Determination of the plutonium content and O/M ratio of (U,Pu)O2-x using Raman spectroscopy

Oxygen stoichiometry in (U,Pu)O2-x nuclear fuels, while of prime interest, is still difficult to be determined at the micrometric scale. It has been recently evidenced that Raman microscopy is a promising tool to characterize (U,Pu)O2-x samples at a microscopic scale by probing the oxygen sublattice. Its use for determining the local O/M ratio was studied in this work on mixed oxide samples mostly containing Pu-239 and natural uranium, in addition to minor traces of other isotopes, including decay products and Am-241. The first step was to dissociate the influence of the Pu/(U+Pu+Am) content, self-irradiation and O/M ratio on Raman spectra and especially on the main Raman band position in fluorite structure, the T-2g. In this aim, freshly annealed and aged U1-yPuyO2-x samples, with 0.19 < y < 0.46 and different O/M ratios, were analyzed by XRD and Raman spectroscopy. After figuring out that self-irradiation alone had no significant impact on the T-2g position, two mathematical relations were determined, linking the T-2g position to the Pu/(U+Pu+Am) content and to the lattice parameter. Finally, the oxygen hypostoichiometry direct impact on (U,Pu)O2-x Raman spectra was determined for the first time. In agreement with past results observed in CeO2-x, a O/M ratio decrease induces a T-2g shift towards lower frequencies in (U,Pu)O2-x. Combining the whole results, an equation allowing to determine the T-2g position according to the O/M ratio and Pu/(U+Pu+Am) content was established. Raman spectrometer associated to a confocal optical microscope can then be used to determine locally (approximate to 1 mu m) either the Pu/(U+Pu+Am) content or the O/M ratio. This study proves the relevance of such method to characterize the fuel pellets in an industrial way. (C) 2020 The Authors. Published by Elsevier B.V.