Studies on spent nuclear fuel evolution during storage

Initially conceived to last only a few decades (40 years in Germany), extended storage periods have now to be considered for spent nuclear fuel due to the expanding timeline for the definition and implementation of the disposal in geologic repository. In some countries, extended storage may encompass a timeframe of the order of centuries. The safety assessment of extended storage requires predicting the behavior of the spent fuel assemblies and the package systems over a correspondingly long timescale, to ensure that the mechanical integrity and the required level of functionality of all components of the containment system are retained. Since no measurement of ''old'' fuel can cover the ageing time of interest, spent fuel characterization must be complemented by studies targeting specific mechanisms that may affect properties and behavior of spent fuel during extended storage. Tests conducted under accelerated ageing conditions and other relevant simulations are useful for this purpose. During storage, radioactive decay determines the overall conditions of spent fuel and generates heat that must be dissipated. Alpha-decay damage and helium accumulation are key processes affecting the evolution of properties and behavior of spent fuel. The radiation damage induced by a decay event during storage is significantly lower than that caused by a fission during in-pile operation: however, the duration of the storage is much longer and the temperature levels are different. Another factor potentially affecting the mechanical integrity of spent fuel rods during storage and handling / transportation is the behavior of hydrogen present in the cladding. At the Institute for Transuranium Elements, part of the Joint Research Centre of the European Commission, spent fuel alterations as a function of time and activity are monitored at different scales, from the microstructural level (defects and lattice parameter swelling) up to macroscopic properties such as hardness and thermal conductivity. In order to reproduce cumulative damage effects expected after very long storage time within acceptable laboratory timescales, accelerated damage build-up conditions are applied, e.g. by using unirradiated (U,Pu) oxide with high specific alpha-activity (alpha-doped UO₂). The measurements performed so far show that saturation of macroscopic hardness and thermal conductivity occurs for a simulated timescale corresponding to spent fuel after decades or centuries of storage (the exact time scale depends on parameters like fuel composition and burnup). The outcome of the studies also show a non-negligible extent of lattice swelling, peaking at an accumulated damage level of ∝1 dpa. The behavior observed in tailor-made samples is compared to actual high burnup fuel (UO₂, MOX), to verify extent and significance of eventual macroscopic alterations of spent fuel during long-term storage. This paper will present the main outcome of the extended storage studies performed on alpha-doped UO₂ and on spent fuel. The experimental approach to investigate the possible impact of spent fuel rods alterations on their retrieval after extended storage, and the expected behavior under accident conditions will be described. Extrapolations to very long timeframes corresponding to spent fuel evolution after emplacement in the geologic repository will be illustrated.