Fusion Science and Technology / Volume 52 / Number 4 / November 2007 / Pages 990-994
Technical Paper / Tritium, Safety, and Environment
Niobium is a potential candidate of tube material in vacuum permeator for tritium recovery from Pb-17Li liquid blanket system. From this viewpoint, the permeation of hydrogen through a Nb membrane was investigated with an ultra-high vacuum apparatus under the conditions relevant to the blanket system where no oxide films could be retained on the membrane surfaces. It was, however, found that the permeation rate sharply decreased with increasing oxygen concentration in the bulk of membrane; at upstream H2 pressure of 1 Pa and membrane temperature of 700°C, for example, the permeation rate at oxygen concentration corresponding to oxygen potential in Pb-17Li (0.054 at%) was evaluated to be 1/5 of the value expected from hydrogen solubility and diffusivity in Nb. Such small permeation rate was ascribed to the presence of oxygen monolayer formed by surface segregation from the bulk. Surface modification by Pd coating was found to give only limited improvement due to degradation in coating effect induced by interdiffusion between Pd and Nb. Methods to improve the high temperature stability of Pd coating was discussed.