In-reactor degradation of fuel and cladding in fuel pins operated with weld defects

A PHWR fuel pin having an incomplete fusion defect in the end plug weld operated in the reactor in the failed condition for a period of 710 days and accumulated a burnup of 4400 MWd/tU. Detailed non destructive and destructive PIE was carried out on the fuel pin to understand the nature and extent of degradation of fuel and cladding in this fuel pin. PIE included visual examination, ultrasonic testing, metallography of a large number of transverse sections of the fuel pin, beta gamma autoradiography, microhardness measurement of the cladding, ring tension test on the cladding. Examination showed secondary hydriding failures, significant rise in fuel temperature resulting in extensive fuel restructuring, severe oxidation of fuel and cladding by steam providing hydrogen for hydride (deuteride) blister formation. Increase in hardness and loss of ductility was also observed in the cladding.