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VIII Environmental Radioactivity: Development of Radioactivity Measurement Methods and Activity of Radionuclides in the Environment Monitored After the Accidents at TEPCO's Nuclear Power Stations

Environmental Transfer of Carbon-14 in Japanese Paddy Fields

Abstract It has been recognized that carbon-14 (14C) is one of the dominant radionuclides affecting dose from transuranic (TRU) wastes. This radionuclide has a decay half-life of 5,730 years, and 14C organic materials have very low sorption properties to clay and rock in the environment, which raises some concerns about the releases of 14C to the biosphere from radioactive waste repositories. For the safety assessment of TRU waste disposal, we studied the behavior of 14C in rice paddy field soils. We also determined key parameters such as soil–soil solution distribution coefficients (Kds) and soil-to-rice plant transfer factors (TFs) of 14C in the field soils. The TFs were obtained in laboratory and field experiments. In our laboratory experiments, we used [1,2-14C] sodium acetate as a source of 14C because it has been suggested that low molecular weight organic-14C compounds are released from metallic TRU wastes. The results showed that 14C-bearing sodium acetate in irrigated paddy soils was rapidly decomposed by indigenous bacteria. Although some of the 14C was assimilated into the bacterial cells, most of the 14C was released into the air as gaseous compounds. The main chemical species of 14C gases was 14CO2, and a part of the released 14CO2 gas was used by rice plants during photosynthesis. Only a negligible amount of 14C was absorbed through the roots. Therefore, the contamination of rice plants is mainly caused by gasification of 14C, and microorganisms are responsible for driving this process. The activity of microorganisms is a key issue in the behavior of 14C in paddy fields.

Keywords Bacteria • Behavior • Degradation • Radiocarbon • Rice paddy fields

• Safety assessment • TRU wastes

Introduction

Transuranic (TRU) wastes contain a variety of radionuclides, for example, Np, Pu, and long-lived radionuclides such as 14C and 129I. In Japan these wastes are categorized into four groups in accordance with their physical properties and the concentration of radioactive materials. Group two waste includes hull and end piece wastes with relatively high amounts of 14C, and leaching of low molecular weight

14C organic materials from simulated hull wastes has been reported [1]. The 14C organic materials have very few sorption properties to clay and rock, and 14C has a relatively long half-life of 5,730 years. These properties raise concerns about releases of 14C to the biosphere from radioactive waste repositories.

Rice is a major agricultural crop throughout Asia, and thus human exposure to 14C through rice intake must be considered. To reduce the risk of the internal radiation dose from 14C, it is important to clarify the behavior of 14C in rice paddy fields. In this study, we determined transfer pathways of 14C through the rice paddy fields to rice grains. Environmental parameters such as soil–soil solution distribution coefficients (Kds) and soil-to-rice plant transfer factors (TFs) of 14C were also determined, because these parameters are often used in transfer models to predict the behavior of radionuclides in the environment. From a series of our experimental results, we describe the behavior of 14C in rice paddy field soils and the importance of microbial activity.

 
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