The environment inside a fusion reactor is uniquely harsh with high temperatures and high radiation fluxes from the fusion reaction. Materials have to be chosen carefully and the research for this is ongoing.

Materials for Fusion Reactors

The choice of materials has many considerations: mechanical property evolution during operation; transmutation and neutron damage; ion impact and sputtering; thermal properties; oxidation behaviour during air or water ingress; as well as the issue of fuel permeation and diffusion. Future fusion reactor designs now concentrate on tungsten for plasma facing surfaces and special steel alloys for structures. The figure below, from Fusion Material, Coenen, indicates how well tungsten satisfies the conditions above. Tungsten is intrinsically brittle, so work on tungsten composites is underway. The ferritic/martensitic steels suffer less than other alloys with the problems of : swelling, inferior thermal properties, helium embrittlement and microstructural instabilities under neutron irradiation and so have been adopted for fusion reactor designs. A commercial alloy of the sort is called Eurofer.

Neutron Flux

The intense flux of neutrons emitted in the D-T fusion reactions is the major challenge for materials in fusion reactors. Although fission reactors are subject to radiation damage, the much higher energies of the neutrons from fusion (14 MeV) compared to fission (<2 MeV) means the problems are different, and much worse, for materials in fusion reactors.

Comparison of neutron flux in fusion and fission reactors of similar power

Radiation Damage in Lattice of Solid Material

The figure below, taken from Knaster et al. , illustrates the multiple possible interactions of an energetic neutron with the lattice of a material. These processes range from primary knock-on atoms (PKA), displaced by a neutron, to transmuted atoms, where the nucleus is converted to that of a different element following absorption of the neutron. The production of helium inside a metal is particularly damaging for the structural integrity. 

Radiation damage is quantified as displacements per atom (dpa) and the production of helium is quantified as atoms parts per million(appm). The plot below shows that both quantities are expected to be higher in fusion reactors compared to fission reactors. 

Radiation damage effects for fusion and fission reactors with indications of regions accessible with testing facilities, like MTS at Los Alamos and IFMIF in construction in Japan.

Needs for Materials Research

The conclusion of basically all fusion materials research, is that it is essential to have appropriate test facilities which produce a neutron flux with the same energy and intensity as a fusion reactor to develop and test new candidate materials. The page Essential Test Facilities discusses the proposals.