UO2 dissolution in high pH conditions of the Belgian Supercontainer

Since a few years, ONDRAF/NIRAS has selected the Supercontainer design with an Ordinary Portland Cement buffer as the reference design for geological disposal of HLW and Spent Fuel in the Boom Clay formation. To assess the long-term performance of the spent fuel disposal in alkaline conditions, a research programme was started to determine the matrix dissolution rate, which governs the long-term release of radionuclides. Static and dynamic dissolution tests were performed with alpha-doped and depleted UO2 in contact with cement water at pH 13.5, 25 - 30°C under argon atmosphere. The (238U or 233U) release in solution was used to estimate the matrix dissolution rate of the UO2. The concentration of dissolved uranium gives the solubility of the controlling phase, which is important for the system understanding. The resulting rates appear to be in good agreement with the dissolution rates reported in the literature for neutral pH conditions. However the α-activity threshold above which radiolytical self-oxidation drives the fuel matrix dissolution may be lower than in anoxic media at lower pH, although the redox conditions in cement water should be considered as weakly reducing. Based on these results, we propose a selection of reference dissolution rates for performance assessment.