J.RADIOANAL NUCL.CHEM.,LETTERS 154 (2) 111-119 (1991)

RADIOACTIVITY IN THE HIGHLY CONTAMINATED AREA NEAR THE CHERNOBYL SITE

T. Imanaka, T. Seo, H. Koide

Research Reactor institute, Kyoto University, Kumatori-cho, Sennan-gun, Osaka, 590-04 Japan

Received 13 March 1991 Accepted 27 March 1991

Field measurements of radioactivity were performed in highly contaminated areas around Chernobyl in the summer of 1990. Six radionuclides including the most dominant 137Cs have been identified in soil samples through y-ray spectrometry. The relation be- tween the y-ray dose rate above the ground and the radioactivity density in soils has been investigated. The external dose from deposited radiocesium for the period of 70 years after the deposition has been evalu- ated to be about 5 mSv per I and 0.5 Ci km -2 of 137Cs and 134Cs deposition, respectively.

INTRODUCTION

Since the USSR report I was presented to the IAEA

meeting held in August 1986, we have had little informa-

tion about the highly contaminated area around Cherno-

hyl. Recently it has been revealed that the severe

radioactivity contamination extends over regions even

300 km away from the Chernobyl site 2'3

In the summer of 1990 we visited Minsk and discussed

the radioactivity contamination with Byelorussian scien-

I l l Elsevier Sequoia S. A., Lausanne A kadgmiai Kiad6, Budapest

IMANAKA el al.: RADIOACTIVITY IN THE AREA NEAR THE ('HERNOBYI

Y N " " 9 N ~ cOW

Fig. I. Location of field measurement (see text). Hatch- ing indicates 137Cs contamination over 5 Ci km -2 (Refs 4, 5)

tists. We also performed field measurements in contami-

nated areas and brought soil samples back to our labo-

ratory. In this paper we present the results of our

measurements.

EXPERIMENTAL

Locations of the field measurements are shown in Fig.

I. Brief descriptions of the locations are given below:

No. I. In a forest in the suburbs of Minsk (350 km

North-West from the Chernobyl site) ;

No~ 2. In a vegetable garden of a farmer's house in

SlavgoL'od (250 km NNE) ;

No. 3. In a fruit garden in Zapaiyani village (250

km NNE )

112

No. 4.

No. 5.

No. 6.

No. 7.

No. 8.

No. 9.

IMANAKA et al.; RADIOACTIVITY IN THE AREA NEAR THE CHERNOBYL

In a farmer's house garden in Klikovka vil-

lage (250 km NNE) ;

In a bush near a check gate to the Chernobyl

site (30 km S) ;

In abush beside a road from Kiev to Chernobyl

(50 km S);

Beside a farmer's house in Fenevitch village

(80 km S);

In a riverside forest of the Dnepr in Kiev

(110 km S);

In a grass area around a hotel in Moscow

(700 km NE).

The inhabitants of Zapalyani had already evacuated in

the spring of 1990 and those of Klikovka were to evacu-

ate within the year. Fenevitch was located within the

region under radioactivity control but farming work

seemed to be normally under way.

The measurement of y-rays was performed with a port-

able spectrometer (HAM~/~ATSU SS-y/C3477, I"~ X 2"

NaI(TI) with 128 ch MCA). Gamma-ray spectra were ac-

quired for three minutes at I m above the flat ground.

A built-in microcomputer calculates the dose equivalent

rate from a 128 ch spectrum using dose conversion coef-

ficients. This detector system was calibrated at the y-

ray field of the Institute of Radiation Measurements in

Japan.

Soil samples were taken at six points (No. I., 3.,

4., 5., 6. and 7.). The soil cores taken with cylindri-

cal samplers (6 cm dia. and 10 cm depth) were dismantled

into five pieces of 2 cm soil layer. Gamma-ray spectrom-

etry was performed with a Ge detector (relative effici-

ency of 19% and energy resolution of 1.8 keV at 1.33

3 113

IMANAKA et al.: RADIOACTIVITY IN THE AREA NEAR THE CHERNOBYL

TABLE I

Dose equivalent rate at 1 m above ground

No Date Dose equivalent rate, pSv h -I

I 25 Aug

2 28 Aug

3 28 Aug

4 28 Aug

5 I Sep

6 I Sep

7 I Sep

8 3 Sep

9 6 Sep

0 O8

0 28

2 44

2 86

0 39

0 23

0.09

0 . 1 3

0 . 0 9

MeV) at our laboratory in Japan. The detection effici-

ency of the Ge detector was calibrated using a CEA

mixed standard solution.

