Structural investigation of (U0.7Pu0.3)O2-x mixed oxides

Uranium-plutonium mixed oxide containing 30% of PuO2 is a candidate fuel for several fast neutron and accelerator driven reactor systems. In this work, a detailed structural investigation on sol-gel synthesized stoichiometric U0.7Pu0.3O2.00 and substoichiometric U0.7Pu0.3O2-x, using X-ray diffraction (XRD), oxygen 17 magic angle spinning nuclear magnetic resonance (17O MAS-NMR) and X-ray absorption spectroscopy is described. As observed by XRD, the stoichiometric U0.7Pu0.3O2.00 is monophasic with a lattice parameter in good agreement with Vegard’s law, while the substoichiometric U0.7Pu0.3O2-x material is biphasic. 17O MAS-NMR confirmed a random distribution of metal atoms and hence an ideal solid solution behaviour for U0.7Pu0.3O2.00. X-ray absorption near-edge structure (XANES) spectroscopy showed the prevalence of plutonium (III) in U0.7Pu0.3O2-x. Extended X-ray absorption fine-structure (EXAFS) spectroscopy indicated a similar local structure around U and Pu, in agreement with the long-range order determined by XRD.