Stability of Studtite in Aqueous Suspension: Impact of HCO3- and Ionizing Radiation on the Dynamics of Dissolution

In a geological repository for spent nuclear fuel, studtite (UO2)O-2(H2O)(4) may form on the fuel surface when in contact with groundwater under certain conditions. Studtite has a very low solubility and could thereby reduce the reactivity of spent nuclear fuel toward radiolytic oxidants. This would inhibit the dissolution of the fuel matrix and thereby also the spreading of radionuclides. It is therefore important to investigate the stability of studtite under conditions that may influence its stability. In this work we have studied the kinetics of studtite dissolution in aqueous suspensions containing no added HCO3- and with 10 mM HCO3-. The same type of experiment was performed also with solutions containing 0.2 mM H2O2. The solubility of studtite in the suspensions containing no added HCO3- is very low as expected, while the solubility in solutions containing 10 mM HCO3- is significantly higher. This is attributed to the formation of uranyl-carbonate and uranyl-peroxo-carbonate complexes. In 0.2 mM H2O2 and 10 mM HCO3- the observed solubility of U(VI) seems unaffected by the presence of H2O2. Again, this can be rationalized by the formation of uranyl-peroxo-carbonate complexes. It is interesting to note that H2O2 appears to be catalytically decomposed in solutions containing uranyl-carbonate complexes. In addition, gamma-radiation-induced dissolution of studtite in HCO3- deficient solutions and in 10 mM HCO3- was studied. The dissolution rate was found to be extremely high in HCO3- under gamma-irradiation. This is attributed to a combination of radiolytic degradation of H2O2 and the formation of uranyl-peroxo-carbonate complexes keeping the concentration of free H2O2 at a very low level and thereby driving the dissolution process.