Behaviour of fallout Tc-99 in soil environment. In the case of paddy field environment

In this study, for focussing to paddy field soil environment particularly, behavior of Technetium-99 (99Tc) emitted into environment was investigated. At first, in order to examine concentration level of 99Tc in actual paddy field soil, quantitative analysis of fallout 99Tc in the paddy field soil adopted from all over Japan was conducted, by which 0.02-0.11 Bq/kg was obtained. And, as a result obtained on a radioactivity ratio of 99Tc/137Cs, it was found that 99Tc was accumulated into the paddy field soil. Then in order to elucidate the mechanism, a tracer experiment was conducted by imitating the paddy field. As a result, it was found that concentration of soluble Tc decreased by reduction of the soil flooding, and that on this process. Tc was gradually absorbed by insolubilization due to bonding with organic materials, formation of difficult-soluble TcO2 and TcS2 and so on due to reduction, adsorption to soil solid phase such an Mn and Fe oxides, and so forth. Therefore, in the soil apt to develop reductive atmosphere such as paddy field soil at breeding period, Tc changes from TcO4- with high solubility to the other physico-chemical form, to be difficult to transfer for plants. And, as a result that it was investigated if the absorbed Tc changes again to soluble one by oxidation of the soil, it was found that change to TcO4- was slow. That is to say, it can be said that Tc once absorbed into the soil is present there stably. (G.K.)