Abstract
10x10 BWR segmented rods with a burnup of about 30 MWd/kgU were subjected to high power irradiation tests in the R2 reactor. Metallography on the cladding showed small incipient cracks with a depth of 10mm at the inner surface of the cladding liner after the irradiation tests. In order to investigate the incipient cracks, several post irradiation examinations were performed. Local micro-hardness of the liner was measured with an indentation force of 1 g. The results showed that the hardness was significantly increased at the inner surface facing the fuel. Electron probe micro-analysis (EPMA) of the liner gave an elemental profile showing an increased amount of fission products at the inner surface and the profile agreed with the micro-hardness profile. Scanning electron microscopy showed propagation of the incipient cracks along grain boundaries. Elemental mapping with EPMA showed concentrated cadmium at the incipient cracks. Considering these observed facts, the cause of the crack formation could be stress corrosion cracking (SCC). Applied hoop stress history to the liner during the irradiation test was calculated with a fuel performance analysis code. Based on the results of the observation and calculation, the process of the crack formation and propagation was discussed.
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