153154148150151147145

4.726.11846

5.1

Activityin sample

IBql958

422285755789

9321179.34.92.63.0

1.771.622.0

1.750.650.980.350.78

Bq/m2mmg/cm2

2 25 i r

I 3<)

I 35

u 40

< 45

50

Cs-137 depth profile

Specific activity [Bq/g]

20 40 60 no

-ZA(M Young Birch Snulh

No.[ ]

1

2

3

4

567

8

9

10

11

12

13

14

15

16

17

Meandepth[mm]

8

26

44

63

81

100

118

136

155

173

191

210

228

247

265

284

303

Acc.ulatedattenuation

[g/cm2]0.2

0.9

2.3

4.0

5.7

7.6

9.5

11.513.515.617.720.122.625.127.730.232.9

Total depositionMean depthMean attenuation

Weightof sample

fel23

62

91

99

102

109

109

116

116

116

129

140

144

144

145

146

153

2.989.62237

2.3

Activityin sample

IBql

458135912201

997

722

343

166

9988823718

9.25.1101074

Bq/m2mmg/cm2

cs£3} ID

I :1 15

i1 -l •i » •* .,„;

Cs-137 depth profile

Specific activity [Bq/gj50 KM) 150 200 250

_--•

—•— ZA12 OU Pine Fomsl

Ris0-R-1O29(EN)

No.1)

123456789

1011121314151617

MeandepthImm]

826446 381

100118136155173192210229247266284303

Acc.ulatedattenuation

[g/cm2)0.10.51.73.45.37.39.5

11.814.116.619.121.623.926.328.631.033.5

Total depositionMean depthMean attenuation

Weightof sample

Ig)103795

1011161)9126132135143144137134135130136144

2.902.8043 31.4

Activityin sample

[Bq|299654312938

626227215

403231211311141235205.6

Bq/m2mmg/cm2

,—,

.o

O

3C

£

td

umu

oo

5

HI

15

20

25

30

35

40

Cs-137 depth profileSpecific activity [Bq/g]

UK) 200 300

1 —

- Z A 13 s. Old Forrest

Ris0-R-1O29(EN) 89

Appendix B. Dose Rate Measurements

Table B. 1. Dose rate measurements with the Renter Stokes ion-chamber.

Pre doserate18/81997

20/81997

X-pos00000000000000001010101010101010101010101010101020202020202020202020202020

Y-pos Describtion010203040506070809010011012013014015001020304050607080901001101201301401500102030405060708090100110120

90

nSv/h8558088739161012901876941937795907913914106088075111511068110810181016105010011048102810261057112310671183120410361142114011121020109611511148118511481127113311091161

nSv/h

1073886769

1197981752

nSv/h nSv/h nSv/h11898778368921046910892966956823931946945109089977111811087112910411024106810001065104310791092116211031198999756118911641101981104010901081111711101156117311531189

Ris0-R-1O29(EN)

20202030303030303030303030303030303030404040404040404040404040404040405050505050505050505050505050505060606060

1301401500 ditch10203040506070809010011012013014015001020304050 Gutter6070809010011012013014015001020304050607080901001101201301401500102030

10821042104896110971116852106691711081132820105770911188471086122712329081214121513261383169313611128

1000

1025

11851152118410091111113510421048109210071090102810991099110010801094117411631086109511851079

1126972869

110912471247

11339808749881025478300280284276313340105072511458511121125912649231129388258264263266304

921

1060

1232119112251044112010718168568767728226929861113114011281143123412201124113512171036

Ris0-R-1O29(EN) 91

60606060606060606060606032.232.235.939.639.6

32.232.235.939.639.6

32.232.235.939.639.6

35.92935.942.7*35.628.835.642.5335.628.835.642.532.232.235.939.639.632.232.2

405060708090

10011012013014015031.236.533.831.336.5

52.257.454.852.257.4

74.379.276.974.379.5

27.93439.8344954.860.654.871.176.982.776.997.3

102.599.997.3

102.5118.6

123.8

house 1 NW gndhouse 1 NE gndhouse 1 C gndhouse 1 SW gndhouse 1 SE gndhouse 1 S 1sthouse 1 C 1sthouse 1 N 1sthouse 2 NWgndhouse 2 NE gndhouse 2 C gndhouse 2 SWgndhouse 2 SE gndhouse 2 S 1sthouse 2 C 1sthouse 2 N 1sthouse 3 NWgndhouse 3 NE gndhouse 3 C gndhouse 3 SWgndhouse 3 SE gndhouse 3 S 1sthouse 3 C 1sthouse 3 N 1st2 m west of h 12 m north of h 12 m east of h 12 m south of h 12 m west of h 22 m north of h 22 m east of h 22 m south of h 22 m west of h 32 m north of h 32 m east of h 32 m south of h 3house 4 NW gndhouse 4 NE gndhouse 4 C gndhouse 4 SW gndhouse 4 SE gndhouse 5 NW gndhouse 5 NE gnd

