Oxalic acid formation by radiation-induced reactions in water under a simple model condition of geological disposal of radioactive wastes

熊谷 友多

no journal, , 

Deep geological disposal has been studied as a safe and reliable technology for the long-term management of vitrified high-level radioactive waste or spent nuclear fuel. In scenarios of failure of the geological repository, direct contact of water with the waste form results in radiolysis of water. Groundwater at the depth of the geological repository is reductive in general, and also anaerobic corrosion of the metallic materials of the canister generates molecular hydrogen. The water radiolysis under such reductive conditions could produce organic acids through reduction of carbonate/bicarbonate ion contained in groundwater. Organic acids may affect the solubility and migration of radioactive elements. Therefore, the formation of organic acids by irradiation of aqueous bicarbonate solution under hydrogen atmosphere was studied. The aqueous solution of sodium bicarbonate was prepared and purged with mixture gas of argon, hydrogen, and carbon dioxide. The samples were irradiated in septum-sealed vials by using an X-ray irradiator. After the irradiation, the sample was analyzed by an ion chromatography and formation of oxalic acid was detected. The formation of oxalic acid was clear in 1 mM sodium bicarbonate solution, but not significant in of 10 mM solution. The results were analyzed by using a kinetic simulation of water radiolysis. The decrease in oxalic acid formation by the increase in the bicarbonate ion concentration can be explained by competition between the OH radical-hydrogen reaction and OH radical-bicarbonate ion reaction. The results of this study indicate that oxalic acid generation by water radiolysis is strongly dependent on concentrations of carbonate/bicarbonate ion and dissolved molecular hydrogen.