The Distribution Coefficients and Gasification Ratios of [1,2-C] Sodium Acetate for Various Paddy Soils in Japan

For appropriate safety assessment of the disposal of TRU waste, distribution coefficients (Kd) and gasification ratios of C labeled [1, 2-C] sodium acetate (C-NaOAc) were determed by batch sorption tests for 85 Japanese paddy soil samples. The soil studied were from four soil types: Andsol; Gley; Gray lowland; and Yellow. The range of Kd values for all soil samples was from 7.5 to 295.2 mL g , and the mean value was 105.6 mL g . This mean value was higher than that of previous study (1). The high Kd values of the present study could be a result of the properties of the paddy soils. The comparison of Kd values by each soil type revealed statistically significant difference between Andsol and Gray lowland soils (P < 0.05). The soil type was one of the factors affecting partitioning of C-NaOAc. Gasification ratios ranged from 29.1% to 83.3%, and its mean value was 66.4% of the total C-NaOAc added. These results suggest that most of the radiocarbon in C-NaOAc will be released from soil into the air as gases. The gasification ratio between soil types was also compared, but no statistically significant differerence was found. Gas production may be controlled by other than physicochemical properties of soil, for expample by factors such as bacterial community. In addition, both the Kd values and the gasification ratios for Gley soil were decreased according to the increase in pH although the underlaying mechanisms for this observation are not clear. INTRODUCTION Transuranic (TRU) waste containing radionuclides is generated during the operation and dismantling of reprocessing facilities and mixed oxide (MOX) fuel fabrication facilities. In Japan, this waste broadly equates to long-lived intermediate level waste (ILW) and low level waste (LLW) with significant alpha content. The dominant nuclides contributing to the dose from TRU waste are C and I. Because these nuclides are long-lived, water-soluble and have very poor sorption properties on soils, they are the key nuclides in safety assessment for geological repositories of TRU waste. Recently, the possibility of leaching of organic carbon compounds from the simulated hull waste has been reported, and carboxylic acids such as formic acid and acetic acid are dominant forms among the radioactive organic carbon compounds (1). For appropriate safety assessment of the TRU waste, understanding of the behavior of these radioactive carboxylic acids is required. As an indicator of the soil-water partitioning behavior of radionuclides in the soil, distribution coefficients (Kd) are commonly used. Because carboxylic acids can be a carbon source for microorganisms, radiocarbon included in carboxylic acid molecules might be released into the air as gases. Therefore, gasification ratios must also be estimated to understand the behavior of the radioactive carboxylic acids. Values of Kd and gasification ratios may be influenced by regional climate and soil types, and thus more reliable data on these parameters should be obtained for specific regional environmental conditions. To obtain basic information on the behavior of radioactive carboxylic acids in the environment of Japan, we measured Kd values and gasification ratios for radioactive sodium acetate using agricultural soils which were collected from various parts of Japan. Relationships between soil types and these two parameters were also investigated. MATERIALS AMD METHODS Soil Samples A total of 85 soil samples were collected by using a shovel from paddy fields (top 20 cm) in different parts of Japan and classified into 4 types: Andosol (n = 15), Gley soil (n = 21), Gray Lowland soil (n = 44), and Yellow soil (n = 5). The soil samples were dried at room temperature, and then passed through a 2-mm-mesh-sieve to obtain a