The use of tungsten in fusion reactors: A review of the hydrogen retention and migration properties

In the past the role of tungsten as a fusion reactor plasma-facing material has been fairly limited. It has appeared sparingly in tokamaks, but usually only for experimental purposes. This is likely to change in the future. Tungsten has a very high threshold for sputtering as well as a high melting point and high thermal conductivity. Applications of tungsten in areas where the energy of the plasma particles can be kept below the sputtering threshold removes the plasma impurity problem often associated with the use of tungsten in fusion reactors. In the area of recycling and retention, tungsten is unlike carbon and beryllium in that hydrogen appears to stay in solution in the metal (at least at low concentrations) and diffuse somewhat classically. This paper presents a review of the hydrogen isotope retention and migration properties of tungsten as they relate to fusion applications. The review is begun with an examination of past experiments on the diffusivity solubility, and permeability of hydrogen in tungsten. Fusion specific topics such as implantation and surface effects are then covered. Trapping is shown to be an important aspect of understanding hydrogen transport in this material. Blister and bubble formation are also addressed.