Key issues from the French R and D project on the long term evolution of the spent nuclear fuel in conditions of interim dry storage

Some of the objectives of the R and D PRECCI project, performed in CEA with the support of EDF and FRAMATOME, are to qualify and quantify the long term evolution of the spent nuclear fuel under conditions of dry storage and its consequence on potential radionuclides release. Chemical and physical states evolution of the spent fuel in a closed system have been studied. In this case, no chemical evolution and no microscopic swelling are expected to occur in the long term. Radionuclides mobility and grain boundaries transport properties need today to be more investigated. Two creep models have been developed for irradiated Zircaloy-4. They take into account the effects of stress, temperature and deformation on the creep rate. A new creep rupture criterion depending on the irradiation dose has also been proposed. The effect of a container failure on the evolution of the spent fuel after the cladding breaching is being investigated. The nucleation-growth process classically assumed for the oxidation from U3O7 to U3O8 has not been observed. Thus, a new modelling approach of the spent fuel oxidation and subsequent radionuclides release is under development. (author)