Soil-to-Rice Transfer of ⁹⁹Tc 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 ⁹⁹Tc because of its very long half-life (2.1x10⁵ years) and relatively high ²³⁵U-fission yield. Accordingly, it is one of the critical radionuclides in an environmental impact assessment for radioactive waste disposal. A significant amount of ⁹⁹Tc can be released into the atmosphere in a severe reactor accident as was shown in the Chernobyl accident. It is a pure better emitter and thus internal exposure via food consumption may be a primary contributor to the ⁹⁹Tc radiation dose to humans. Paddy rice fields can be contaminated with ⁹⁹Tc in various ways. In the present study, greenhouse experiments were conducted to investigate the transfer of ⁹⁹Tc from four paddy soils contaminated in two different ways. One was to simulate plowing the topsoil after a pre-transplanting deposition of ⁹⁹Tc, whereas the other was to simulate a ⁹⁹Tc deposition onto the surface water shortly after transplanting. Soil-to-rice transfer of ⁹⁹Tc in paddy soils was experimentally investigated for two different scenarios of the paddy-field contamination. It was demonstrated that a post-transplanting deposition of ⁹⁹Tc onto the surface water can lead to a much higher transfer than a pre-transplanting deposition followed by plowing. The surface-water concentrations of ⁹⁹Tc following the post-transplanting deposition was markedly higher than those following the pre-transplanting deposition, possibly indicating a much higher plant-base uptake of ⁹⁹Tc following the post-transplanting deposition. The present results can be referred to in a radiological impact assessment for the contamination of paddy fields with ⁹⁹Tc by chronic or acute releases from nuclear facilities