Radiation induced dissolution of model compounds for spent nuclear fuel : mechanistic understanding of oxidative dissolution and its inhibition

Abstract: This doctoral thesis is focused on radiation induced oxidative dissolution of UO2, Pd-doped UO2, SIMFUEL (as model substances for spent nuclear fuel) and UN (a possible future fuel) and inhibition of the oxidative dissolution. H2O2 is assumed to be the most important oxidant for spent nuclear fuel dissolution under deep repository conditions. The dissolution of uranium has been studied by oxidation by added H2O2 and by gamma irradiation in the presence and absence of carbonate. In carbonate free solutions very low amounts of uranium are dissolved from UO2 due to formation of metastudtite, UO4·2H2O on the UO2 surface which blocks the surface from further oxidation. Metastudite formation was confirmed with Raman spectroscopy. In the presence of carbonate, the concentration of dissolved uranium increases linearly over time for UO2 and UN, due to the complex formation between carbonate and oxidized uranium. For SIMFUEL a large fraction of H2O2 is consumed by catalytic decomposition under all conditions examined. This results in very low amounts of uranium released. Metastudtite formation was not observed on SIMFUEL. The oxidation during gamma radiolysis shows a larger difference in dissolution rates between UO2 and UN in carbonate solutions compared to upon oxidation by added H2O2. UN was found to have a lower dissolution rate, most probably because 50 % more oxidant is needed to reach the soluble U(VI). It was shown that the redox reactivity of UO2 appears to increase ~1.3 times, after being irradiated at doses > 40 kGy. The effect is permanent and delayed. The presence of sulfide shows an inhibiting effect on radiation induced dissolution due to scavenging of radiolytic oxidants and reduction of U(VI). The catalytic properties of Pd (as a model for the noble metal particles containing Mo, Ru, Tc, Pd and Rh, formed by the fission products) are examined. It was found that Pd has a catalytic effect on the reaction between H2O2 and H2 and the second order rate constant is determined to (2.1±0.1)x10-5 m s-1. The reaction between UO2 and H2O2 is catalyzed by Pd. Pd also has a catalytic effect on the reduction of U(VI) by H2 both in aqueous solution, rate constant (1.5±0.1)x10-5, and in the solid phase, rate constants 4x10-7 m s-1 and 7x10-6 m s-1 for pellets with 1 and 3 % Pd respectively. These values are very close to the diffusion limit for these systems. The catalytic effect was not influenced by the presence of sulfide. The catalytic effect in the solid phase reduction shows that the expected conditions in a deep repository, 40 bar H2 and 1 % noble metal particle content, is sufficient to stop the dissolution.