[en] 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.g_UO2^-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.)