Microstructural examination of fuel rods subjected to a simulated large-break loss of coolant accident in reactor

A series of tests have been conducted in the National Research Universal (NRU) reactor, Chalk River, Canada, to investigate the behavior of full-length 32-rod PWR fuel bundles during a simulated large-break loss of coolant accident (LOCA). In one of these tests (MT-3), 12 central rods were prepressurized in order to evaluate the ballooning and rupture of cladding in the Zircaloy high-α/α + Β temperature region. All 12 rods ruptured after experiencing < 90 pct diametral strain, but there was no suggestion of coplanar blockage. Postirradiation examination was carried out on cross sections of cladding from selected rods to determine the azimuthal distribution of wall thinning along the ballooned regions. These data are assessed here to check whether they are consistent with a mechanism in which fuel stack eccentricity generates temperature gradients around the ballooning cladding and leads to premature rupture during a LOCA. After anodizing, the cladding microstructures were examined for the presence of prior-beta phase that would indicate the α/α + Β transformation temperature (1078 K) had been exceeded. These results were compared with isothermal annealing test data on unirradiated cladding from the same manufacturing batch.