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

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 U 3 O 8 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 U 3 O 8 fraction in some samples calcined under argon resulted in a higher normalized dissolution rate. For comparison, the normalized dissolution rate of a pure U 3 O 8 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 U 3 O 8 of the transition to a kinetic dissolution regime autocatalyzed by nitrogen species. • A panel of various UO 2 was prepared by hydroxide co-precipitation route then converted in two different atmospheres at various temperatures • Dissolution tests of the powdered UO 2 were performed in 0.1 mol L −1 HNO 3 and 60 °C • Different normalized dissolution rates were obtained depending on the heating atmosphere and temperature of the samples • The presence of U 3 O 8 in the fired samples is linearly correlated to the increase of the normalized dissolution rate