UCLA

A one dimensional numerical model is presented to predict oxide scale growth and failure in zirconium clad exposed to water in out-of-pile as well as in-pile conditions. A Stefan model coupled to diffusion kinetics tracks the interfaces between the two oxide sublayers formed on top of the metal clad. The model involves a temperature dependence to account for the thermal gradient inside the clad. A mechanical failure criterion incorporates the accumulation of compressive stresses in the oxide near the metal interface. In Chapter 2, the results of oxygen diffusion spanning the clad, time-dependent oxide formation and stress induced oxide fragmentation are presented. The results show that the oxide grows as the cubic root of time due to a charge distribution near the oxide interface. Alloying is capable of suppressing this charge distribution which accounts for square root growth in certain Zr alloys. A sensitivity study has shown that � 15% variations on relevant model parameters produce � 5% changes in model predictions.

In Chapter 3, the oxidation model adds the effect of radiation enhanced diffusion (RED). The RED assumes a linear formulation for low dose rates due to defect recombination while it assumes a quadratic formulation for high damage rates due because of defect annihilation. The results have found that the corrosion is accelerated and the oxide growth rate increases fivefold as a function of dose rate.

In Chapter 4, the effects of hydride formation are incorporated to the oxidation model. The hydride extension is coupled to the oxidation interface tracking model to follow hydrogen diffusion in the oxide and the metal. The terminal solid solubility for precipitation (TSSP) criterion accounts for hydride growth since hydrogen diffusion in the oxide is the rate-limiting step. The flux boundary condition in the water-oxide interface is incorporated to account for the ability of the oxide to impede hydrogen diffusion. This set of results found that hydride formation is possible above the TSSP while hydride growth is instantaneous in the metal. A desorption analysis has shown that effect of hydrogen resorption is minimal for hydride nucleation.