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Nuclear Submarine Decommissioning. Radiation Risk Assessments.

A.Ja. Blekher1, V.V. Dovgusha1, N.L. Kuchin2

1Research Institute of Industrial and Marine Medicine, St.-Petersburg, Russia2Krylov Institute, St.-Petersburg, Russia

INTRODUCTIONFrom a beginning of atomic shipbuilding (the 1956) in Russia built about 250 atomic submarines (NPS),

5 ships and 9 civil atomic vessels. In connection with expiry of the term of maintenance and conclusion of thecontracts about limitation and cutting of offensive arms of Russia from the extremity of the 1980-th years acutelythere was a problem of their utilization.

Utilization of the ships and vessels with nuclear powered plants (NPP) on the complexity, dearness andsocial importance represents a problem of world importance and is priority in activity of naval fleet (NF).

It is possible to judge its scales that to a start of 1999 from battle composition of NF is output about 180NPS (60 % - from composition of Northern Fleet, 40 % - from composition of Pacific Fleet). Among NPS,output from battle composition, 30-40 % are of a first generation and more than 60 % - of second generation.

The removal from operation and utilization NPS is predetermined necessity of the decision of a seriesof problems, connected to development and realisation technologies for handling with the radioactive equipmentand radioactive waste of the ships with NPP and safe for the personnel, population and environment.

MAIN STAGES of a NPS's UTILIZATIONThe process of a NPS's utilization represents a sequence of stages, basic of which are:

• conclusion NPS from maintenance;• storage NPS with the not unloaded spent fissile material (SF);• unloading SF;• storage NPS after unloading SF;• translocation NPS on ship repair facility;• demounting radioactive equipment NPS;• cutting and preparation for long-termed storage of a reactor compartment;• transporting a reactor compartment to a place of temporary or long-termed storage;• disjunction on scrap of bow and stern compartments.

The most complicated is the problem of handling with RC, in which radioactive equipment NPP andbiological shielding is located. This problem includes the decision of the following basic questions:• the preliminary estimation of time period after removal of concrete object out of operation on the expire of

which reactor's compartment (RC) utilization will come possible, and utilization technology option;• the choice of version for RC temporary storage or burial.

The key moment under these questions decision is the radiation state of removed out of exploitation andprepared for utilization of ship NPP. After discharge of liquid and solid radioactive waste there remain thesources of 2 types radiation in reactor compartment volume: volumetric-radiating-activated material of NPPequipment and superficial-radiating - contaminated by radionuclides equipment's and constructions' surfaces.

Thus the integrated activity of these sources at time of endurance 1-3 year reaches (1-2).1015 Bq. Morethan 90 % of long-termed radionuclides are localised in intrareactor constructions, that forming not more 5-7 %of total radioactive materials mass.

Radiation contamination’s level of intrareactor constructions surfaces may reach 2 millions Bq/sm2, ofinternal surfaces of the 1-st circuit equipment elements - 100 thousand Bq/sm2, of the surfaces of the 3-d circuitequipment elements, of desiccation and repair cooling tanks - 20 thousand Bq/sm2. Depending on constructionperiod the surfaces contamination’s activity can be conditioned by corrosion products (by 60Co on the whole) orby radionuclides of fragmentation origin: 137Cs, 134Cs, 144Ce, 106Ru, 90Sr (1).

Actinide radionuclides activity in the 1-st circuit communications volume of water-cooled ship's NPPcan reach 4.5.1011 Bq.

By 1-3 year delay-time induced activity of construction of exploitation ship's NPP in the places mostclose to the active zone for reactor casing reaches 2.6.1011 Bq/kg order value, for reactor caisson - 1.5.1011 Bq/kg,for steam generator case - 4.108 Bq/kg, for pressure hull under reactor - 2.1010 Bq/kg.

Two conceptions of handling with RC radioactive saturation are discussed.The first one is, owing to the absence of enterprises with appropriate equipment for disjunction of high-

active equipment at present, the long-term keeping (during 50-80 years) for natural activity decay is necessary.After long-term keeping they may be disintegrated without large dose expenditures, and, as a rule, withoutspecial remote and protective equipment.

