The pathway of iodine-129 mainly discharged into the atmosphere from nuclear fuel reprocessing plants can be copied by that of stable iodine because of its long half-life (1.6×107y). Among the routes of entry of iodine into the human body, the largest contribution seems to be made by the ingestion of rice in Japan. In view that the determination of 129I in rice is still not successful, the transfer of stable iodine from paddy field to rice grain has been studied by comparing the concentration in rice grain with that in soil on which the rice was grown. Polished rice and soil samples were collected in 1979 and 1980 from 14 Agricultural Experiment Stations throoghout this country. The methods of analysis are a radiochemical neutron activation technique developed by the authers. The results obtained are as follows:
1) Fluctuations of the iodine concentrations between 1979 and 1980 are very small both in rice and soil.
2) Except Iwate and Ibaraki Prefectures where the soil concentrations exceed some 10mg/kg, most soil contained around 1mg/kg (0.41-2.33 in 1979 and 0.61-2.46 in 1980) and general trend seems higher levels in Kanto and northern prefectures than in western ones.
3) Rice level varied between 1.6-4.0μg/kg in 1979 and 2.5-5.0μg/kg in 1980. Coefficient of variation of rice level throughout this country is only 25% in both years.
4) The average concentration ratio iodine in polished rice/total iodine in soil was calculated as 2×10-3, on the other hand, the ratio iodine in polished rice/water soluble iodine in soil was calculated as 2×10-1. It was suggested that the latter ratio was better to assess the iodine level in rice on the basis of soil levels.
