Radiolytic oxidation of UO₂ pellets doped with alpha-emitters (^238/239Pu)

To assess the impact of alpha radiolysis of water on the oxidative dissolution of UO₂ under anoxic conditions, two series of plutonium-doped samples (specific alpha activity 385 and 18MBq g_{U O2} ^{- 1} ) were fabricated, characterized and leached in water of varying complexity (pure water, carbonated water, dissolved hydrogen). Given the very high reactivity of these samples in the presence of air and in order to minimize any prior surface oxidation, a strict experimental protocol was developed based on high-temperature annealing in Ar + 4% H₂ with preleaching cycles. Failure to follow this protocol prevents absolute quantification of oxidation of the UO₂ surface by water radiolysis in solutions. Preoxidation of the pellet surface can lead to uranium release in solution that is dependent on the alpha particle flux, revealing initial oxidation by radiolysis in air including potential traces of water. This makes difficult the accurate quantification of the radiolytic oxidation in water solutions. Controlling the initial surface condition of the samples finally allowed us to demonstrate that radiolytic oxidation in water-saturated media is governed by several threshold effects for which the main parameters are the sample alpha activity and the hydrogen concentration.