Impact of Lanthanide and PGM Elements on the Chemical Durability and Surface Modifications During Leaching Tests of UO₂ Pellets Mimicking Interim Repository

Managed by ANDRA, the European collaborative research program EURAD aims to federate research efforts and to share scientific and technical knowledge of some 100 European players in the field on the long-term management of radioactive waste. In this context, ICSM/LIME is interested in the chemical durability of spent nuclear fuels under interim storage conditions (i.e. in conditions mimicking their stay in cooling pool).<br/>Due to the complex structure and microstructure of spent nuclear fuels (SNF), the global understanding of their behavior when leaching is very difficult to analyze, although it appears as a very important task. In this study, the role of two families of fission products (FP, i.e. lanthanide and PGM elements) was thus specifically examined. Indeed, in SNF, lanthanide elements are incorporated in the UO₂ structure, inducing redox constraints or formation of oxygen vacancies while PGM elements (Ru, Rh, Pd) are forming metallic inclusions in contact with UO₂ grains.<br/>In order to discriminate the structural, microstructural and chemical impacts of these elements on the chemical durability, a series of simplified model samples has been prepared by direct precipitation of hydroxide-based precursors. A calcination step (800°C, reducing atmosphere) led to fluorine type oxides. After shaping by uniaxial pressing, a sintering step (1600°C, 8 hours, reducing atmosphere) led to dense pellets (up to 90 % and 96 % for PGM and lanthanide doped samples, respectively, considering geometrical measurements and He pycnometry). In addition, the pellets were characterized in terms of microstructure and cationic homogeneity by SEM and X-EDS.<br/>At the macroscopic scale, leaching tests were performed on all the model compounds in conditions mimicking an interim storage, i.e. at pH = 5 and 7, at 50 and 70°C and in the presence (or not) of boron (5.0.10^-2 M). Simultaneously, the monitoring of the solid/liquid interface was performed by ESEM. The normalized dissolution rate of UO₂ during leaching test in 5.0.10^-2 M boric acid, at pH = 7 and 50°C reached about (5.5 ± 0.1) × 10^-4 g.m².d^-1 (i.e. 0.04% of dissolved material after 300 days of alteration). The presence of PGM elements induced the significant increase of the alteration rate (2.0 ± 0.1) × 10^-4 g.m².d^-1 (i.e. 0.2% of dissolved material after 300 days of alteration). Moreover, this dissolution rate reaches 3.3% in the medium at pH = 5. On the other hand, lanthanide elements induced a particular behavior. Indeed, after 80 days of leaching, the uranium concentration was found to be constant in the leachate, suggesting the precipitation of a secondary U-based phase onto the surface of the pellets. This phase is now under characterization.