Thus, during 70 years of keeping, the rate of activated gamma-radiation sources will reduce in 1000

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times average.Exclusively compact arrangement of the ship NPP main radioactive equipment and relatively small

reactor compartment mass (1000 tons order), makes reasonable to assemble NPP in containers. As a containerthere can be used the pressure hull of removed reactor compartment with established on end faces (in places willbe used are sharp) sealing hermetically enclosures.

For normalising radiation situation in accordance with sanitary requirements in the lower part ofcompartment, in the reactors zone arrangement, the separate sections of pressure hull (container) must becovered by appropriate biological shielding.

According to the second conception, is immediate upon termination of period of a storage NPS (2-3years) radioactive equipment and reactor compartment constructions are disassembled, extracted from thecompartment, inserted in pressurised containers and are directed to a long-term delay or to burial ground. Theadvanced variant of the given concept provides a complete utilization (remelting) of equipment and hull of areactor compartment.

. However, the reactor's compartment dismantling immediately after leading out of exploitation isclosely connected with considerable dose loading for environment and necessity of using of special equipmentwith remote control. Really, at being repaired NPS of the 2-nd generation dose rate in reactor caisson was up to400 mSv/h, close to reactor body - 20-40 mSv/h, in steam generator caisson - 2 mSv/h. Thus realisation ofpresent conception is possible only after creation of specialised ship disintegrating enterprise, equipped withappropriate technology that needs large-scale capital investment. Moreover, there remains the problem of storageof disembarked equipment and left part of reactor compartment, including reactor itself.

Owing to the told at present both in Russia and in USA (2) in practice the method of containerising theequipment of ship NPP using the NPS’s pressure hull and sealed bulkhead over its ends is used. Fornormalisation of the radiation situation in conformity with sanitary requirements in the lower part ofcompartment, in the reactor's arrangement zone, separate areas of pressure hull (container) are covered by properbiological protection. The thickness and configuration in every concrete case can be determined by dosimetricinspection results.

Prepared for burial compartment must correspond to IAEA requirements (3) for type "B" radiationpacking (by tightness and impact strength) and PTBRV-73 (4) for the 4-th transport category (by externalgamma-radiation levels).

In this case equipment of bow-and stern compartments is disassembled, extracted and either comes forutilization or is reserved for the second employment. Stage by stage ship's constructions are disintegrated inblocks.

The second major problem is the organization of storage of the cut RC. By maintain of main principlesof conception various versions of prolonged RC keeping organising can be offered:• afloat at allocated region of water area;• wreck in shallow water of allocated region of water area;• in underground workings-galleries;• in concrete trenches or on the surface-type sites protected from precipitation;• in encirclement of artificially frozen ground, in conditions of permafrost and polar climate (at present thus

proposal is being working out in reference to conditions of Novaya Zemlya archipelago).Afloat storage of ship NPP removed out of exploitation (forming part of NPS, three - compartments

blocks or reactor compartments) are of radiation-ecological danger and contradict standards and internationalrecommendations (5-8), prohibiting storage of radioactive waste on flooded and waterlogged territories, what ismore afloat or under water, in shallow water.

So, this practice should be regarded as forced and provisional. The terms of afloat storage as well asquantity of objects should be reduced to a minimum.

Sanitary regulations and ICRP recommendations exclude the possibility of radioactive waste depotsarrangement in flooded and waterlogged places, in regions situated nearer than 500 m from the reservoirs, aswell as by ground waters level above 4 m from the depot floor (5,6). Specified requirements should be carriedout beyond dependence upon the degree of depot waterproofing, that significantly makes difficult the practicalrealisation of version NPS' RC arrangement in underground workings-galleries.

The most reasonable and ecologically safe version in organising RC long-term storage of NPS removedout of exploitation is their arrangement in concrete trenches (holes), protected from atmospheric precipitation, orin polar climate conditions in permafrost or artificially frozen ground.

Organising of RC storage on the ground surface or in mild holes (10-20 m deep) and in permafrostground encirclement also does not contradict with sanitary-hygienic documents requirements.

