Behaviour of Iodine-129 in Rice Paddy Fields
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BEHAVIOUR OF IODINE-129 IN RICE PADDY FIELDS SHIGEO UCHIDA* AND YASUYUKI MURAMATSU* *Division of Radioecology, National Institute of Radiological Sciences, 3609 Isozaki, Nakaminato-shi, Ibaraki, 311-12 Japan. ABSTRACT Transfer factors of iodine from soil to rice were obtained by laboratory experiments using 1251 tracer. Two typical soil types in Japan, Andosol and Gray lowland soil, were used. The transfer factor (TF) is defined as 'concentration of the nuclide in a plant organ at harvest' divided by 'concentration of the nuclide in dry soil'. The TFs for brown (hulled) rice were 0.006 for Andosol and 0.002 for Gray lowland soil. The TFs for different organs of rice plants decreased in the order of blade > stem > rachis > unhulled rice >> brown rice. The concentration of iodine in soil solution under flooded conditions varied with time during cultivation. The iodine concentration in rice plants seemed to be influenced by the soil solution. The effect of removal of 1-129 from paddy fields by harvesting rice plants was also modelled. Even assuming continuous deposition of 1-129 onto the field, annual harvesting of the blades and stems of rice plants could effectively reduce the amount of the nuclide in the root zone.
INTRODUCTION Although the fission yield of 129I is low, this isotope has accumulated in the environment due to its long radiological half-life (1.6 x 107 y). It is known that iodine deposited on farm land is retained within the surface layer and that very little elutes into the soil solution. By contrast, under submerged conditions, this element is soluble in the soil solution and possibly taken up by vegetation through the roots [1]. For rice plants cultivated under flooded conditions, the iodine uptake is expected to be higher than that of the crops grown on upland fields. In Japan and other Asian countries, rice plants are grown through the countries and the consumption of rice is very high. Therefore, the transfer of radioiodine from paddy soil to rice is thought to be one of the most important pathways for assessing radiation doses due to the 129I released into the environment in these countries. For estimation of the amounts of 129I in rice, the soil-to-plant transfer (TF) factor of iodine is the most important parameter. The purpose of our present study is to obtain transfer factors through laboratory experiments. The TF is defined as 'concentration of the nuclide in a plant organ at harvest' divided by 'concentration of the nuclide in dry soil'. Removal of 1-129 from paddy fields by harvesting of the rice plants is also estimated by a mathematical model with the transfer factors obtained in this study. In some cases, elements taken up by roots are accumulated more in inedible parts than in edible parts. Attention, however, has scarcely been paid to transfer to the inedible parts. For example, the leaves and stem, which are inedible parts of cereals or fruit vegetables, could play an important role in lowering the radionuclide amounts in the root zone of soil by harvesting these crops. In this pa
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