Oxide growth and oxygen transport in proton irradiated Zircaloy-4 oxide layers

In order to investigate the impact of irradiation damage on oxygen transport and oxide growth in Zircaloy-4, two sets of Zircaloy-4 samples were corroded in flowing autoclaves containing simulated PWR coolant for 52 and 131 days at 350 °C before being proton irradiated to 0.25 and 0.75 dpa at 350 °C and returned to a static autoclave containing water spiked with H218O for 40 days at 320 °C. This produced two sets of irradiated and corroded oxides, one pre-transition and one immediately before/immediately after transition, depending on local conditions in regions of the sample. These samples were then analysed using NanoSIMS to assess how the oxide morphology, oxide thickness and oxygen diffusivity varied with irradiation, damage levels and time in autoclave. In pre-transition oxides, irradiation was observed to increase both oxide diffusivity and oxide thickness, with small isolated regions containing large amounts of cracking perpendicular to the oxide-metal interface and greatly increased local oxide thickness. In contrast, for post-transition oxides irradiation damage resulted in slower oxide growth through exacerbated cracking parallel to the oxide-metal interface, leading to spallation in the 0.75 dpa sample.