Diffusion of gases in solids. Tritium diffusion in stainless steel, Zircaloy-2, and niobium. Period covered: November 1, 1972--October 31, 1973

Abstract

A surface effect which retards the release of recoil-injected tritium was observed for austenitic stainless steel, Zircaloy-2, and niobium. Tritium release from the surface is characterized by diffusion coefficients which are lower than the bulk diffusion coefficients by approximately two orders of magnitude in stainless steel and seven or eight orders of magnitude in Zircaloy-2 and niobium. The transport rate through the surface films appears to be the rate- limiting process for transport through thin specimens of the latter two materials. The effect appears to be associated with the transport through surface oxide films and is not simply a surface-absorption, desorption process. A two-region diffusion model was used to calculate apparent diffusion coefficients and activation energies for transport in the surface films. The surfacelayer diffusion coefficients determined for niobium exhibit a change in activation energy at temperatures above 500 deg C, an effect attributed to the dissolution of the oxide film at high temperatures. The niobium-release results are interpreted in terms of several possible models, and it is shown that the data can fit each model, thereby yielding tritium diffusion coefficients within the surface film and through the film. A three-region diffusion model, which includes the effect of surface filmsmore » at both specimen faces, was applied to stainless steel and Zircaloy-2 reactor fuel cladding using typical reactor temperatures and times. Predicted tritium release was high for stainless steel- clad elements and low for Zircaloy cladding, in agreement with experimental observation. Tritium diffusion coefficients within the zirconium-oxide surface films on Zircaloy-2 were observed to be essentially independent of oxide film thickness. (16 figures, 30 references) (auth)« less