Alteration mechanisms of UOX spent fuel in aqueous media

The mechanisms of spent fuel alteration in aqueous media need to be understood on the assumption of a direct disposal of the assemblies in a geological formation or for long duration storage in pool. This work is a contribution to the study of the effects of the alpha and/or beta/gamma radiolysis of water on the oxidation and the dissolution of the UO2 matrix of UOX spent fuel. Several experimental approaches have been developed for the study of alteration of the UO2 matrix of the spent fuel under water and two types of materials have been chosen: 1)UO2 pellets doped with alpha emitters (238U and 239Pu); they have been leached in the experimental conditions representative of a storage (water in equilibrium with air and under gamma irradiation) and of a disposal (anoxic medium under argon or reducing agent in presence of H2 able to be formed in great quantity by the anoxic corrosion of the containment); 2)UOX fuel fragments irradiated at 60 GWj.t-1, real material leached in the same conditions for comparison. The inhibiting effect of hydrogen has been studied too in leaching these samples under an Ar/H2 (30%) atmosphere. The uranium releases in solution have shown a control by the solubility for specific activities until 385 MBq.gUO2-1. Several leaching experiments of spent fuel fragments have been carried out in varying the oxidizing conditions: leaching under air with and without addition of H2O2, leaching under external gamma irradiation under air or in presence of radicals captors (N2O, HCOONa, tertio-butanol). The experimental obtained results are given and described. (O.M.)