Radiation induced dissolution of U3Si2 - A potential accident tolerant fuel

To assess the integrity of the accident tolerant fuel candidate U3Si2 under geological repository conditions, the kinetics of γ-radiation- and H2O2- induced oxidative dissolution was studied. The experiments were performed in aqueous solutions containing 10 mM HCO3− and in solutions without added HCO3−. The same experiments were also performed on UO2 for comparison. All experiments were performed using powder suspensions. The experiments show that U3Si2 is less than one order of magnitude more reactive towards H2O2 than is UO2. The dissolution yield of U3Si2 slightly exceeds the theoretical yield (23%). In experiments with consecutive additions of H2O2 in HCO3− solutions, the reactivity remains constant implying that no significant amount of a secondary phase is formed on the U3Si2 surface. The dissolution of Si closely follows that of U in HCO3− solution. In solutions without added HCO3− the reactivity towards H2O2 is reduced by a factor less than 2. The dissolution is slightly slower than in HCO3− containing solutions but precipitation of U is observed after some time. The results of consecutive additions of H2O2 to the HCO3− free system shows that the reactivity is decreasing for every addition. This indicates that a secondary phase is formed. XRD shows that the secondary phase is studtite. The irradiation experiments show that the surface area normalized radiation chemical yields for uranium dissolution for U3Si2 and UO2 in HCO3− solution differ by a factor 5–10 in favour of UO2. This difference can largely be attributed to the difference in dissolution yield.