radiocesium concentration in milk after the chernobyl accident in japan

J.RADIOANAL.NUCL.CHEM~ 145 /2[ 151-157 [1990/ J

RADIOCESIUM CONCENTRATION IN MILK AFTER THE CHERNOBYL ACCIDENT IN JAPAN

T. Imanaka, H. Koide

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

Received 19 March 1990 Accepted 27 March 1990

Radiocesium concentrations in cow's milk from two producing districts in Japan were measured monthly for three years following the Cherno- bvl accident. The Chernobyl contribution in ~Cs Concentration was evaluated f~om the lJaCs concentration and the 137Cs/l~4Cs ratio. The highest 137Cs concentration of 0.6 Bq 1 -I was observed in May 1986 and the Chernobyl contribution has decreased during three years to levels corresponding to the contribution from past nuclear weapons fallout. Annual values of child internal dose through milk consumption were estimated at 0.6, 0.3 and O.i uSv for the first, the second and the third year following the accident, respectively.

INTRODUCTION

The Chernobyl accident in April 1986 caused widespread

radioactivity contamination throughout the northern hemi-

sphere. Radioactive plumes arrived in Japan on 3 May,

twenty kinds of y-emitting nuclidebeing identified in 1

air . Among the nuclides released from the Chernobyl re-

15 | E~evier Sequom S. A., L~sanne A k ~ m m i Ki~6, B ~ e s t

IMANAKA, KOIDE: RADIOCESIUM IN MILK AFTER CHERNOBYL ACCIDENT IN JAPAN

actor, two cesium isotopes, 137Cs and !34Cs are most im-

portant to evaluate the long-term radiological effect of

the accident 2'3. The average value of the 137Cs deposition

in Japan is estimated to be 180 Bq.m -2 /Ref. 3/. Although

the contamination level in Japan were not as serious as

those in European countries, radioactivity contamination

has also become a matter of public concern.

~e commenced the monthly measurement of radioactivity in

cow's milk from two producing districts in Japan just af-

ter the accident. This paper presents the results of our

measurement for three years following the accident.

MATERIALS AND METHODS

Samples of commercial milk were collected monthly throus

dairy co-operatives in two milk producing districts: Toka-

chi, plain fields in the Hokkaido island and Hiba, a moun-

tainous district in Hiroshima prefecture in the Honsyu

island /Fig. i/. Tokachi is a district where large-scale

dairy farming is extensively carried on. In Hiba, however,

dairy farming with large meadowland is not available and

hence imported crops are fed to dairy cows to cover the

shortage of local fodder.

Milk samples before December 198~ were measured in a

l-litre Marinelli beaker without any pretreatment. There-

after milk samples of one or two litre were dried under

infrared lamps and reduced to ashes in an electric fur-

nace at about 450 ~ to concentrate nuclides before the

activity measurement.

Gamma-ray spectra from liquid milk or milk ashes were

obtained with an intrinsic Ge detector /relative efficien-

cy 19%, resolution 1.8 keV at !.33 MeV/. The detector was

connected to a 4k channel analyzer and shielded from room

152


_...... &&~

1.5.

~onsyu Is.

pacific ,OceOr~

~ o

132 ~ 136 ~

Fig. i. Location of milk producing districts; Tokachi and Hiba

background with iO cm of lead. Efficiency calibration for

each geometry was carried out with a CEA standard solution

of mixed nuclides. Counting times ranged from about one

day for initial samples to more than four days for later

samples. Detection limits of radiocesium activity are

about 0.04 and O.O1 Bq.l -I for liquid milk and milk ashes,

respectively.

RESULTS AND DISCUSSION

Radioactivity in milk

The radiocesium concentration in milk obtained from

May 1986 through April 1989 is shown in Fig. 2, all values

being decay-corrected to the date of milk production.

153


1 ! , j .

