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New insight in the uranium valence state determination in U$_y$Nd$_{1−y}$O$_{2 \pm x}$

R. Bès 1 K. Kvashnina 2, 3, * A. Rossberg 2, 3 G. Dottavio 4 L. Desgranges 4 Y. Pontillon 5 P.L. Solari 6 S. Butorin 7 P. Martin 8
* Corresponding author
5 LAMIR - Laboratoire d'Analyse de la Migration des Radioéléments
SA3E - Service d'Analyses, d'Elaboration, d'Expérientations et d'Examens des combustibles : DEN/DEC/SA3E
8 LFC - DEN/DMRC/SFMA/LFC
DMRC - Département de recherche sur les procédés pour la mine et le recyclage du combustible : DEN/DMRC/SFMA/LFC
Abstract : The charge compensation mechanisms in U$_y$Nd$_{1−y}$O$_{2 \pm x}$ and its consequence on the overall O stoichi-ometry (or O/M ratio where M = Nd + U) have been studied through the uranium valence state mixture evolution as a function of Nd content up to y = 0.62 by means of high energy resolution fluorescence detection X-ray absorption spectroscopy (HERFD-XAS) at the U M$_4$-edge. Our results clearly demonstrate the formation of U$^{5+}$ at low Nd content (y < 0.15). Upon increasing the Nd content, oxygen vacancies and the formation of U$^{6+}$ appear as competing mechanisms for intermediate Nd concentrations, leading to the coexistence of U$^{4+}$/U$^{5+}$/U$^{6+}$ mixed valence and an overall hypostoichiometry (O/M < 2.00). Finally, the formation of U 6þ associated with strongly distorted U local environment is observed for high Nd concentrations (y = 0.62), leading to an overall hyperstoichiometry (O/M < 2.00).
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R. Bès, K. Kvashnina, A. Rossberg, G. Dottavio, L. Desgranges, et al.. New insight in the uranium valence state determination in U$_y$Nd$_{1−y}$O$_{2 \pm x}$. Journal of Nuclear Materials, Elsevier, 2018, 507, pp.145-150. ⟨10.1016/j.jnucmat.2018.04.046⟩. ⟨cea-02429995⟩

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