Tritium Release from Niobium

Tritium diffusion measurements in niobium were carried out over the temperature range 400 to 950°C by direct measurements of both concentration profiles and surface release rates. The ⁶Li(n,α)³H reaction was used to inject tritium into the specimens and produce an initial tritium atom fraction lower than 0.01 ppm. The concentration profiles showed a high surface concentration in a surface region 1 to 2 µm thick and a nearly flat bulk diffusion profile deeper into the sample. Surface release rate measurements of tritium verified the existence of a surface trapping layer. The surface trapping was attributed to oxide films formed at room temperature. The surface release data were analyzed using diffusion models to determine tritium diffusion coefficients within the surface film and the diffusion coefficients controlling release from the bulk through the film. The tritium diffusion coefficients within the surface film are about eight to ten orders of magnitude lower than the bulk diffusion coefficients. Between 600 and 900°C, the film barrier to tritium diffusion appears to change and surface layer diffusion coefficients approach the bulk diffusion coefficients at higher temperatures.