Studtite formation assessed by Raman spectroscopy and $^{18}$O isotopic labeling during the oxidative dissolution of a MOX fuel

The formation of studtite has been studied during the oxidative dissolution of a MIMAS MOX fuel disc in aerated water enriched in $^{18}$O under a gamma radiation source, coupling Raman spectroscopy and solution analyzes. The use of isotopic labeling allowed following the reactions responsible for the precipitation of studtite. At the beginning of the experiment, different $^{18}$O enrichments in the uranyl and peroxide bonds of the studtite were observed. While the uranyl bond was primarily enriched in $^{18}$O, the peroxide bond contained large amounts of $^{16}$O. This result suggests an oxygen contribution coming from different radiolytic species for each bond: H$_2$O$_2$ and radicals. The comparison with a UO$_2$ sample leached in similar conditions ruled out a role of the Pu alpha self-irradiation in this different behavior. Yet, the influence of the MOX MIMAS heterogeneous microstructure and chemistry is observed with the preferential dissolution of the UO$_2$ grains, the Pu-rich areas being much more stable with regard to the dissolution. In addition, the studtite first precipitates preferentially on the Pu-poor areas of the sample before covering the entire surface, including the plutonium-enriched aggregates, at the end of the experiment.