Permeation of hydrogen through the materials used for nuclear fusion reactors, (1)

Discussion is made on the problems of the permeation of hydrogen through metals for nuclear fusion reactors, theoretical treatment and experimental methods of the hydrogen permeation. The most promising fuel cycle in nuclear fusion is deuterium-tritium cycle. Tritium is not only a very expensive isotope but is radioactive. Therefore, the permeation of tritium through fusion vessels must be prevented from the viewpoints of economy and safety. Besides the permeation, the brittleness of metals caused by hydrogen absorption is also hazardous. At present, stainless steel plated with gold is used most often. Tungsten and molybdenum are also used as limiters. Theory of the permeation of hydrogen is described on the subjects of solubility at equilibrium state, solubility in non-equilibrium state, diffusion, permeation, the effect of surface processes on the rate of permeation, and permeation through surface coating materials. The permeation of hydrogen through a material takes place in 5 steps, namely, adsorption, permeation of 1st stage, diffusion, permeation of 2nd stage, and desorption. For the experiment on hydrogen permeation, a vacuum system, a sample holder, a heating system, and a pressure gauge for hydrogen are necessary. The presence of water and oxygen in hydrogen largely affects the rate of permeation of hydrogen. The dew point of sample hydrogen must be below -600C. (Fukutomi, T.)