1023108210561136892

1023116611211205117011631171449465332560615588468551479511334608580598494494495483344579605563473524789923

10611054941914

11691017872922972

1009519540349538570529525

298

221264275407362422365450259275326422386424

246250213259263401350401258253209252269387331361

1021109811641158899

104812031164126912231217

254243213254243392347393237236

236230372322358

212199157204196265212265

92 Ris0-R-1O29(EN)

35.9 121.2 house5Cgnd39.6 118.6 house5SWgnd39.6 123.8 house5SEgnd10 44.5 line hi - h212 44.5 line hi - h214 44.5 line hi - h216 44.5 line hi - h218 44.5 line hi - h220 44.5 line hi - h222 44.5 line hi - h224 44.5 line hi - h226 44.5 line hi - h228 44.5 line hi - h230 44.5 line hi - h232 44.5 line hi - h234 44.5 line hi - h236 44.5 line hi - h238 44.5 line hi - h240 44.5 line hi - h242 44.5 line hi - h244 44.5 line hi - h246 44.5 line hi - h248 44.5 line hi - h250 44.5 line hi - h252 44.5 line hi - h254 44.5 line hi - h256 44.5 line hi - h210 145 Green plot at river12 145 Green plot at river14 145 Green plot at river16 145 Green plot at river18 145 Green plot at river20 145 Green plot at river22 145 Green plot at river24 145 Green plot at river26 145 Green plot at river28 145 Green plot at river30 145 Green plot at river32 145 Green plot at river

House 1 2-m EastHouse 1 2-m SouthHouse 1 2-m WestHouse 1 2-m NorthHouse 2 2-m NorthHouse 2 2-m EastHouse 2 2-m SouthHouse 2 2-m West

359667679 15/8 97 16/8 97 18/8 97 20/8 971192 1200 1183 11791249 1249 1233 12391200 1203 1185 11841137 1133 11011206 1199 1142 11441202 1181 1100 10891162 943 9491078 579 5771183 1103 419 4011213 1094 363 3311213 1000 358 3301196 625 312 2941188 394 291 272 2471151 361 301 280 2511170 331 292 270 2441179 351 306 279 2591217 398 321 297 2891203 357 327 352 3431153 442 422 408 4151085 646 624 597 5901120 952 950 918 9251189 1094 1093 1089 10771233 1149 1149 1149 11491220 1155 1159 1160 11581204 7681305 5411163 4701144 4731211 5421189 8451049 9451105 10511225 12001230 12161255 12531270 1272

236290255305294254296260

Ris0-R-1O29(EN) 93

Table B.2. Exposure rate inside house 1, ground floor at different stages of in-tervention measured with El-1101 (JJR h'1).

Point N

ClC2C3C4

123456789

Average123456789

Average

Floor

groundgroundgroundgroundgroundgroundgroundgroundgroundgroundgroundgroundgroundgroundfirstfirstfirstfirstfirstfirstfirstfirstfirstfirst

Before

6054768949364633574066453052.457575453636064605057.6

Soilremoving

2226232421202421242223221922.440384039424039383438.9

Pouringsand

2224212223222520232223211922.139393939434039383438.9

Roofremoving

1921221819172118191920181518.927232827282625252025.4

New roofput on

2022222019162118201820171519.124242625262525232024.2

Table B.3. Exposure rate inside (ground floor) and outside House 2 before andafter intervention, East corner - west corner direction

Distance fromcentre of house

-8.2-6.8-6.4-6-4.6-3.203.24.666.48.2

DR before, ixRJh

1432031648158382735485982

123

DR after, jjR/h

16171614171819

94 Ris0-R-1O29(EN)

Table B.4. Influence of roof contamination on exposure rate formation aroundHouse I, Chernobyl component.

