Degraded fuel cladding fractography and fracture behavior

An increase in the frequency of fuel degradation following a primary breach in boiling water reactor (BWR) rods was observed during the late-1980s and early 1990s; these rods were often found to contain axial splits. Several failed rods containing splits have been returned to the Vallecitos Nuclear Center (VNC) for pc,st-irradiation examination (PIE). This paper describes in detail the characteristics of fracture in these rods and relates the observations to postulated mechanisms of split formation. Crack initiation invariably occurred at localized hydrides well away from the primary defect. Crack: propagation was almost entirely brittle except for fracture of the zirconium liner, which was typically ductile. Fractography of the matrix exhibited nonsymmetrical chevron features with the crack front generally leading near the outside of the cladding. Detailed metallography demonstrated that cracks propagated through regions of both high and low (i.e., 150 ppm) hydride concentrations with the same brittle features. A modified Charpy test was developed to elucidate the effects of metallurgical and stress state variables on the crack phenomena and provide relevant fracture toughness measurements as a function of hydride level fur thin-walled tube specimens. A parallel series of tests examined the role of the environment on cracking behavior. The sum of the information provides valuable insight into the mechanism of split formation in degrading fuel rods.