The development of materials which can survive the intense flux of neutrons emitted in the D-T fusion reactions is a unique challenge for 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
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, as well as hydrogen, inside a metal is particularly damaging for the structural integrity.
Extensive research is underway to find materials which can survive in the fusion reactor environment. The plot below indicates the status of materials development for EU DEMO from : Materials for in-vessel components . Although progress has been made on basic materials much more remains to be done to qualify materials for a full scale fusion reactor.