Thermal Diffusion of Hydrogen in Zircaloy-2 Containing Hydrogen beyond Terminal Solid Solubility

Abstract

The thermal diffusion of hydrogen is one of causes of uneven hydride precipitation in zircaloy fuel cladding tubes that are used in water reactors. In the diffusion model of hydrogen in zircaloy, the effects of the hydride on the diffusibility of hydrogen has been regarded as negligibly small in comparison with that of hydrogen dissolved in the matrix. Contrary to the indications given by this model, phenomena are often encountered that cannot be explained unless hydride platelets have considerable ostensible diffusibility in zircaloy.In order to determine quantitatively the diffusion characteristics of hydrogen in zircaloy, a thermal diffusion experiment was performed with zircaloy-2 fuel cladding tubes containing hydrogen beyond the terminal solid solubility. In this experiment, a temperature difference of 20"-30"C was applied between the inside and outside surfaces of the specimen in a thermal simulator.T o explain the experimental results, a modified diffusion model is presented, in which the effects of stress are introduced into Markowitz's model with the diffusion of hydrogen in the hydride taken into account. The diffusion equation derived from this model can be written in a form that ostensibly represents direct diffusion of hydride in zircaloy. T h e apparent diffusion characteristics of the hydride a t around 300°C are D,=2.3 ~10~exp(-32,000/RT), (where R : gas constant, T : temperature) and the apparent heat of transport Qp*= -60,000 cal/mol. The modified diffusion model well explains the experimental results in such respects a s reaches a steady state after several hours.

Behavior of Zircaloy Fuel Cladding Tubes

“…As the oxide film grows thicker, the cladding would become significantly affected in mechanical strength, with the reduction in effective clad wall thickness and increase in cladding temperature. Chapter vn will present the results of measurements made on (6) The apparent thermal conductivity of the oxide film formed on the surface of the cladding obtained by experiment.…”