Specific outcomes of the research on the spent fuel long-term evolution in interim dry storage and deep geological disposal

This paper presents an overview of the main results of the French research on the long-term evolution of spent fuel. The behavior of the spent fuel rods in the various conditions likely to be encountered during dry storage and deep geological disposal, i.e., in a closed system, in air and in water were investigated. It appears that in a closed system the effects of helium production on the mechanical stability of grain boundaries remain the major unanswered question. In air, microscopic characterization of the UO2 oxidation leads to introduce a new phase in the classical oxidation scheme. The limiting step assumption on which the oxidation kinetics are based is only partially valid. In water, the effect of the alpha radiolysis which accelerates UO2 dissolution was demonstrated for anoxic conditions. However this effect could be counteracted by the environmental conditions, such as the presence of H2 produced by the container corrosion. The effects of the environmental parameters on the fuel matrix dissolution still need to be assessed.