MICROSTRUCTURE AND MACROSCOPIC ALTERATION AFFECTING NUCLEAR FUEL DURING EXTENDED STORAGE

Cumulative effects associated with the alpha-decay and radiogenic helium may affect the mechanical integrity of spent fuel rods during storage and handling/transportation after storage. Accelerated decay damage build-up studies performed on unirradiated alpha-doped UO2 indicate that swelling induced in the UO2 lattice would reach a maximum at a level of damage of approximately 1.2 displacement per atom (dpa). This alpha-damage level would be reached in used UO2 fuel after one or more centuries of storage, depending on the fuel burnup and composition. Other fuel properties affected by the alpha-decay damage, such as hardness and thermal conductivity, show maximum alteration at ~0.2-0.4 dpa, corresponding to storage times of decades. The evolution of the macroscopic properties correlates with microstructure changes caused by the decay process and by the accumulation of helium. At damage levels in excess of 3 dpa, evidence is shown that the fuel microstructure undergoes restructuring and some grains subdivision is observed. This paper describes the alterations in the UO2 microstructure and the corresponding macroscopic property changes induced by the alpha-decay process as a function of time, hence of accumulated damage and helium. The range of alpha-damage considered covers short, medium and long term storage. The applicability of results obtained under conditions of accelerated decay damage accumulation to the case of used fuel rod stored for different times is discussed on the basis of comparison with irradiated UO2 and MOX data. Gaps in the available knowledge and future studies are described. The final goal of these studies is to assess the mechanical integrity of nuclear and high level waste forms during and after extended storage in view of further handling/processing.JRC.E.2-Hot cell