Distance from thecentre of House 1

[m]

-15.8-14.8-13.8-12.8-11.8-10.8

-9.8-8.8-7.8-6.8-5.8-4.8-3.803.84.85.86.87.88.89.8

10.8

After soilremoval[MR h"1]

21212018191822212121171815151921212121242525

After roofremoving[MR h"1]

18 Direction18 to House 2(east)19191919181817161615141017191918202121 Direction23 to gate(west)

Appendix C. Miscellaneous Meas-urements

Table C.I. ^-Measurement results of roof contamination of House 1 after cov-ering with clean sand. In this table gamma refers to the background responseof the j3-detector caused by y-rays.

N of sheet

1

234567

Average

B+gamma[Bq cm'2]

10.5

7.812.88.9

10.110.710.110.1

East side

gamma[Bq cm"2]

4.0

2.83.43.22.53.23.33.2

6[Bq cm"2]

6.5

5.19.45.77.57.56.86.9

B+gamms[Bq cm"2]

7.2

7.15.67.79.3

11.09.48.2

West side

i gamma[Bq cm"2]

2.9

3.52.92.73.24.03.83.3

6[Bq cm"2]

4.3

3.52.75.16.17.05.64.9

Ris0-R-1O29(EN) 95

Table C.2. ^-Measurement results of roof contamination of House 2 after cov-ering with clean sand. In this table gamma refers to the background responseof the fi-detector caused by y-rays.

N of sheet

1

23456

Average

B+gamma[Bq cm"2]

10.7

6.53.85.1

11.1

4.57.0

East side

gamma[Bq cm"2]

4.0

3.72.31.74.62.03.0

13[Bq cm"2]

6.6

2.91.63.56.52.53.9

13+gamma[Bq cm"2]

10.6

10.611.39.6

11.711.110.8

West side

gamma[Bq cm"2]

3.7

3.94.04.54.13.84.0

6[Bq cm"2]

6.9

6.77.35.17.67.36.8

Table C.3. Decontamination experiment on one roof sheet from house 1. A 50% reduction was obtained by scraping with a simple steel instrument. In thistable gamma refers to the background response of the (5-detector caused by J-rays.

Sheet 5

Measuring point

123456

Average

Before scraping

B+gamma[Bq cm"2]

10.469.358.83

11.4110.610.310.2

gamma[Bq cm"2]

2.962.872.53.242.882.632.8

B[Bq cm"2]

7.56.486.338.177.727.677.3

After scraping

13+gamma gamma[Bq cm"2] [Bq cm"2]

6.295.355.047.096.215.846.0

2.452.462.272.272.252.462.4

fi[Bq cm"2]

3.842.892.774.823.963.383.6

Table C.4. Decontamination of two roof sheets determined by Ji- and y-measurements of the U7Cs contamination. The ^-measurements were done insitu, 50 cm from the edge of the lowest sheets

B-measurements[kBq m"2]

y-measurements[kBq m"z]

Object Average

Sheet 21, before scraping 91

Sheet 21, after scraping 60

SD

11

11

Average

196

152

SD

18

11

96 Ris0-R-1O29(EN)

Table C.5. Angle distribution of flux of gamma quanta with energy 661 keV inthe middle of house 1. Shielded measurements.

Direction

rooffloorsouth wallwest wallnorth walleast wallSum40° up west40° up eastNo shielding

Beforeintervention

0.3800.590.80.230.532

4.27

After soilremoving and

sanding

0.3800.340.230.170.121.24

2.27

After roofremoving

0.0500.240.220.170.20.680.010.161.5

Total reductionfactor

7.602.53.61.42.72.9

2.8

i

I

30-

25 -

^ — K -WallofHouse2 ^ \

East wall

5 -

1 1 1 ft-

/ West wall

-•— After soil decontamination

-*— After roof removing

1 1

-20 -15 -10 -5 0 5

Distance from center of House 1, m

10 15

Figure C.I. Exposure rate from house 2 through house 1.

