EPRI--NASA cooperative project on stress corrosion cracking of Zircaloys. Final report

The results of research aimed at developing improved understanding of the stress corrosion cracking (SCC) mechanism considered responsible for pellet-cladding interaction (PCI) nuclear fuel failures are presented. PCI failures originate at the fuel side of the Zircaloy cladding, and therefore, the major objective was to define the SCC mechanism of Zircaloy in environments relevant to those at the inside surface of operating fuel cladding. The results of examinations of the inside surfaces of irradiated fuel cladding from two power reactors showed that Zircaloy cladding can be exposed during service to a number of potentially aggressive substances. Zircaloy samples were screened at reactor operating temperatures for susceptibility to SCC induced by such substances. The most aggressive were iodine, cadmium, and iron-contaminated cesium. Detailed studies were made of iodine-induced SCC of well-characterized samples of Zircaloy sheet and tubing. The results on pressurized, cladding-grade tubes indicated that a threshold stress must be exceeded for iodine SCC to occur. The threshold stress was sensitive to the microstructure of the Zircaloy and probably to in-reactor irradiation, but was not affected by the presence of an oxide film or by iodine concentration within the range studied. The existence of a threshold stress indicates that crack formationmore » probably is the key step in iodine SCC. A detailed investigation of the crack formation process showed that the cracks responsible for SCC failure nucleated at locations in the metal surface that contained higher than average concentrations of alloying elements and impurities. A four-stage model of iodine SCC is proposed based on the experimental results and the relevance of the observations to PCI failures is discussed.« less