Reduction in the onset time of breakaway oxidation on Zircaloy cladding ruptured under simulated LOCA conditions

Breakaway oxidation, which is characterized by sudden increases in oxidation rate and significant hydrogen absorption after longer exposure in high-temperature steam, was examined for the ballooned and ruptured cladding. Short test rods with PWR-type Zircaloy-4 cladding were heated in steam and oxidized at 1273 K for periods from 1200 to 3600 s after ballooning and rupture. The breakaway oxidation was observed after more than 1500 s in the ruptured cladding whereas it was observed after more than 3600 s in the non-ruptured cladding at this temperature. The cause of the reduction in the onset time of breakaway oxidation was investigated with changing possible influential parameters. The onset time of breakaway oxidation was reduced only in the case where thick and porous oxide was formed on the inner surface of the cladding after rupture. Results of the parametric experiments suggest that the tensile stress in the circumferential direction generated by the thick oxide film formed on the inner surface is a main cause of the reduction in the onset time of breakaway oxidation. (C) 2013 Elsevier B.V. All rights reserved.