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Quasi-Effective Energy of NaturalRadiation around Nuclear Power PlantsToshiyuki NAKAJIMA aa Division of Physics , National Institute of Radiological Sciences ,Anagawa, Chiba-shi , 260Published online: 15 Mar 2012.

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Journal of NUCLEAR SCIEKCE and TECHNOLOGY, 23[1], pp. 44-52 (January 1986).

Quasi-Eff ective Energy of Natural Radiation around Nuclear Power Plants

Toshiyuki NAKAJIMA

Division of Physics, National Institute of Radiological Sciences*

Received January 9, 1985 Revised A p r i l 18, 1985

As an application of the twin filter TLD system, estimates of the quasi-effective energy of natural radiation were obtained a t 43 monitoring points around nuclear power plants of eight prefectures in Japan. Quasi-effective energy was defined a s energy corresponding to the effective energy of photon radiation determined using the half value layer method. The quasi-effective energy of natural radiation, except for the hard component of cosmic rays, was in the range of approximately 850keV to 2.5MeV, with 85% of all the monitoring points giving quasi-effective energies from 1.0 to I.5MeV. This result shows that the cali- bration source for natural radiation dosimeters must be a 6oCo source.

KEYWORDS: thermoluminescence dosimeter, twin filter method, quasi-effectiue energy, natural radiation, nuclear power plants, emergency exposure, geological information, energy distribution, radiation monitoring, radioactiuity

I. INTRODUCTION Observations of natural radiation exposure have been madeover a long period of time

at monitoring points around nuclear power plants by a number of prefectural Institutes for Public Health and other organizations, using a variety of radiation dosimeters, as part of regular monitoring of operating conditions at the plants. Thermoluminescence dosimeters (TLDs), one of such radiation dosimeters, have been widely used to measure only the ex- posure. However, when an abnormal signal from such a TLD is observed, the cause of the abnormal signal cannot be inferred using only ordinary TLD monitors.

In an emergency or accident, identification of radionuclides leaking from the plant must be carried out at all monitoring points, in order to provide proper medical treatment to exposed persons around the monitoring points. In such cases, it is important to evaluate changes in the effective or quasi-effective energy of radiation. However, the ordinary TLD monitor is inadequate for this purpose and the use of spectrometers such as semiconductor detectors or scintillation detectors a t all monitoring points is not economically practical.

Exposure from natural radiation has been researched in studies of the distribution of both natural radiation dose and national population In order to obtain more exact measurements of exposure, absorbed dose and national population dose, the quasi- effective energy of natural radiation, as well as the exposure from natural radiation, needs to be researched.

For this purpose, the quasi-effective energy of natural radiation needs to be researched, in advance, under normal conditions. However, effective energy or quasi-effective energy of natural radiation has not been researched at all monitoring points using spectrometers such as semiconductor or scintillation detectors. It has also yet to studied using an inte-

* Anagawa, Chiba-shi 260.

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Vol. 23. No. 1 (Jan. 1986) 45

grated monitor such as the TLD. If the TLD system could give information about energy of radiation, it would be a most useful instrument, able to estimate both quasi-effective energy of natural radiation and effective energy of accidental radiation. Thus, a method for estimating effective energy of radiation has been developed using a TLDc6’-c8’.

In a previous work@), it was reported that the twin filter TLD method could be used for evaluating the effective energy of photon radiation. Thus, as one application of the twin filter TLD method, evaluation of the quasi-effective energy of natural radiation has been carried out a t monitoring points around some of the nuclear power plants presently operating in Japan.

Natural radiation consists of photons and charged particles from natural radionuclides in the soil and rock, and cosmic rays. Therefore, the definition of the effective energy used for photon radiation is not applicable to natural radiation. Notation of quasi-effective energy is introduced into natural radiation, and in this work, the quasi-effective energy of natural radiation is defined as the energy corresponding to the effective and the most pro- bable energiesc9’ of photon radiation determined using the half-value layer method and the depth dose measurement method, respectively.

II. EXPERIMENT The twin filter TLD method has been used to evaluate the quasi-effective energy of

natural radiation. The twin filter TLD system used in this experiment consists of a cylindrical lead filter of 10g/cmz in the wall thickness and 5cm in height, with a cylin- drical lucite filter of 0.45g/cm2 in the wall thickness and 5cm in height, in which the TLD elements are mounted. Observation of radioactive elements such as 210Pb in all the lead filters was carried out over 1,000 min, using a whole body counter and a semi- conductor detector, but measurements of radioactive elements could not be obtained from these lead filters. Internal structural defects of all lead filters were checked by radio- photography using 6oCo 7-rays, and defective filters were eliminated from the twin filter TLD system.

