Experimental studies of radiation-induced dissolution of UO2 The effect of intrinsic solid phase properties and external factors

Abstract: Dissolution of the UO2 matrix is one of the potential routes for radionuclide release in a future deep geological repository for spent nuclear fuel. This doctoral thesis focuses on interfacial reactions of relevance in radiation-induced dissolution of UO2 and is divided in two parts:In the first part, we sought to explore the effects of solid phase composition:The impact of surface stoichiometry on the reactivity of UO2 towards aqueous radiolytic oxidants was studied. H2O2 reacts substantially faster with stoichiometric UO2 than with hyperstoichiometric UO2. In addition, the release of uranium from stoichiometric UO2 is lower than from hyperstoichiometric UO2. The behavior of stoichiometric powder changes with exposure to H2O2, approaching the behavior of hyperstoichiometric UO2 with the number of consecutive H2O2 additions.The impact of Gd-doping on the oxidative dissolution of UO2 in an aqueous system was investigated. A significant decrease in uranium dissolution and higher stability towards H2O2 for (U,Gd)O2 pellets compared to standard UO2 was found.In the second part, we sought to look at the effect of external factors:The surface reactivity of H2 and O2 was studied to understand the overall oxide surface reactivity of aqueous molecular radiolysis products. The results showed that hydrogen-abstracting radicals and H2O2 are formed in these systems. Identical experiments performed in aqueous systems containing UO2 powder showed that the simultaneous presence of H2 and O2 enhances the oxidative dissolution of UO2 compared to a system not containing H2.The effect of groundwater components such as bentonite and sulfide on the oxidative dissolution of UO2 was also explored. The presence of bentonite and sulfide in water could either delay or prevent in part the release of uranium to the environment. The Pd catalyzed H2 effect is more powerful than the sulfide effect. The poisoning of Pd catalyst is not observed under the conditions studied.