Résumé
Although a spent nuclear fuel is composed of about 96 wt. % of UO2, its composition and microstructure display an extreme complexity due to the presence of more than 30 fission products. Fission products found as metallic precipitates are mainly composed of the noble metals Ru, Rh, Pd, Mo (platinium group metals = PGM's). The first step of the spent nuclear fuel reprocessing is the dissolution in a concentrated nitric acid solution. In this context, this Ph. D. work focuses on the impact of these elements on the kinetics of dissolution of the UO2 matrix. Model samples of UO2 incorporating 3 mol. % of Ru, Rh, Pd or Mo were prepared using a wet chemistry route, then were sintered at high temperature under reducing atmosphere. The speciation, morphology as well as the spatial distribution of the elements in the sintered samples were determined. The pellets exhibited similar microstructure. Then, the synthesized samples were submitted to dissolution tests in order to quantify the impact of the presence of the metallic particles on the dissolution rate of UO2. Dissolution experiments were carried out under conditions representative of the reprocessing of SNF. The dissolution rate of each model compound was determined at macroscopic scale and compared to that of UO2. At microscopic scale, the evolution of the solid/liquid interface was monitored by MEBE. Additionally, specific dissolution experiments were designed to identify the mechanisms involving these elements. The obtained results allows to compare the influence of each element of interest and to improve our understanding of their specific role during the dissolution of UO2 in nitric acid. dissolution in a hydrochloric medium will also be carried out to erase the impact of nitrate ions on the dissolution