At organization of storage on the specially equipped site of the shore the depot should have not less thantwo bars on the possible environmental radionuclides route entry. The first includes engineering equipment ofstorage place for prevention of meteorological and hydrological factors influence, which may breakcompartment's tightness (covering shield) and engineering system for prevention of radionuclides entry in

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environment in case RC loss of sealing (spreading shield). Geological formations in depot's arrangement place,which must limit radionuclides dissemination, are the second bar. Requirements, similar to arrangement andequipping of points for radioactive waste burial are brought for plot choice. By execution of all necessaryrequirements for the storage place the danger of activity dissemination in environment is insignificant and isclearly defined primary by possibility of outward reasons influence (hurricane, tornado, aircraft's fall).

As the advantage of version for RC burial in permafrost is the unlimited heat capacity of surroundingrocks and absence of activity dissemination with ground waters as in conditions of frozen rocks of arcticgeocryologic region free water freezes completely, forming so called constitutional ice in which migrationprocess physically cannot be realised.

RC burial in five meters deep from the top part of compartment down to permafrost surface groundsatisfy IAEA requirements and Russian regulating documents for radioactive waste burial (5-7).

The final decision about carrying out of utilization of equipment materials and constructions of RCremoved out of exploitation NPS may be accepted only on the basis of results of materials radiometricexamination.

RADIOECOLOGICAL SITUATION on TODAYOur ratings (1) have shown, that for known methods of metal's cutting the greatest exit of radioactive

substances in an environment has a place at plasma cutting, and least - at hydrocutting and mechanical cutting.Composition of aerosols mainly contains radionuclides 55Fe and 60Co. According to results of calculations, in allcases at the most conservative suppositions already apart 1,5 kms the volumetric activity of air does not exceed1.10-3 of valid concentration for the population of mixture of radionuclides of the given composition atendurance 2 years and 1.10-5 of valid concentration - at endurance 50 years. The soil pollution also is extremelyunsignificant.

The detailed analysis of observed data of objects of an environment on enterprises of atomicshipbuilding occupied with a utilization NPS, quits for frameworks of the present message. However, it ispossible to make of available materials a main conclusion that the radiation-dangerous operations, conducted onthese enterprises, to the present time have not rendered significant effect on a radiocontamination of free air, soil,vegetation, seawaters, algae and bottom sediments. Thus it is essential, that practically it is impossible to isolatethe contributions to an available radiocontamination of repair operations and operations on a utilization NPS,therefore, at the marked considerable increase of scales of operations on a utilization, this favourable picture it isimpossible to extrapolate in time.

TOTAL CHARACTERISTIC of POSSIBLE FAILURESAt realization complex decommissioning of NPS, as well as at their repair, there are the emergencies,

accompanying with an output of radionuclides, including uncontrollable, in objects of an environment. Theemergencies are possible as at storage of NPS and overloading of nuclear fuel, and directly at realization ofdismantling work. Their reasons can be refusals of technological means and equipment, infringement oftechnological process, error of the personnel, and also external reasons - extreme weather phenomena, collisionof the ships and courts, fall of flying apparatus, explosions (including, diversions) and other. Irrespective of theprimary reason of an emergency, danger to the population and environments represent air and liquid discharges,activity of which exceeds discharges, carried out at normal technological process.

The list of typical failures, resulting to increase (emergency) discharges in an environment is in thetable. The expected consequences of each failure are compared to a known scale IAEA of classification ofnuclear incidents and failures for atomic power stations (9,10).

The scale IAEA is developed for atomic power stations (APS) and until is not adapted for the ships andvessels with NPS, and also for the enterprises, on which are carried out nuclear- and radiation-dangerous works.In view of much smaller capacity both other of constructive and operational peculiarities of ship' NPP incomparison with APS, scale of expected consequences at identical damages of reactor installation and otherequipment on APS is much higher, than on nuclear vessels and ships. Nevertheless, parity of failures on thenuclear ships and ship repair enterprises with failures similar on character on APS allows really estimate scale ofdanger of failures, especially heavy, for the population and environment.

