Structural Investigation of Uranium–Neptunium Mixed Oxides Using XRD, XANES, and ¹⁷O MAS NMR

Uranium–neptunium mixed dioxides are considered as fuels and targets for the transmutation of the minor actinides in fast neutron reactors. Hereafter, a local and atomic scale structural analysis was performed on a series of U_1–<i>x</i>Np_<i>x</i>O₂ (<i>x</i> = 0.01; 0.05; 0.20; 0.50; 0.75; 0.85) synthesized by the sol–gel external gelation method, for which longer range structural analysis indicates that the process yields solid solutions. The oxidation state of IV for uranium and neptunium cations was confirmed using U L_III and Np L_III edge X-ray absorption near edge structure (XANES). The atomic scale structure was probed with ¹⁷O magic angle spinning nuclear magnetic resonance (MAS NMR) for the anion. Structural distortions due to the substitution of U by the smaller Np cation were detected by ¹⁷O MAS NMR