Zircaloy-4 Oxidation in Steam Under Transient Oxidizing Conditions

Abstract

Oxidation of Zircaloy-4 in unlimited steam has been studied under conditions simulating a loss of coolant accident (LOCA) using a specimen heating system (“Gleeble”). Simple linear ramp transients have been analyzed as well as a few postulated LOCA transients for currently operating light water nuclear power plants. Comparison is made between computer code prediction using isothermal oxidation kinetics and experimental observation with regard to the extent of oxidation. Simulation of Zircaloy oxidation assuming a stepwise isothermal model using isothermal kinetics developed by the authors is compared to experimental measurements of the extent of oxidation. This computer code, designated TRANS 1, accounts for prior oxidation present and results in agreement with experimental measurements of oxide and oxygen stabilized α-thicknesses for high heating and slow cooling rate transient profiles. However, as heating rate decreases and cooling rate increases, this model becomes increasingly conservative. Modeling, using either a series-expansion or a slope-based isothermal kinetics model, results in essentially the same prediction as TRANS 1. A one-dimensional finite-difference moving-boundary diffusion model has been developed and found to predict accurately isothermal oxidation behavior of Zircaloy-4 in steam. This model also can be used to predict transient behavior. In addition to predicting oxide and stabilized a thicknesses, this code is capable of predicting oxygen concentration gradients in all phases present, total oxygen as the summation of oxygen present in each phase, and formation of oxygen stabilized α-incursions in the β-phase if accurate diffusion coefficients and appropriate phase equilibrium information are known.