Distribution and behavior of transuranium elements in paddy surface soil

Global fallout transuranium elements 237Np, 239,240Pu and 241Am in surface soils from paddy fields of various locations in Japan have been measured for studying their integrated deposits, distribution, time variation and behavior of these elements. The full set of radioanalytical date obtained showed that (1) large differences in local or regional fallout deposition were observed: 239,240Pu and 241Am deposits on the Japan Sea coast were 2-3 times higher than those ones on the Pacific coast, (2) 241Am/239,240Pu activity ratio was about 0.3 in 1984 and will reach a maximum of 0.41 in the year 2033, and (3) 237Np/239,240Pu activity ratio was in the range of (3-4)x10-3. By using a simple model for surface soils, the behavior of 237Np, 239,240Pu and 137Cs in the soil of the past 30 years was evaluated in terms of apparent residence time. The apparent residence time for 237Np was found to be shorter than those for 239,240Pu and 137Cs, indicating greater mobility of 237Np than of 239,240Pu and 137Cs. (author)