Impact of firing temperature and atmosphere on the chemical reactivity of UO2+x powders in nitric acid

Abstract

The aim of this work is to study the dissolution kinetics of a series of powdered UO_2+x samples with different structural and microstructural properties. For this purpose, UO_2+x powders were prepared by hydroxide precipitation and then heat-treated at different temperatures under argon and reducing atmospheres. The calcined UO_2+x samples were first investigated ex-situ by several physicochemical techniques in order to highlight the dependence of the normalized dissolution rates on various parameters. The PXRD experiments showed the preservation of the fluorite structure under reducing atmosphere over the whole temperature range studied, while the formation of a U₃O₈ phase was highlighted under argon at T ≤ 1100 °C. The study of the dissolution of UO_2+x samples first highlighted the effect of increasing the calcination temperature (decrease of S_SA), which significantly improves the chemical durability of the solids. The higher the calcination temperature, the lower the reactivity of the sample and the longer the time required to reach full dissolution. Secondly, the presence of a U₃O₈ fraction in some samples calcined under argon resulted in a higher normalized dissolution rate. For comparison, the normalized dissolution rate of a pure U₃O₈ sample reached R_L = (5.6 ± 1.1) × 10⁻¹ g m⁻² d⁻¹, a higher value than that of UO_2+x, R_L = 5.5 × 10⁻² g m⁻² d⁻¹ on average. Furthermore, these samples showed no change in kinetic regime during dissolution, which could be explained by the blocking by U₃O₈ of the transition to a kinetic dissolution regime autocatalyzed by nitrogen species.