RESULTS

The results of y-ray dose measurements are shown in

Table 1. At points No. 2. to No. 6. the dose rates ap-

parently exceed the level of the natural background.

But they do not show a simple dependence on the distance

from the site, reflecting the complex nature of the ra-

dioactivity deposition. According to a Byelorussian

scientist who accompanied us in the severe contaminated

area (Nos 3. and 4.), the dose rate reached 800-1500

mR h -I there just after the accident.

Examples of y-ray spectra with the NaI(TI) spectrom-

eter are shown in Fig. 2. The peak of 662 keV y-ray

from 137Cs is clear in these spectra and the peak of

114

IMANAKA el al.: RADIOACTWITY IN THE AREA NEAR THE CHERNOBYL

~ 104 v/h

i r"

- ~ ' ' li nl 104- No. 6:0.23 ,uSv/h

InF hnr nn ~ 102 o U 1 [I

104-- No. 8:0.13 ,uSv/h

1 Ilfl 0 200 400 600 800 1000 1200 1400 1600

Energy;keV

Fig. 2. Gamma-ray spectra taken with an NaI(TI) detector at I m above ground. Measurement time 3 min

TABLE 2

Radioactivity density in 0-10 cm soil (Ci km -2 )

No 137Cs 134Cs I06R u 125Sb 144Ce 60Co

I 0.097 0.004 ND ND ND ND

3 38.6 5.03 0.32 0.13 ND ND

4 46.5 6.09 0.70 0.42 ND ND

5 3.83 0.45 0.61 0.09 0.98 0.005

6 2.32 0.28 0.68 0.18 0.98 0.004

7 0.83 0.09 0.08 0.02 0.26 0.002

ND; not detected. All values are decay-corrected to the date of sampling.

796 keV y-ray from 134Cs is also found in Nos 4. and 6.

In the spectra measured in Minsk and Moscow (No. I. and

9.) , the peak of 137Cs was not distinguished.

Table 2 summarizes the results of the measurement of

the soil samples with the Ge detector. In addition to

~* 115

IMANAKA et al.: RADIOACTIVITY IN THE AREA NEAR THE CHERNOBYI_

I 2 Activity density in each 2-cm ,ayer~Ctlkm

O] 013 5 ~ ~2 . . . . . S 10 -~ B 1 5 ~ 0 ~ ~ ~ ' I ~ ' I l

ONo. 1 �9 No. 3 %

2 aNo. 4 / ~ /

E �9 No. 5 u

4_a No. 6 , / No. 7

~ 6

E s

.12

K c~ 10

F i g . 3 . D e p t h d i s t r i b u t i o n o f 137Cs i n s o i l . A l l v a l u e s a r e d e c a y - c o r r e c t e d t o t h e d a t e o f s a m p l i n g

137Cs and 134Cs, 106Ru, 125Sb, 114Ce and 60Co were de-

tected in the soil samples. According to 137Cs contam-

ination maps 4'5 written by Russian scientists, our -2

sampling points correspond to areas over 40 Ci km for

Nos 3. and 4., between I and 5 Ci km -2 for Nos 5., 6.

and 7., and within I Ci km -2 for No. I. Our values in

Table 2 are consistent with these maps.

The depth distribution of 137Cs is shown in Fig. 3.

