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) , Yves Pontillon (5) , P.L. Solari (6) , S. M Butorin (7) , P. Martin (8)

1 Aalto University
2 ESRF - European Synchrotron Radiation Facility
3 HZDR - Helmholtz-Zentrum Dresden-Rossendorf
4 DEC - Département d'Etudes des Combustibles
5 LAMIR - Laboratoire d'Analyse de la Migration des Radioéléments
6 SSOLEIL - Synchrotron SOLEIL
7 Uppsala University
8 LFC - DEN/DMRC/SFMA/LFC

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).

Domaines: Physique Nucléaire Expérimentale [nucl-ex]

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, 2018, 507, pp.145-150. ⟨10.1016/j.jnucmat.2018.04.046⟩. ⟨cea-02429995⟩