Abstract
In France, the single recycling of irradiated fuel leads to the medium-term storage of irradiated Mimas® MOx (Mixed Oxide) fuel assemblies in pools, with a view to recycling or final disposal. In this context of underwater storage, it is important to consider an scenario involving a defect in the cladding. Given that the pool water is oxidizing due to the presence of radiolytic products such as H2O$_2$ and OH°, leaching by this water promotes the dissolution of uranium from the fuel through an oxidative dissolution mechanism, followed by its reprecipitation as studtite [(UO$_2$)(O$_2$)(H$_2$O)$_2$]·2(H$_2$O). It is therefore important to study these degradation mechanisms with the aim of recovering the rods after several years and decades of storage and limit contamination of the pool water by the release of radionuclides into the water.To this end, a three-step scientific approach has been implemented. First, we studied the dynamic leaching of model materials (U, Th)O$_2$ using a multiparametric approach aimed at establishing a set of kinetic laws that can be extrapolated to real fuel, in order to feed chemistry-transport models. In parallel, studies on the static leaching of MIMAS MOx fuel under gamma radiolysis were conducted. These studies, which coupled Raman spectroscopy and image analysis, allowed us to propose a precipitation process for studtite on the surface of Mimas® MOx fuel.Finally, an original experimental methodology involving the use of 18O isotopes and Raman spectroscopy has been developed. These studies were conducted to enhance our understanding of the mechanisms of studtite precipitation on the fuel surface. The results obtained are consistent with each other and have allowed us to propose a mechanism for the formation of studtite, which is presented in Figure 1. The link between the formation of uranyl-peroxide bonds in the solid phase and the nature of the oxidizing species produced by water radiolysis has been elucidated.It appears that there is a direct chemical relationship between the peroxo ligands within studtite and the peroxide entities of the H$_2$O$_2$ molecule in the solution. Under alpha radiolysis, the formation of H$_2$O$_2$ results from the primary radiolytic yield during the heterogeneous chemistry step involving the recombination of OH° + OH°. Meanwhile, under gamma radiolysis, the formation of H$_2$O$_2$ results from a reaction mechanism involving dissolved oxygen in the solution. Regarding the uranyl bond, the characterization of its isotopy has allowed us to specify the oxidation mechanism occurring at the UO$_2$/water interface. It appears that under gamma radiolysis, the primary oxidation mechanism of UO$_2$ involves the incorporation of oxygen atoms from OH° radicals. In contrast, under alpha radiolysis, there is a significant contribution from H$_2$O$_2$, whose oxidation mechanism occurs in two steps. The first step involves a simple electron transfer between UO$_2$ and H$_2$O$_2$, followed by a second step where an oxygen atom is incorporated via OH° radicals produced as a reaction product from the first step.