Tritium diffusion through oxide surface films on niobium

Diffusion of tritium in niobium has been investigated using recoil implantation with particular emphasis on the role of surface oxide films. In conformity with previously reported data, bulk diffusion in niobium is found to be extremely rapid and surface oxide films significantly retard the release rate of tritium. The observed tritium release data indicate diffusion through cracks or defects in the oxide film below 500°C and transport through an intact film at temperatures higher than 500°C. The release rates are described in terms of a mathematical model with the necessary model parameters determined from experimental data. The oxide films could be removed with a reducing hydrogen atmosphere, thereby resulting in classical bulk diffusion controlled release above 700°C. Below 700°C the oxide layers were stable in hydrogen, presumably because of a moderately high oxygen potential resulting from minute amounts of moisture in the sample chamber.