A kinetic study of UO2 dissolution and H2O2 stability in the presence of groundwater ions

Current reactors spent nuclear UO2 fuel, that will probably enter geologic formations for disposal, is assumed to get into contact with groundwater of a repository earliest after several thousands of years. After this time, its alpha activity will be still high enough to induce a process of water radiolysis that leads to production of oxidants and, probably, to the oxidative dissolution of the material, that could start a mobilisation of radiotoxic nuclides. The main oxidant produced during this process would be hydrogen peroxide (H2O2), which is thermodynamically unstable and can decompose during reactions with certain groundwater ions. Information on the stability of H2O2 is therefore important for assessing the behaviour of the material in the repository. We investigated the stability of H2O2 and the correlated dissolution of U from UO2 as a function of the concentration of H2O2 and of the groundwater ions carbonate, sulphate, and silicate. Decomposition of H2O2 was measured in the presence and absence Of UO2(cr). We monitored the concentrations of H2O2, U-diss, groundwater ions, O-2 (diss), and the pH in aqueous solution under Ar atmosphere. The lowest stabilities (in average) for H2O2 were found in carbonate solutions. When comparing the effect of carbonate systems containing UO2(cr) to carbonate solutions without a solid phase, we found that in some of the homogeneous tests H2O2 was consumed faster. This effect was not observed with the other groundwater ions. Also, U dissolution rates were significantly lower in the carbonate experiments than in those with sulphate or silicate. The results suggest that a radical-controlled mechanism (similar to the Halpern - Smith-mechanism of dissolved U(IV) oxidation) takes place at the UO2 surface during dissolution. The hypothesis is supported by ESR measurements on hydroxyl radical scavenging by the carbonate ion.