The Modulated Permeation Technique Used at the Open University

Abstract

The permeability of hydrogen isotopes in first wall materials is a crucial research subject in fusion reactor technology [1]. Preventing the loss of tritium is required to increase the economic viability of the fusion reactors and also permeation of tritium to the surroundings poses a radiological hazard. Recent strategies to reduce hydrogen permeation combine low diffusivity/permeability coatings on a mechanical substrate. The coatings can be put into two categories; one is the compact oxidation films formed by oxidation of metal construction materials, and the other is the thin films created by chemical vapor deposition, ion sputtering, ion implanting, plasma assisted deposition etc. TZM, a molybdenum alloy with small additions of Ti and Zr, owing to its very high tensile strength and high thermal and electrical conductivity, has been proposed as a candidate divertor material for future fusion reactors. A rather large amount of permeation data, showing wide scatter, particularly in solubility and diffusivity of hydrogen in molybdenum, suggest that the permeation process may be convolved with processes other than pure lattice transport [2—4]. Recent reports on hydrogen permeation in TZM show that the permeation data are not so different from those of pure molybdenum. It is claimed that slow surface processes and/or trapping of hydrogen is important in these materials [5—7].