Abstract
Although spent fuel reprocessing remains the reference scenario in France, direct disposal in deep geological formations is also studied as a potential option. While the alteration of UOx spent fuel is well documented, this is not the case for (U,Pu)O2 fuels. During this PhD, a particular attention was given to the identification of the oxidative dissolution mechanisms due mainly to the presence of radiolytically produced H2O2. The inhibition of these mechanisms by species present in the environmental water such as silicate and calcium ions was also studied. This work focuses on the alteration of heterogeneous MIMAS® MOx fuel and U1-xCexO2 homemade surrogate materials. Alteration experiments were conducted with the (U,Ce)O2 materials under dynamic conditions and in the presence of H2O2. The specific role of calcium and silicate ions was investigated by tests under static conditions with both categories of materials in solutions with simplified compositions. Altered surfaces were characterized by the means of ESEM, Raman and XPS, whereas aqueous samples were analyzed by ICP-AES & MS. These experiments were supported by speciation calculations performed with Phreeqc and GWB software.Concerning the impact of H2O2, the alteration rates were found to decrease with increasing cerium content in homogeneous solid solutions (U,Ce)O2. The formation of a cerium (IV) rich layer passivating the surface was also revealed by XPS analyses. Alteration of the heterogeneous material in these conditions resulted in studtite precipitation localized only on the UO2 matrix and not on the cerium rich clusters. These results are similar to those observed in the MOx MIMAS experiments and attest to the good analogy of cerium behavior towards plutonium under oxidizing conditions. In environmental water, silicate ions were found to play an important role in decreasing release rates and concentrations during the thermodynamic regime, but also participate in the formation or stabilization of uranium or cerium based colloidal species. Experiments carried out at alkaline pH and in the presence of silicate ions revealed that the alteration of MIMAS MOx in this medium is independent of its alpha activity. Silicate ions are also responsible for the inhibition of oxidative dissolution at alkaline pH. The blocking of reactive sites on the surface of the materials via the adsorption of silicate ions is a satisfactory mechanism with regard to the observations made in this work.