Tc in Paddy Soils Contaminated in Two Different Ways

Rice is one of the most important food crops in the world. All isotopes of technetium (Tc) are radioactive, and the environmentally most important one is 99 Tc because of its very long half-life (2.1x10 5 years) and relatively high 235 U-fission yield [1,2]. Accordingly, it is one of the critical radionuclides in an environmental impact assessment for radioactive waste disposal. A significant amount of 99 Tc can be released into the atmosphere in a severe reactor accident as was shown in the Chernobyl accident [3]. It is a pure better emitter and thus internal exposure via food consumption may be a primary contributor to the 99 Tc radiation dose to humans [1]. Paddy rice fields can be contaminated with 99 Tc in various ways. In the present study, greenhouse experiments were conducted to investigate the transfer of 99 Tc from four paddy soils contaminated in two different ways. One was to simulate plowing the topsoil after a pre-transplanting deposition of 99 Tc, whereas the other was to simulate a 99 Tc deposition onto the surface water shortly after transplanting.