Abstract
A model has been developed to predict the long-term oxidation rate of Zircaloy-4 for ex-reactor (autoclave) and in-reactor (PWR) environments and operating conditions. A computer program has been written to solve the oxygen diffusion equation by employing a fully implicit finite difference method for a one dimensional cylindrical geometry. The moving boundary of the oxide is treated by deriving a time dependent relationship to control the speed of the interface motion. New diffusion coefficients for the oxygen in the oxide film have been developed. The proposed diffusion coefficients are a function of temperature and oxygen concentration gradient in the oxide and are derived for both the pre-transition and post-transition regions. To include the in-reactor corrosion behavior, additional factors have been considered, specifically the influence of the heat flux on the oxide-metal interface temperature, the fast neutron flux effect on the acceleration of the corrosion rate, and the effect of the lithium hydroxide concentration on the enhancement of the corrosion rate. This model allows the prediction of the long-term oxidation weight gain and the oxide thickness on Zircaloy-4 in operating conditions which simulate the PWR conditions. The results obtained for ex-reactor (autoclave) and in-reactor oxidation are compared with experimental data existing in the literature. The predictions are found to be in good agreement with the experimental data.