All materials have problems with fusion neutrons. The neutrons from DT fusion are sufficiently energetic to cause (n,alpha) and (n,p) reactions, causing hydrogen and helium gas to accumulate. Helium in particular is a problem, because it collects into tiny very high pressure bubbles that rip the material apart from the inside. Simple elastic scattering of neutrons off the atoms cause them to scatter many atomic diameters in the material, scrambling its crystalline structure.

There are also just a few elements that do not produce unacceptably long lived radioisotopes under fusion neutron bombardment. This greatly limits the choice of elements from which to make the reactor structure. Right now, the best choice is RAFM steel, but it has a number of serious drawbacks.