The CaSO,(Tm)-TLD elements of UD-110s were used for the twin filter TLD system in this experiment. After thermal treatment a t 450°C for 30min to eliminate the radiation history of the TLD elements, these TLD elements were mounted in the lead and lucite filters of the twin filter TLD system, and were covered with Al-foil to protect them from the illumination effect of light on thermoluminescence emission(1o) during the monitoring period a t the monitoring points.

The covered twin filter TLD systems were immediately sent by mail to perfectural Institutes of Public Health in eight prefectures in which nuclear power plants are located or being built. The twin filter TLD systems sent to the Institutes consisted of two types. One of the twin filter TLD systems was used as a monitor for evaluating both the quasi- effective energy and exposure of natural radiation a t each monitoring point. The other was used as a background monitor for evaluating the background radiation dose, such as natural radiation during the mailing of the system, the hard component of cosmic rays in the shielded lead box after arrival a t each prefectural Institute, and the self dose of the TLD elements. After arrival a t the prefectural Institutes, the background monitors were immediately kept in a shielded lead box of 5 or 10 cm in the wall thickness until their return together with the other monitors to author’s Institute.

Monitoring points in each prefecture were selected at random by staff members of the

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46 J . Nucl. Sci. Technol.,

1.5

0 .- L

L 1.25

* L)

'; 1.0 .- L) .- (D

0.75 In 2 + 0.5

prefectural Institutes. Two of the twin filter TLD systems were left in a wooden or plastic box for about 3 months, a t 1.5m above ground level a t the monitoring points. About 3 months later, both the monitor and the background monitor were returned to author's Institute.

The T L intensity of the returned twin filter TLDs was measured with a commercially available TLD reader, purchased from Kasei-Optonics Co. There were six TLD elements in each of the filters a t each monitoring point. The mean of the six T L intensities from each filter was used as the value in a quarter year and for evaluating the quasi-effective energy of natural radiation at each monitoring point.

m. EXPERIMENTAL RESULTS AND DISCUSSION 1. Calibration Curve of Quasi-effective Energy The photon energy dependence of the twin filter CaSO,(Tm)-TLD system was obtained

for evaluation of the quasi-eff ective energy of natural radiation. The calibration sources used were r-rays from 13'Cs and T o , X-rays from an ordinary X-ray generator and 10 MeV X-rays from a linear accelerator. In the case of X-rays from the linear accelerator, the

-

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-

-

-

energy is determined using the depth dose meas- urement method") and a value of 10 MeV indicates the most probable energy in front of the accelerator window (9).

Figure 1 shows the energy dependence of the ratio between T L intensities of CaSO,(Tm)-TLD elements in the lead and lucite filters of the twin filter TLD system used in this work. Each ob- served point shown in Fig. 1 is the mean value of twelve T L ratios obtained from the T L elements of three different batches. The coefficient of variation for each mean ratio was within 3.0%.

As shown in Fig. 1, the ratio is presented in an energy region from 250 keV to about 10 MeV as a semi-logarithmically increasing function of the effective energy of photon radiation. The curve in Fig. 1 was used as a calibration curve for evaluating the quasi-effective energy of natural radiation.

O . j 5 I 0 - 0 - 0 9 0 , , , , I,,, , , , , , , , , , ,

0 . 0 1 0.1 1 1 0

E f f e c t i v e energy ( M e V )

Fig. 1 TL sensitivity ratio of twin filter CaSO,(Tm) -TLD system as function of effective energy of photon beams

2. Quasi-effective Energy at Monitoring Points The quasi-effective energy of natural radiation was evaluated as follows : When TLPbr

TLI, TL(BGIpb and TL(BG), are presented as the mean T L intensities of six T L elements in each of the lead and lucite filters of both the monitor and the background monitor, respectively, the quasi-effective energy is evaluated from a ratio of (TLPb-TL(BG)Pb)/ (TL,-TL(BG),) using the calibration curve in Fig. 1. As..given in the equation of the above- mentioned ratio, the evaluated quasi-effective energy of natural radiation does not contain the hard component of the cosmic rays during the monitoring period.

Table 1 gives the location of monitoring points, evaluating conditions, and the means and standard deviations of the evaluated quasi-effective energy of data obtained from 4 to 11 measurements.

