Insights into the UO2+x/U4O9 phase characterization in oxidized UO2 pellets as a function of hyper-stoichiometry

We present new insights into the study of the UO 2+x /U 4 O 9 equilibrium in UO 2 as a function of the hyper-stoichiometry (x) by coupling HERFD-XANES at the uranium M 4 -edge with micro-Raman spectroscopy mapping. XANES allowed the measurement of uranium speciation in the samples, while Raman spectroscopy was used to individually characterize the composition and localization of the different oxide phases. UO 2 pellets were oxidized under dry conditions at temperatures above the UO 2+x /U 4 O 9 phase transition to reach hyper-stoichiometries in the range of 0.01 ≤ x ≤ 0.1. Combining both techniques, we could determine the proportions of U 4 O 9 and UO 2+x . We show that at a low O/U ratio, U 4 O 9 is present as small clusters inside UO 2 grains. As the O/U increases, we found evidence of the formation of a network of U 4 O 9 crystallized inside the UO 2+x grains. The variation of the UO 2+x phase hyper-stoichiometry (x) was evaluated as a function of the sample oxidation.