Investigating iodine-induced stress corrosion cracking of zirconium alloys using quantitative fractography

Pellet Cladding Interaction (PCI) is a fuel failure phenomenon that limits power manoeuvrability of light water nuclear reactors with Iodine-Stress Corrosion Cracking (I-SCC) the most commonly hypothesised cause. To increase mechanistic understanding, I-SCC tests have been performed by compressing zirconium alloy c-rings at fixed load while submerged in iodised ethanol. Samples left until failure produced fracture surfaces subsequently analysed using quantitative Scanning Electron Microscopy (SEM) fractography, and samples with cracks arrested part way produced incipient cracks assessed by optical imaging. The influence of microstructure, stress and iodine concentration on I-SCC performances has been assessed by comparison between cold-worked and recrystallised material. Increasing the stress resulted in faster time to failure and produced smaller regions of intergranular cracking; explained using the stress intensity factor and its control over the prevalent mechanism, further evidenced by tests that removed either iodine or stress part way through cracking. Increasing iodine concentrations resulted in faster times to failure and more intergranular cracking indicating the importance of an incubation period and a possible crack blunting effect. Cracks in cold-worked material progress at a non-radial angle, whereas cracks within recrystallised and stress-relieved material cracks progress radially. Analysis suggests that the mechanisms that lead to this difference may be related to grain morphology rather than residual stress as is typically suggested in the literature, with fractography and incipient cracks used as evidence.