Except for the point No. 7., the 137Cs density de-

creases with the depth from the surface and 70-95% of

the total 137Cs was retained within the upper two

layers of 0-4 cm. A strange pattern found in No. 7. is

considered to be due to the turnover treatment of the

surface soil that was commonly adopted around contam-

inated areas in order to reduce the effect of the ra-

dioactivity contamination. As for other nuclides, their

depth distribution is found to be very similar to that

of 137Cs, as seen in the example of No. 6. shown in

Fig. 4~

116

1MANAKA el a].: RADIOACTIVITY IN THE AREA NEAR THE CHERNOBYL

Act iv i t y dens i ty in each 2 -cm t a y e r ; C i / k m 2

(~0 -4 s ~0 -3 s 10 -2 5 ~0 -1 5 ~ s 10 | i 1 ~ I ' I ' l '

I // / / o1~Cs b F , / / / / a l~'Ce

r' o- , 60co

I0 ~

Fig. 4. Depth distribution of radioactivities at site No. 6. All values are decay-corrected to the date of sampling

DISCUSSION

It is important to know the relation between the ra-

dioactivity density and the y-ray dose above the ground

in order to evaluate the long term effect of the radio-

activity contamination. The dose rate from deposited

radiocesiums is proportional to their initial deposi-

tion density and can be evaluated by a function of the

elapsed time after the deposition. We used the following

equation 6 in which the effect of radioactivity migra-

tion into the ground is incorporated into two exponen-

tial terms:

R (t) =D.K (flexp (-i I "t) +f2exp (-12 .t) ) exp (-l.t)

where R(t) is the dose rate at time t after the deposi-

tion, D the initial deposition density, K the dose rate

conversion factor for surface deposition, fl and f2 the

I17

IMANAKA et al.: R A D I O A C T W I T Y IN THE A R E A N E A R THE C H E R N O B Y L

~s , ...... ID > �9

.~ ~ ," �9 o_ 3 - /,

1 - llJ ! i 21

i i I , , 0 10 20 30 40 50 60

In i t ia l 137Cs deposi t ion densi ty ~ Ci, krn 2

Fig. 5. Dose rate above ground as a function of initial 137Cs deposition density. (Solid line - theoret- ical values with migration into the gound; dashed line - theoretical values without migration; dots - measured values.) Initial ~34Cs depcsition was assumed to be one half of 137Cs in the theoretical value and the natural background of 0.I gSv h -I was subtracted from the measured value

fractions of fast and slow migration components with

migration constants of 11 and 12, respectively, and 1

is the radioactivity decay constant.

The dose rates calculated with the theoretical values

of K = 0.111 and 0.291 (~Sv h-1)/(Ci km -2) for 137Cs

and 134Cs, respectively, for infinite plain sources, and

with fl = 0.63, f2 = 0.37, 11 = 1.13 y I and 12 =0.0075 -I

y (Ref. 6) are compared with the measured values in

Fig. 5. Here we assumed the deposition density of 134Cs 137

to Cs to be 0.5. As is seen in the figure the migra-

tion model reproduces the measured values fairly well.

The cumulative dose above the ground was calculated by

integrating the dose rate R(t) for the period after the

deposition and is shown in Fig. 6. The external dose to

human bodies is evaluated by multiplying the above cu-

mulative dose by a shielding factor of buildings. If

IL8

IMANAKA el al.: RADIOACTIVITY IN THE AREA NEAR THE CHERNOBYL

3 E

o ~o- d

I I I I0 20 30 40 50 60 70

Time ,ufter deposilion,yeor

F i g . 6. C u m u l a t i v e dose f o r t h e p e r i o d a f t e r t h e d e p o s i - t i o n of I and 0.5 Ci km -2 of 137Cs and 134Cs, respectively

the effective shielding factor of 0.4 (Ref. 7) is as-

sumed, the external dose for the pexiod of 70 years is

evaluated to be 0.0051Sv per I Ci km -2 of initial

137Cs deposition.

Further work to evaluate the contribution of such

short-lived radioiodines as were dominant at earlier

times is now in progress.

REFERENCES

I. USSR State Committee on the Utilization of Atomic Power, The Accident at the Chernobyl Nuclear Plant and Its Consequences, Report to IAEA, Vienna, 1986.

2. Sovietskaya Byelorussia, 9 Feb. 1989.

3. Pravda, 20 March 1989.

4. Sovietskaya Byelorussia, 7 March 1990.

5. Molodi Ukraini, 24 April 1990.

6. USNRC, Reactor Safety Study, App. VI, WASH-1400, 1975.

7. UNSCEARr Ionizing Radiation: Sources and Biological Effects, UN Publ., 1982.

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