Radiation induced dissolution of U3Si2 - A potential accident tolerant fuel

To assess the integrity of the accident tolerant fuel candidate U ₃Si ₂ under geological repository conditions, the kinetics of γ-radiation- and H ₂O ₂- induced oxidative dissolution was studied. The experiments were performed in aqueous solutions containing 10 mM HCO ₃ ⁻ and in solutions without added HCO ₃ ⁻. The same experiments were also performed on UO ₂ for comparison. All experiments were performed using powder suspensions. The experiments show that U ₃Si ₂ is less than one order of magnitude more reactive towards H ₂O ₂ than is UO ₂. The dissolution yield of U ₃Si ₂ slightly exceeds the theoretical yield (23%). In experiments with consecutive additions of H ₂O ₂ in HCO ₃ ⁻ solutions, the reactivity remains constant implying that no significant amount of a secondary phase is formed on the U ₃Si ₂ surface. The dissolution of Si closely follows that of U in HCO ₃ ⁻ solution. In solutions without added HCO ₃ ⁻ the reactivity towards H ₂O ₂ is reduced by a factor less than 2. The dissolution is slightly slower than in HCO ₃ ⁻ containing solutions but precipitation of U is observed after some time. The results of consecutive additions of H ₂O ₂ to the HCO ₃ ⁻ 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 U ₃Si ₂ and UO ₂ in HCO ₃ ⁻ solution differ by a factor 5–10 in favour of UO ₂. This difference can largely be attributed to the difference in dissolution yield.