Stress corrosion crack initiation of Zircaloy-4 cladding tubes in an iodine vapor environment during creep, relaxation, and constant strain rate tests

During accidental power transient conditions with Pellet Cladding Interaction (PCI), the synergistic effect of the stress and strain imposed on the cladding by thermal expansion of the fuel, and corrosion by iodine released as a fission product, may lead to cladding failure by Stress Corrosion Cracking (SCC). In this study, internal pressure tests were conducted on unirradiated cold-worked stress-relieved Zircaloy-4 cladding tubes in an iodine vapor environment. The goal was to investigate the influence of loading type (constant pressure tests, constant circumferential strain rate tests, or constant circumferential strain tests) and test temperature (320, 350, or 380 °C) on iodine-induced stress corrosion cracking (I-SCC). The experimental results obtained with different loading types were consistent with each other. The apparent threshold hoop stress for I-SCC was found to be independent of the test temperature. SEM micrographs of the tested samples showed many pits distributed over the inner surface, which tended to coalesce into large pits in which a microcrack could initiate. A model for the time-to-failure of a cladding tube was developed using finite element simulations of the viscoplastic mechanical behavior of the material and a modified Kachanov's damage growth model. The times-to-failure predicted by this model are consistent with the experimental data

Development and validation of a new experimental device for studies of iodine stress corrosion cracking of zirconium alloys

International audienceZirconium alloys are widely used in the nuclear industry as cladding material due to their good thermo-mechanical, chemical and neutronic properties as well as an excellent general corrosion resistance in many environments. Nonetheless, Iodine-induced Stress Corrosion Cracking (ISCC) is an identified failure mechanism that can occur in nuclear fuel cladding as a result of Pellet Cladding Interaction (PCI) during incidental power transients in the reactor. In this study a new experimental device allowing studies of ISCC in a controlled chemical environment is presented. The experiments are performed in an environment that is free of contaminants. First results show that the time to rupture of the specimen is inversely related to the iodine partial pressure. The lowest tested iodine partial pressure, 150 Pa, resulted in an ISCC crack. Post-Mortem microstructural observations provide evidence of iodine corrosion of Zircaloy-4