XAS study of (U1-yPuy)O2 solid solutions

A new procedure for (U,Pu)O2 nuclear fuel manufacturing based on the oxalic coprecipitation of U(IV) and Pu(III) followed by the thermal conversion of the coprecipitate into oxide is under development. In order to fully investigate the ideality of solid solution with Pu content equal to 50, 30, 15 and 7at.%, X-ray powder diffraction (XRD) and X-ray absorption spectroscopy (XAS) characterizations at uranium and plutonium LIII edges have been undertaken. Using XRD, a face centred cubic structure was observed in each case, and the cell parameter deduced follows satisfactorily the Vegard's law. However, extended X-ray absorption fine structure (EXAFS) measurements moderate these results; only the (U0.5Pu0.5)O2 sample leads to the same conclusion as XRD. For the lower plutonium concentration, a disordered hyperstoichiometric structure (U1-yPuy)O2+x has been revealed. In those compounds, cuboctahedral oxygen defects are only located around uranium atoms and not in the Pu environment. A much more complex structure than that suggested by the XRD is thus observed with a non-random distribution of plutonium atoms within the uranium sites of the (U1-yPuy)O2+x structure.