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Vol. 23, No. 1 (Jan. 1986) 47

Table 1 Mean and standard deviation of estimating quasi- effective energy (MeV) a t each monitoring point

Monitoring Number of Number of Quasi-effective energy (MeV) Prefecture point repeats TLDs Mean Standard deviation

Fukushima

__ Fukui

Niigata

Shimane

-~ .

Kagoshima

-~

Shizuoka

Oragahama Ohono-sho Yorunomori Kumagawa Jigan-Temp.

Urazoko Irogahama Tenoura Yoshiko Fukui Kohono Yamanaka Shimo Shiraki

Niigata Ohominato Nishiyama Kariwa Ohozumi

T a i Kataku Mitsu Ohowashi Nishi hamasada

Fukiba Kodaira Iorinodaira Kanda

~~-

__- -

-

~~ ~-

B w p u ~ ~~~~ __

Sakura Uenohara Hiraba

10 10 10 10 9

128 1.43 0.21 128 1.35 0.20 128 1.46 0.18 128 1.54 0.52

1.48 0.25 116 .~ -

6 11 11 11 9 8 7 8 4

5 4 5 5 5

4 4 4 4 4

6 6 6 6 6

6 6 6

.~

~~~

~

78 150 150 150 126 96 84 96 48

60 48 60 60 60

___~._

48 48 48 48 48

72 72 72 72 72

-

72 72 72

0.81 0.86 0.84 1.00 1.16 1.53 1.59 1.23 0.84

0.07 0.09 0.05 0.09 0.09 0.44 0.21 0.18 0.17

1.02 0.07 1.55 0.07 1.25 0.18 1.57 0.08 1.44 0.27

1.67 0.33 1.48 0.16 1.34 0.24 1.08 0.09 1.32 0.14

1.50 0.11 1.34 0.11 1.43 0.22 1.16 0.13 1.22 0.14

1.18 0.16 1.07 0.05 1.05 0.13

~~~ ~ .~

~-

. - __ - .. ~-

Figure 2(a)-(f) show the quasi-effective energy of natural radiation obtained at monitoring points in each of the prefectures, using the twin filter method.

In the present result, as shown in these figures and Table 1, the quasi-effective energy of natural radiation was approximately 800 keV or more at all of the monitoring points.

The quasi-effective energy of natural radiation was characterized by and varied with geological conditions at the monitoring points, as given in Table 1 and Figure 2(a)-(f). The quasi-effective energy of natural radiation at points located in a granite area, for example, one in Fukui Prefecture, is lower than that in other areas. According to refer- ences on geology"", acid rock, such as granite, contains 1 . 3 ~ l O - ~ g / g of T h , 3XlO-'g/g of U and O.O4g/g of K, while basic rock, such as basalt, contains 3 ~ l O - ~ g / g of Th, lo-' g/g of U and O.O12g/g of K. This variation in the content of natural radioactive ele- ments evidently affects the quasi-effective energy as well as the exposure rate of natural radiation.

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48

Ohano

• 1.31

Fukushima Prefec.

Kurnegewa

• 1.25

Yorunomori

0 1.46

Oragllhll'l'llt

1.43 °

5 km

10 km

...... , 0 1.48

(a) Fukushima Prefecture (Period Oct. 1979- july 1984)

lzumozaki 0

1.32

~' " '

Nishiyama\,

\ ..... \ .... w. ) 0 ' ) ·~:::;

/1

(c) Niigata Prefecture (Period Dec. 1982- Oct. 1984)

(e) Shizuoka Prefecture

c .. .. u 0

(Period Mar. 1983- Dec. 1984)

]. Nucl. Sci. Techno/.,

(b) Fukui Prefecture (Period Oct. 1981- Oct. 1984)

Shimane

Nillhl\amaaada

(d) Shimane Prefecture (Period Jan. 1983- Oct. 1984)

Kagoshlma Prefec.

\ \

(f) Kagoshima Prefecture

\

(Period Apr. 1983- Oct. 1984) The house symbol represents the nuclear plants.

Fig. 2(a)-(f) Mean quasi-effective energy (MeV) of natural radiation obtained at monitoring points in six prefectures

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Vol. 23, No. 1 (Jan. 1986) 49

This shows that when the dose due to cosmic rays is assumed to be a constant, in- creasing the content of natural radioactive elements contributes to a decrease in the quasi- effective energy of natural radiation, and that the quasi-effective energy of natural radia- tion in high dose areas is lower than that in low dose areas.

