Hydrogen Permeation in Fusion-Relevant Materials

A viable nuclear fusion reactor will be operated with a magnetically confined hydrogen plasma as its fuel. The energy is gained by the fusing hydrogen isotopes deuterium (D) and tritium (T). These fusion processes take place in the core of the plasma at temperatures of 108 K. The significantly cooler outer regions of the D-T plasma will be interacting with the reactor inner walls. The wall materials must therefore withstand heat and particle loads from the plasma.
It is important to know as much as possible about the behaviour of these materials with respect to permeation of hydrogen isotopes in order to assess their suitability for containing and isolating the tritium fuel from the surrounding components. Of particular concern is the trapping and retention of tritium inside the reactor wall materials, and the possibility of tritium diffusing through the material, finding its way into the coolant water of the reactor components where it would pose a potentially serious environmental hazard.
Although the precise mechanism is not understood, it is well-known that trapped hydrogen reduces the ductility of many materials, including steels, a phenomenon known as hydrogen embrittlement or hydrogen-induced cracking. For safety and operational reasons it is important to understand and mitigate the potential damage that could be caused to key components in a fusion reactor, including the large number of diagnostic ports needed in experimental fusion reactors such as ITER.
The study of hydrogen permeation in these materials is further complicated by the elevated temperatures present under normal operation of a reactor and the anticipated damage they will suffer due to irradiation by the energetic neutrons produced in the fusion reaction. Also of relevance is the nature of the material surface, which is altered by interaction with the plasma through erosion and preferential sputtering of certain component atoms.
The overall objective is to provide evaluated experimental data on hydrogen permeation in fusion-related in-vessel materials. Furthermore, the data will be used in the benchmarking of modelling codes for hydrogen permeation in fusion-relevant conditions.