The influence of helium on the bulk properties of fusion reactor structural materials

Radiation-induced deterioration of fission reactor materials is dominated by displacement damage. In fusion reactors, the influence of (n, α) produced helium upon material deterioration is regarded to be of equal importance because of the high nuclear reaction rate caused by the high-energy fusion neutrons. In this review, the mechanisms and the anticipated rates of helium generation in fusion materials are discussed; helium introduction techniques simulating fusion conditions are reviewed in some detail and the atomistic behaviour of helium in metals as well as the nucleation and growth of helium bubbles are briefly surveyed. These phenomena are the main cause for the influence of helium on macroscopic material properties such as tensile strength, creep and fatigue behaviour and swelling. Typical examples of experimental results of material deterioration and first attempts at their theoretical modelling are given in the main part of the review. It is shown that helium effects can be the determining factor for the lifetime of fusion reactor components, particularly at high temperatures. The review concludes with an outlook on future investigations of helium effects and a call for a systematic approach in the development of helium-resistant alloys.