Operational model for the safety of defective spent fuel rods in dry storage

摘要

In dry storage, the breach of the container and cladding will expose the spent fuel to oxygen. The oxidation of UO_2 up to U_3O_8 leads to crystalline swelling and sample bulking that may increase the initial defect in the cladding and enhance the release of fission products. A model has been developed in order to describe the oxidation of spent fuel fragments. The oxidation model is able to reproduce the different weight gain curves provided in literature with diffusion coefficients of oxygen in the intermediate structural phases, consistent with literature data. Furthermore in-situ SEM investigations during oxidation of UO_2 grain has led to propose a new physically-based criterion for spent fuel degradation which is consistent with previous phenomenological criterion on its validity range. The time to achieve this criterion is calculated by using the oxidation model. The time before consequent fracturing of spent fuel in air, is thus deduced. Sensitivity and uncertainty analyses allow us to express this duration as a simplified analytical formula of the input parameters of the oxidation model, such as temperature of storage and spent fuel characteristics. The duration of safety can be qualitatively linked to irradiation parameters such as the burn up, the linear power or else the Fission Gas Release in reactor (FGR).