3. Distribution of Quasi-effective Energy Figure 3 shows the distribution of quasi-effective energies of natural radiation at 43

monitoring points. The experimental result shows that about 85% of the total number of the observed

monitoring points are in an energy region of 1-1.5 MeV. The evaluated quasi-effective energy of natural radiation was larger than the mean energy of terrestrial radiation, which is about 500 keV or The difference between the two may be eva- luated as follows:

In general, the mean energy of cosmic rays is much larger than that of the photon energy from terrestrial radiation, and their dose is about 20 to 50% of total natural radiation, when the dose from cosmic rays is assumed to be about 3.3pRlh. In the case of the twin filter TLD system, the TLD elements are exposed to natural radia- tion, including cosmic rays. The quasi- effective energy of natural radiation ob- served with the twin filter TLD system is, therefore, larger than the mean energy of terrestrial radiation.

1 .o 1.5 2 . 0 2 . 5 Quasi-ef f e c t i v e e n e r g y ( M e V )

Fig. 3 Numerical distribution of quasi-effec- tive energies of natural radiation a t surveyed monitoring points

When the biological effect of natural radiation is discussed, the dose from natural radiation reasonably includes the dose from cosmic rays. In the case of notation of the energy of natural radiation, the quasi-effective energy, as well as the mean energy of natural radioactive elements, is useful.

The response of certain integrating dosimeters is calibrated using a 226Ra or 137Cs source. However, the distribution of the quasi-effective energy reveals that signal output -such as TL intensity, luminescence intensity and ionization-from the environmental radiation monitor is more useful when calibrated using a source,

4. Seasonal Variation of Energy The quasi-effective energy of natural radiation varies with changing meteorologicaI

conditions, especially the amount of rainfall and snowfall. Thus, the effects of meteor- ological conditions on quasi-effective energy will be discussed.

Figure 4 shows the seasonal variation of the quasi-effective energy of natural radia- tion at one of the monitoring points, a t which there is no snow during the winter season ; that of background radiation from the hard component of cosmic rays in the shielded lead box during storage; natural radiation during the mailing, and the self-dose of the back- ground monitor.

If there is no snow in the winter, the quasi-effective energy is not affected by season. As shown in Fig. 4, minimal seasonal variation of quasi-effective energy of natural radia- tion was observed at these monitoring points. However, the quasi-effective energy from

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50 J. Nucl. Sci. Technol.,

background radiation and other sources varied, because of the variation of the period between annealing and measurement of T L intensity. This shows that the quasi-effec- tive energy of natural radiation obtained with the twin filter method gives one value a t the monitoring point, and that the sea- sonal variation of that value is small.

In winter 1983, there was heavy snow- fall at some of the monitoring points in Niigata Prefecture. In this case, the quasi- effective energy of natural radiation would be expected to increase, because of snow's shielding effect on the terrestrial component of natural radiation. Therefore, a large amount of snowfall should lead to an in- crease in the quasi-effective energy.

Figure 5 shows a comparison of the quasi-effective energy of natural radiation between the snow season and other seasons at some of the monitoring points in Niigata Prefecture. The solid and open circles in Fig. 5 represent the mean quasi-effective energy of natural radiation for winter 1983, and other seasons, respectively. The num- bers in Fig. 5 give the average amount of snowfall a t the monitoring points"').

As shown in Fig. 5, the quasi-effective energy during the snow season was larger than that in other seasons. This shows that snowfall affects quasi-effective energy more strongly than rainfall, and that, dur- ing the snow season, exposure decreases while quasi-effective energy increases.

5. Effect on Various Types of Construction Materials

Contents of natural radionuclides vary with the type of material, including con- struction materials. Therefore, the quasi- effective energy, as well as the exposure rate of natural radiation, will change ac- cording to the type of material used. Thus, a comparison of the quasi-effective energies of natural radiation of a wooden house, a concrete building, and a field was carried out using the twin filter TLD system.

5

4

3

2

1

0 . 5

0.4

0.3

0 . 2

a .

0 0

a .

6

O 0 0

A

.

0 0

a

a

0 0

Monitoring period

Fig. 4 Typical seasonal variation of quasi- effective energy of natural radiation (curve A) and background radiation a t one of monitoring points (curve B)

Niigata Prefec.

+ + t

0

- Niigata Izumozaki Nishiyama Kariwa ( 1 . 1 3 m) (0.69 m)

Location

Numbers in parentheses are average amounts of snowfall. Error bars represent standard devia- tions divided by the square root of sample sizes.

