A model for the oxidation of zirconium-based alloys

Abstract

A model has been developed to predict the oxidation rate of zirconium-based alloys exposed to hot water such as in a nuclear power reactor. The model assumes that the migration of oxygen ions along the grain boundaries of the oxide from the water coolant to the oxide-metal interface is rate controlling. As the oxide thickens, a self-generated biaxial stress (due to atomic mismatch between the oxide and the metal) is developed in the oxide which interacts with the activation volume of the oxygen ion to limit the oxygen diffusion coefficient. The above interaction results in a change in the slope of the plot of log weight gain versus log time from one half as observed at low weight gains to one third observed at weight gains between approximately 10 mg/dm2 to 30 mg/dm2 at PWR operating temperatures. Such a model also explains the slope of one half observed by Bradhurst and Heuer at high temperatures (where the stresses in the oxide are observed to be much less). At weight gains above 30 to 40 mg/dm2 the weight gain accelerates and becomes nearly linear with time. At this point (called transition) it is assumed that the water coolant reaches the oxide-metal interface through pores and/or cracks in the oxide and the above process repeats itself. A computer program has been developed based on the above model, and comparisons made in the pre-transition region with data are good. © 1983.