Photon irradiation effects on oxide surface electrochemistry and oxide microstructure of zircaloy 4 in high-temperature water

Although there exists a correlation between autoclave and in-reactor zirconium alloy performances, consistent oxidation kinetics discrepancies in these two environments have been observed and a fundamental understanding of the oxidation kinetics enhancement under irradiation is still lacking. Recent results obtained at the Advanced Test Reactor by the Naval Nuclear Laboratory show that photon irradiation significantly affects zirconium corrosion kinetics. In reactors, various photon sources are present in the core from ultraviolet (UV) to gamma (c) rays. This study aims at characterizing the effect of UV and c rays on the corrosion mechanism of Zircaloy-4. To this end, a state-of-the-art autoclave equipped with sapphire windows and connected to a recirculation loop has been installed. Zircaloy-4 coupons were exposed for 7 days at 260^◦C with and without recirculation or UV irradiation (or both). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) oxide characterizations show the presence of iron (Fe)-rich oxide deposits on top of the zirconium oxide where the sample has been irradiated by UV. The deposit concentration is larger in the static corrosion case and does not significantly influence the zirconium oxidation kinetics. A mechanism is proposed to explain the nucleation of these deposits and the relationship to Chalk River Unidentified Deposit nucleation is discussed. In another experiment, Zircaloy-4 coupons have been irradiated at the MIT reactor in neutronþgamma, gamma, and unirradiated loop conditions. The in-core specimens were exposed to ~10²¹ n/m² fast neutron fluence in 290^◦C water at 7 MPa. Oxide layers have been characterized by SEM and TEM. The oxide grain size, t-ZrO₂ fraction, fiber texture, and m-ZrO₂ twin boundaries’ density were characterized. The results indicate that, at low dpa, the neutron þ c irradiated sample has a more protective oxide than the c-irradiated sample, which has a more protective oxide than the nonirradiated sample.