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 239Pu and natural uranium, in addition to minor traces of other isotopes, including decay products and 241Am. 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 T2g. 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 T2g position, two mathematical relations were determined, linking the T2g 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 T2g shift towards lower frequencies in (U,Pu)O2-x. Combining the whole results, an equation allowing to determine the T2g 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 (≈1 µm) 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.