Ris0-R-1O29(EN) 97

Table C.6. CORAD measurements 1997, Guta Miiravinka, Briansk Region.

x-coordinate

[m]

0000000

101010101020202020203030303030404040404040505050505060606060606040.460101214161820101214161820

y-coordinate

[m]

02040608080

10010305070900

204060

1001030507090

020406070901030507090

020406080

10050.680

145145145145145145145145145145145145

Equivalentdeposition[]XG m1]

24.516.22218.719.219.226.126.723.523.725.622.930.124.224.731.732.622.919.814.929.525.91731.728.73923.216.830.324.428302624.631.718.126.811.229.1

13712.325.538.31426.933.639.411.21.87002.15

17.7

Total deposition[|xCi m"2]

35.221.830.923.428.828.833.533.431.628.233.627.244.240.934.838.638.727.22917.7363431.939.640.452.435.524.139.734.43539.43031.541.660.331.915.841

17919.234.256.326.334.549.466.422.5

7.14007.2

35

Ratio

1.300.811.140.871.071.071.241.241.171.041.241.011.641.511.291.431.431.011.070.651.331.261.181.471.491.941.310.891.471.271.301.461.111.171.542.231.180.581.526.620.711.272.080.971.281.832.460.830.260.000.000.271.30

98 Ris0-R-1O29(EN)

Bibliographic Data Sheet Ris0-R-1O29(EN)Title and authors

Mechanical decontamination tests in areas affected by the Chernobyl accident

J. Roed, K.G. Andersson, A.N. Barkovsky, C.L. Fogh, A.S. Mishine, S.K. Ol-sen, A.V. Ponamarjov, H. Prip, V.P. Ramzaev, B.F. Vorobiev

ISBN

87-550-2361-4

Department or group

Nuclear Safety Research and Facilities

Groups own reg. number(s)

Pages Tables

98 24

Department

Illustrations

75

ISSN

0106-2840

Date

August 1998

Project/contract No(s)

MIKA-By

References

33

Abstract (max. 2000 characters)

Decontamination was carried out around three houses in Novo Bobovichi, Russia,in the summer of 1997. It was demonstrated that significant reductions in the doserate both indoor (DRF = 0.27) and outdoor (DRF = 0.17) can be achieved when acareful cleaning is undertaken. This report describes the decontamination work car-ried out and the results obtained. The roof of one of the houses was replaced with anew roof. This reduced the Chernobyl related dose rate by 10 % at the ground floorand by 27 % at the first floor. The soil around the houses was removed by a bobcat,while carefully monitoring the ground for residual contamination with handhelddose meters. By monitoring the decline in the dose rate during the different stagesof the work the dose reducing effect of each action has been estimated. This reportalso describes a test of a skim&burial plough developed especially for treatment ofcontaminated land. In the appendices of the report the measurement data is avail-able for further analysis.

Descriptors INIS/EDB

CESIUM 137; CHERNOBYL; DECONTAMINATION; DOSE RATES;HOUSES; INTERNATIONAL CO-OPERATION; RADIATION DOSE DIS-TRIBUTIONS; REACTOR ACCIDENTS; RURAL AREAS; RUSSIAN FED-ERATION; SOILS; SURFACE CONTAMINATION; SKM-AND-BURIALPLOUGHING; FORESTS; EXCAVATION; REMEDIAL ACTION; LANDRECLAMATION

Available on request from Information Service Department, Ris0 National Laboratory,(Afdelingen for Informationsservice, Forskningscenter Ris0), P.O.Box 49, DK-4000 Roskilde, Denmark.Telephone +45 46 77 40 04, Telefax +45 46 77 40 13

Ris0 National Laboratory carries out research within science and technology,providing Danish society with new opportunities for technological development. Theresearch aims at strengthening Danish industry and reducing the adverse impact onthe environment of the industrial, energy and agricultural sectors.

Ris0 advises government bodies on nuclear affairs.

This research is part of a range of Danish and international research programmes andsimilar collaborative ventures. The main emphasis is on basic research andparticipation in strategic collaborative research ventures and market driven tasks.

Research is carried out within the following programme areas:

• Industrial materials• New functional materials• Optics and sensor systems• Plant production and circulation of matter• Systems analysis• Wind energy and atmospheric processes• Nuclear safety

Universities, research institutes, institutes of technology and businesses areimportant research partners to Ris0.

A strong emphasis is placed on the education of young researchers through Ph.D.and post-doctoral programmes.

ISBN 87-550-2361-4ISSN 0106-2840

Copies of this publicationare available from

Ris0 National LaboratoryInformation Service DepartmentP.O. Box 49DK-4000RoskildeDenmarkTelephone+45 4677 4004risoe@risoe.dkFax +45 4677 4013Website www.risoe.dk