Post irradiation examinations of twenty years stored spent fuel

Full text: The post irradiation examinations (PIE) of stored spent fuels were carried out to evaluate fuel integrity during storage. The spent MOX (Mixed Oxide) fuels irradiated in commercial BWR and the spent PWR-UO2 fuel irradiated in commercial PWR were used. The burnup of MOX fuels was about 20 MWd/kgHM and that of PWR-UO2 was about 58MWd/kgHM. Five fuels from MOX fuel assembly were stored under two different storage conditions for twenty years: Three fuel rods of them were stored under wet condition (in water). Other two rods were stored under dry condition (in air) in capsule after cutting to short length segments. For PWR-UO2 fuel, a fuel pin was stored under dry condition (in air) for twenty years. For MOX fuel, the pre-storage characterization data of fuels, which were symmetry position in the fuel assembly during irradiation, were used to compare with the post-storage PIE results. The following PIE items were carried out for MOX fuel in this study. For wet storage fuel : a) Visual inspection of the cladding outer surface, b) Puncture test. For dry storage fuel : c) Atmosphere gas analysis in capsule, d) Ceramographic examination to observe oxide layer thickness on outside/inside cladding and pellet microstructure such as grain size. For PWR-UO2 fuel, PIE items including cladding were carried out to evaluate fuel integrity during storage. The results show no marked difference after storage of MOX and PWR-UO2 fuels. However, further examination is required to conclude on spent fuel integrity during long term storage. (author)