Studtite Formation Assessed by Raman Spectroscopy and 18O Isotopic Labeling during the Oxidative Dissolution of a MOX Fuel

The formation of studtite was studied during the oxidative dissolution of a MIMAS MOX fuel disk in aerated water enriched in O-18 under a gamma radiation source, coupling Raman spectroscopy and solution analyses. The use of isotopic labeling enabled the reactions responsible for the precipitation of studtite to be followed. At the beginning of the experiment, different O-18 enrichments in the uranyl and peroxide bonds of the studtite were observed. While the uranyl bond was primarily enriched in O-18, the peroxide bond contained large amounts of O-16. This result suggests an oxygen contribution coming from different radiolytic species for each bond: H2O2 and radicals. The comparison with a UO2 sample leached in similar conditions ruled out a role of Pu alpha self-irradiation in this different behavior. Nevertheless, the influence of the MOX MIMAS heterogeneous microstructure and chemistry was observed with the preferential dissolution of the UO2 grains, the Pu-rich areas being much more stable with regard to the dissolution. In addition, the studtite first precipitated 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.

Automated summary

The formation of studtite was studied using isotopic labeling during oxidative dissolution of a MIMAS MOX fuel disk in aerated water enriched in O-18, showing different enrichments of O-18 in the uranyl and peroxide bonds of studtite. Additionally, the influence of MOX MIMAS heterogeneous microstructure and chemistry was observed with preferential dissolution of UO2 grains and stability of Pu-rich areas during the experiment.