No abstract

The formation of elongated zirconium hydride platelets during corrosion of nuclear fuel clad is linked to its premature failure due to embrittlement and delayed hydride cracking. Despite their importance, however, most existing models of hydride nucleation and growth in Zr alloys are phenomenological and lack sufficient physical detail to become predictive under the variety of conditions found in nuclear reactors during operation. Moreover, most models ignore the dynamic nature of clad oxidation, which requires that hydrogen transport and precipitation be considered in a scenario where the oxide layer is continuously growing at the expense of the metal substrate. In this paper, we perform simulations of hydride formation in Zr clads with a moving oxide/metal boundary using a stochastic kinetic diffusion/reaction model parameterized with state-of-the-art defect and solute energetics. Our model uses the solutions of the hydrogen diffusion problem across an increasingly-coarse oxide layer to define boundary conditions for the kinetic simulations of hydrogen penetration, precipitation, and dissolution in the metal clad. Our method captures the spatial dependence of the problem by discretizing all spatial derivatives using a stochastic finite difference scheme. Our results include hydride number densities and size distributions along the radial coordinate of the clad for the first 1.6 h of evolution, providing a quantitative picture of hydride incipient nucleation and growth under clad service conditions.

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.