Fig. 5 Effect of snowfall on quasi-effective energy of natural radiation a t some monitoring points in Niigata Prefecture

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Vol. 23, No. 1 (Jan. 1986) 51

Table 2 gives the quasi-effective energy of natural radiation, excluding the hard component of cosmic rays, inside a wooden house, a concrete building and at a field in Chiba City. In Table 2, the numbers in parentheses are the quasi-eff ective energies of natural radiation including the hard component of cosmic rays. In this case, the twin filter TLD systems were sub- sequently set up, after annealing, at moni- toring points a t author’s Institute in Chiba City.

As shown in Table 2, the quasi-effective energy inside the concrete building was less than that found in the wooden house and at the field. However, in Chiba City, quasi-effective energy in the wooden house was nearly the same as that found at the

Table 2 Mean and standard deviation of quasi- effective energy (MeV) of natural radiation excluding or including hard component of cosmic rays in National Institute of Radiological Sciences (NIRS)

(Points A, B and C are the monitoring point in a field.)

Quasi-effective Location Number Number energy (MeV)

in NIRS repeats T L D ~ Standard Mean deviation

of of used ~~~ -___

Concrete 8 92 0.87 0.11 bldg. (I . 52) (0.14)

Wooden 8 92 1.58 0.45 house (2.28) (0.32)

Point A 6 72 1.35 0.50 (2.10) (0.47)

Point B 7 84 1.45 0.44 (2.42) (0.37)

Point C 3 36 1.26 0.28 (2.20) (0.31)

field. This result shows that the difference among these measurements is dependent on the content of radionuclides in the construction materials. Namely, the content of U, Th and K elements in concrete is larger than that in wood. Quasi-effective energy at the field was more affected by cosmic rays than that inside the concrete building.

The quasi-effective energy of natural radiation including the hard component of cosmic rays was larger than that excluding it. This shows that if the mailing method for such observations is used, the quasi-effective energy of natural radiation will be smaller than the actual energy. Therefore, any estimation of the actual quasi-effective energy of natural radiation must consider both the hard and soft components of cosmic rays.

IV. CONCLUSION Natural radiation consists of many types of radiation, including photon beams and

charged particles. An evaluation of the quasi-effective energy, as well as energy spectra, of each type of radiation is also useful in discussion of the biological effect of natural radiation Qn living organisms.

A large part of the quasi-effective energy of natural radiation was in the energy range of 1-1.5 MeV. Thus, the signal from the dosimeter must be calibrated using a G°Co y-source.

In this work, the contribution of the hard component of cosmic rays has been ignored in evaluating the quasi-effective energy of natural radiation at the monitoring points, except in those cases where the mailing method for observation of quasi-effective energy was used. However, as shown in Table 2, the actual quasi-effective energy of natural radia- tion will be larger than the present result.

The measurement of quasi-effective energy of natural radiation or environmental radia- tion is significant in the following respects :

(1) A more exact dose of natural radiation and accidental radiation can be evaluated, because conversions to the dose or dose equivalent are varied with the energy of radiation. However, the ordinary TLD monitor can not estimate the energy of

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52 J . Nucl. Sci. Technol.,

radiation. (2) Estimation of released nuclides, such as gaseous radioisotopes, in a nuclear power

plant accident may be possible. (3) Knowledge of the form of exposure will contribute to more precise medical’ treat-

ment of patients in an accident or emergency. For example, if the exhausted nuclide is primarily 133Xe or l 3 I I , medical countermeasures for exposure from nuclides will be concentrated on the whole body or thyroid gland, respectively. Furthermore, the periodic variation in such countermeasures for radiation exposure can be estimated from the variation of effective energy and dose from the radiation sources.

ACKNOWLEDGMENT The author wishes to express his thanks to the staff members of the following pre-

fectural Institutes of Public Health and other agencies in Fukushima, Fukui, Shimane, Niigata, Shizuoka, Kagoshima, Ehime and Saga Perfectures in Japan, for their support and assistance related to the present work. The author also wishes to express his thanks to Dr. T. Kumatori, Director General of his Institute, Dr. T. Terashima, Deputy Director General of his Institute, and Dr. E. Tanaka, Director of his Division, for their encourage- ment on the present work. The author wishes thank Mr. M. Okazaki for his assistance with the radiography of the lead filters; Mr. K. Nemoto for his assistance with the ob- servation using whole body counters and semiconductor spectrometers ; Dr. T. Koshijima, Section Chief of the Training School, and Dr. K. Fujitaka, Senior Investigator, for their discussion and encouragement on this work.

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