In order to preserve the conditions for an environmentally safe machine, at present the selection of materials for the structural components of fusion reactors is made not only on the basis of adequate mechanical properties, behaviour under irradiation, and compatibility with other materials and cooling media, but also on their radiological properties, i.e. radioactivity, decay heat and radiotoxicity. These conditions strongly limit the number of suitable materials to a few families of alloys, generically known as low activation materials. The criteria for making decisions about such materials, the alloys resulting from the application of these ideas and the main issues and problems with their use in a fusion environment are discussed.

Reduced activation ferritic/martensitic (RAFM) steels are the reference as structural materials for the future fusion reactors. They have proven to be a good alternative to austenitic steels for their higher swelling resistance. However, RAFM steels exhibit irradiation-induced low temperature hardening and increase in the ductile-to-brittle transition temperature, which imposes a severe restriction on their reactor applications at temperatures below 300°C. Furthermore, a high density of small helium bubbles has been recently evidenced in specimens proton-irradiated at about 300°C to a dose of 10 dpa, which could affect their fracture mechanical behavior at intermediate temperatures. Their temperature window of use is presently limited by a drop in mechanical strength at about 600°C. So, new variants that can better resist at high temperatures, are currently being developed, mainly using a stable oxide dispersion. The potentiality of using present RAFM steels and the variants being developed for the first wall of future fusion reactors are reviewed below.

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