From considered failures the nuclear failure on the ship, destruction of storehouse liquid radioactivewaste (LRW), wreck of the ship with an active zone, fire in RC concern to over-design failures. The otherconsidered failures concern to a category design.

The maximum design failure (MDF) on expected consequences and scale of influence on the populationand environment is failure with loss of the heat-carrier on the nuclear submarine at a presence it in water area ofthe enterprise. On the IAEA scale this failure can be classified as "serious incident" (level 3 scales).

The failure with destruction of temporary storehouse LRW results to greatest possible discharge ofradionuclides in water area of the enterprise, but the consequences of discharge for an environment areinsignificant. This failure also can be referred by a level 3 IAEA scales "serious incident".

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The heaviest failures are implemented on objects with the not unloaded active zones. First of all, it isself-any chain reaction in process of overloading of an active zone, fire on utilized NPS and NPS or reactorcompartment wreck at their storage.

Table 1

The list of emergencies, possible at repair and decommissioning of NPS

№ The name of failure The possible reasons (basic) Scale of consequences (level)on a IAEA scale

Design failures1 Ventilation of industrial

premises(rooms) of acontrollable zone withoutclearing on filters

Refusal of filters; infringement ofwork of system of ventilation, error ofthe personnel

Level 1 "insignificant incident"

2 Discharge LRW in water areaat transfer

Infringement of technological process,error of the personnel, adverse weatherconditions

Level 2 "incident of averageweight"

3 Fire in RC atdecommissioning of NPS

Infringement of technological process,non-observance of rules of safety,error of the personnel

Level 3 "serious incident" -level 4 "failure in limits ofinstallation"

4 Wreck of prepared totransportation cut out RC

Infringement of technological processat transportation, errors of thepersonnel, adverse weather conditions,navigating failures

Level 1 "insignificant incident"

5 Failure with loss of the heat-carrier on NPS

Refusal of means, error of personalstructure

Level 3 "serious incident"

Over-design failures6 Wreck of the NPS with

unloaded activeNavigating failure, extreme weatherconditions, error of personal structure,diversion

Level 2 "incident of averageweight"

7 Destruction of storehouseLPW

Diversion, fall of the flying apparatus,extreme natural situations

Level 3 "serious incident"

8 Fire at decommissioning ofthe NPS (general fire)

Infringement of technological process,infringement of rules of safety, errorof the personnel

Level 4 "failure in limits ofinstallation" -Level 5 "failure, dangerous toan environment"

9 Self-any chain Infringement of technological processat overloading of an active zone, errorof the personnel, diversion

Level 5 "failure, dangerous toan environment"

RADIATION RISK of LONG-TERMED STORAGE NPS or REACTOR COMPARTMENTwith NOT UNLOADED SF

The radiation risk is the complex characteristic of radiation effect, which is taking into accountpossibility of origin radiation danger and a consequence of an exposure.

According to the guidelines ICRP (11) and NRB-99 (12), in the field of small doses (less than 0,5 Sv)the individual risk of origin of stochastic effects is determined by the formula

r = ρ(E) . rE . E

where E - individual effective dose; R (Е) - probability of occurance of event creating a dose Е; RE - coefficient of risk from a fatal cancer, serious heritable effects and no lethal cancer (for thepopulation rE = 7,3.10-2 1/man. Sv).

Potential source of input of activity in an environment at a stage of long-termed storage can be reset ofthe heat transfer medium, and also corrosion of promoted construction materials of a reactor and, probably, fuelcomposition at direct contact with seawater as a result of NPS wreck.

The probability of unsanctioned reset of the heat transfer medium can be rated by value 4.10-6 1/yearson one reactor (13). The maximal amount of the heat transfer medium, which can thus be poured out in waterarea, makes 2 м3. The activity of the heat transfer medium is defined first of all 137Cs and (in view of derivednuclide 137mBa) makes 5.109 Bq. The radiation dose of the population is shaped by consumption of a seafood and,

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by ratings, does not exceed 10-6 Sv.The most probable reason of an NPS wreck during long-termed storage is the damage of a pressure hull

or its structural members as a result of processes of corrosion (14).The probability of such event rated as the ratio of an amount of cases, having a place, of an wreck

during repair or a storage to general operating time NPS, makes ~ 8.10-4 1/years (13). The wreck as a result ofexternal effects (falling of the aircraft, earthquake, wrecked, fire) or damage of a pressure hull at ram by othervessel is represented less probable.

