Long-term safety of radioactive waste disposal: Reaction of high burnup spent fuel and UO(sub 2) in saline brines at room temperature

摘要

In order to determine the long-term performance of spent fuel during direct disposal, high burnup fuel (50 MWd/kg U) has been exposed for 500 days to non-buffered saline solutions and to deionized water under static, anaerobic conditions at 25 C. In the absence of saturation effects (large solution volumes) a dissolution rate of 0,3% p.a. has been measured. Under more realistic conditions of large sample surface areas and small solution volumes, reaction rates were found to be orders of magnitude lower. The pH values remained neutral to slightly alkaline. Radiolysis did not lead to an oxidation of the solution. The releases of Sr90, Tc99, Np237 and Sb125 were controlled by the rate of dissolution of the fuel matrix, whereas the solution concentrations of U, Pu, Am and REE were governed by sorption and solubility effects. In the presence of iron, radiolytically produced oxygen has been consumed by iron corrosion and sorption on iron corrosion products effectively reduced the solution concentrations of most radionuclides. Studies of the corrosion of unirradiated UO(sub 2) have shown strong indications for the formation of U(IV) hydroxide. Under oxidizing conditions, only in few cases solubility limits were reached. In the determination of UO(sub 2) corrosion rates one has to distinguish between the initial fast process of dissolving an oxidized surface layer of UO(sub 2,6) and the process of dissolution of the uranium dioxide beneath and the formation of a new layer of UO(sub 2,4). Under anaerobic conditions reaction rates were significantly lower than under oxidizing conditions. (orig.) (ERA citation 20:008126)