It has been found that metallic TRU waste releases considerable portions of C-14 in the form of organic molecules such as lower molecular weight organic acids, alcohols and aldehydes. Due to the low sorption ability of organic C-14, it is important to clarify the long-term behavior of organic forms under waste disposal conditions. From investigations on radiolytic decomposition of organic carbon molecules into inorganic carbonic acid, it is expected that radiation from TRU waste will decompose organic C-14 into inorganic carbonic acid that has higher adsorption ability into the engineering barriers. Hence we have studied the decomposition behavior of organic C-14 by gamma irradiation experiments under simulated disposal conditions. The results showed that organic C-14 reacted with OH radicals formed by radiolysis of water, to produce inorganic carbonic acid. We introduced the concept of “decomposition efficiency” which expresses the percentage of OH radicals consumed for the decomposition reaction of organic molecules in order to analyze the experimental results. We estimated the effect of radiolytic decomposition on the concentration of organic C-14 in the simulated conditions of the TRU disposal system using the decomposition efficiency, and found that the concentration of organic C-14 in the waste package will be lowered when the decomposition of organic C-14 by radiolysis was taken into account, in comparison with the concentration of organic C-14 without radiolysis. Our prediction suggested that some amount of organic C-14 can be expected to be transformed into the inorganic form in the waste package in an actual system.

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