Determination of dose rate effects on Zircaloy oxidation using proton irradiation and oxygen transport modeling

Abstract

While much of our knowledge about zirconium corrosion at high temperatures comes from out-of-pile experiments performed over the last several decades, understanding the behavior of Zr clad in light water reactor (LWR) conditions requires studying corrosion acceleration under irradiation. In-pile tests are slow and costly to perform, and only allow limited flexibility when it comes to exploring key parameters such as dose, dose rate, temperature, and alloy composition. In contrast, proton irradiations provide a good match for LWR dose rates and for the spatial uniformity of neutron irradiation, and thus constitute a very robust experimental platform from which to explore these parameters. In this work, we present an extension of a recently proposed Zr oxidation kinetic model that accounts for acceleration of oxide layer growth due to irradiation, accompanied by a set of controlled experiments carried out in a corrosion loop coupled to an accelerator beam line for parameterization and validation. The model includes radiation enhanced diffusion (RED) of oxygen in the oxide as the main effect of irradiation, and uses the experimental results of oxide layer growth as a function of dose rate to parameterize the RED coefficients. We show that these coefficients are strongly dose rate-dependent, which is quantitatively consistent with a negligible effect of RED on corrosion acceleration in the pre-transition regimes of samples irradiated under LWR conditions. We also find a smooth transition from columnar to equiaxed oxide grain growth as the proton irradiation dose rate increases.