The feature of NPS wreck at storage, as against an wreck while in service, is, that it happens in a near-shore area on a shallow water in water area of bases or repair factories. All this creates the premises for operatingrise of wrecked NPS. The dwell-time in the wrecked status is valued by the order 2 months. For obtaining aconservative rating starts, that the seawater has input in a reactor of wrecked NPS and has a place direct contactof seawater to fuel composition. Under these conditions the speed of a fission yield from fuel reaches 9.109

Bq/hours, actinides - 7,5.108 Bq/hours (15). Accordingly, the activity of the fission products, which have issuedfuel for 2 months of stay in a wrecked status, will not exceed 1,3.1013 Bq, actinides - 1,1.1012 Bq. The radiationconsequences of an output of activity in water area are valued by a radiation dose of the population no more than0,5 mSv and completely are stipulated by consumption of a seafood.

Thus, the radiation risk at long-termed storage NPS with not unloaded SF makes:• at the expense of probable reset of the heat transfer medium in water area - 3.10-13 1/years;• at the expense of a probable NPS wreck - 3.10-8 1/years.

Therefore, risk of long-termed NPS storage with the not unloaded fuel makes 3.10-8 1/years andcompletely is defined by consequences of an NPS wreck. The value of risk is lower than boundary of certainlyreasonable risk, what is, according to NRB-99, 1.10-6 1/years (11).

RADIATION RISK of SF UNLOADINGThe radiation risk of failures at SF unloading is defined by risk failures, accompanying with a self-any

chain reaction (SCR). The failures with origin SCR, though concern to an over-design category, took place onNF objects, for example, in a Chajma-bay in 1985 (16). The probability of origin SCR at fuel unloading on NPSis valued by 2,2.10-7 1/years on one reactor (17). In view of predicted rates of unloading the complete probabilitySCR in each of locales (Northern and Pacific) with a store is valued by 3,3.10-6 1/years.

According to the carried out calculations, under the worst weather conditions on an early phase of afailure the effective dose on an axis of a cloud outside enterprise can exceed a maximal annual limit of a dose forthe population (5 mSv) in two and more times (18). The zone of a radiation failure on an early phase will have anexpansion from 1500 up to 3000 meters from a place of ejection. More than on 80 % the effective dose isstipulated by inhalation input of radionuclides. According to the Russian criteria for decision making (11), thespecial measures of protection of the population on an early phase of a failure are not required, however it isnecessary both notice about a failure, and deleting of the people from open places on an axis of a track.

The annual doses (late phase of a failure) exceed an annual limit of a dose at 5-10 of time. The zone of aradiation failure has an expansion from 3500 m (unstable status of an atmosphere) up to 10 kms (at a steadystatus of an atmosphere) (18).

Thus, the radiation risk of failures, accompanying SCR, order 2.10-8 1/years that also is lower thanboundary of certainly reasonable risk.

THE CONCLUSIONThus, greatest probable on radiation consequences for the population and environment the failure with

SCR on the unloaded active zone is. On a IAEA scale it concerns to a level 5 - " a failure dangerous to anenvironment ".

The consequences, appropriate to levels 6 - "heavy failures" and 7 - "global failures" IAEA scaleessentially cannot be realised at decommissioning of the ships with NPP even at imposing the most adverseconditions of development of failures.

Radiation risk of the most dangerous stages of a utilization – NPS storage with not unloaded SF andunloading SF are valued by 3.10-8 1/years and 2.10-8 1/years respectively. General value of risk belowestablished by NRB-99 boundary of certainly reasonable risk 1.10-6 1/years. At the same time is apparent, thatthe further long-termed storage afloat NPS with not unloaded SF will reduce in rise of probability of their wreckand, accordingly, to increase of radiation risk.

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