I 7 0 i I I I I I I I I 1 1 I I I I I 1 1

Fig. 2. Concentration trends of ra~iocesium in cow's milk from May 1986 to April 1989: 137Cs in Tokachi / ~. /, 134Cs in Tokachi / O/, 137Cs in Hiba / m / and 134Cs in Hiba / ~ /. Bars indicate the standard er- ror

At the time of the fallout arrival, the beginning of

May 1986, it was already in season of fresh green in Hiba

but was not yet in Tokachi. Thus, at the early stage of

the contamination, cows in Hiba had more chance of grazing

contaminated pasture than those in Tokachi, which led to

higher values of radiocesium concentration in the former

district. The highest 137Cs concentration in Hiba milk,

0.6 Bq.l -I, was observed in May 1986, while in Tokachi

milk the highest concentration, 0.4 Bq.l -I, was in July

1986. We also detected 131I activiEy in initial samples,

but the concentration decreased below the detectable limit

withintwo months; maximum concentrations were 2.6 and

0.9 Bq.l -I in Hiba and Tokachi, respectively.

After the first half-year, however, radiocesium con-

centration in Tokachi milk was consistently higher than

154


in Hiba milk /except in April 1987 when a significant

increase was observed in Hiba/. This trend may be explained

with the difference in the feed contents; the major part

/about 50 wt%/ of feed is a blend of imported crops /most-

ly from the United States/ in Hiba, while in Tokachi suf-

ficient pasture areas are available locally.

Time-integrated concentration

Since the 137Cs avtivity includes the contribution

from past nuclear weapons tests, it is necessary to know

the Chernobyl contribution in 137Cs concentration for

the evaluation of the effect of the accident. The activi-

ty ratio of 137Cs to 134Cs fromChernobyl was 2.0 at the

time of the fallout arrival in Japan I'4. Therefore, the

Chernobyl contribution in 137Cs concentration was obtained

by multiplying the 134Cs concentration by the 137Cs/134Cs ratio, wfth the decay correction of the difference in

their half-lives.

Table 1 summarizes values of time-integrated concentra-

tion observed during each year following the accident; va-

lues in Hiba for the third year /May 1988 to April 1989/

were obtained by extrapolating the preceding values since

134Cs activity in Hiba milk decreased below the detectable

limit of the measurement by the summer of 1988. As shown

in Table i, the Chernobyl contribution in 137Cs concentra-

tion decreased during three years to levels corresponding

to the contribution from past nuclear weapons fallout.

Internal dos E from milk consumption

The estimation of internal dose due to Chernobyl-de-

rived 137Cs and 134Cs was made for a childrwho had been

consuming Tokachi milk after the accident. A milk consumption

rate of 0.5 1 d -I and dose conversion factors of 0.0086

uSv. Bq -I for 137Cs and 0.O12 uSv. Bq -I for 134Cs were as-

155

IMANAIC&, KOIDE: RADIOCESIUM IN MILK AFTER CHERNOBYL ACCIDENT 1N JAPAN

TABLE 1

Time-integrated radiocesium concentrations in milk during each year following the accident; contribution from

Chernobyl and past weapons tests. /Bq.y-l-i/

Chernobyl

District and period 134Cs 137Cs

Weapons tests

137Cs

Tokachi:

May 1986-April 1987

May 1987-April 1988

May 1988-April 1989

Hiba:

May 1986-April 1987

May 1987-April 1988

May 1988-April 1989

O.10+0.O3 0.24+0.07 0.06+0.05

0.04+0.02 0.12+O.06 0.07+0.02

0.02+0.O1 0.07+0.02 0.07+0.03

O.13+0.10 0.28+0.20

0.02+0.O1 0.05+0.02

O.005 x 0.02 ~

0.04+0.03

0.02+0.02

0.02 ~

XEstimated by extrapolating s preceding values.

sumed in accordance with the recommendation by Japan Nuc-

lear Safety Committee 5. Thus annual values of effective

dose equivalent were estimated to be 0.6, 0.3 and 0.i

uSv for the first, the second and the third year following

the accident, respectively.

x

Thanks are due to staffs of Kansai-Yotsuba Co-operative

for providing us with milk samples and to Mr. K. Ii, the

chief director of the dairy farming co-operative in Hiba,

with kindly information on their dairy farming.

156


REFERENCES

i. T. Imanaka, H. Koide, J. Environ. Radioact., 4 /1986/ 149.

2. The Radiobiological Impact of the Chernobyl Accident in OECD Countries, OECD/NEA, Paris, 1987.

3. Sources, Effects and Risks of Ionizing Radiation, UNSCEAR, New York, 1988.

4. M. Aoyama, K. Hirose, Y. Suzuki, H. Inoue, Y. Sugimura, Nature, 321 /1986/ 819.

5. Japan Nuclear Safety Committee, Gensiryoku Anzen Iinkai Geppou 126, 1989.

157

radiocesium concentration in milk after the chernobyl accident in japan

Documents