Aside from materials exposed to plasma and large heat fluxes, structural materials have to be chosen to support the huge weight of the reactor elements — the vacuum chamber, magnetic coils, breeding blankets, and so forth. Normally one would use steel; but for fusion, the type of steel has to be carefully designed. The neutrons bombarding the structure will make it radioactive. Only the following elements can be used: iron, vanadium, chromium, yttrium, silicon, carbon, tantalum, and tungsten. Elements like manganese, titanium, and niobium used in other steels would result in long-lived radioactive isotopes. Two Reduced Activation Ferritic Martensitic Steels have been designed: Eurofer (in Europe) and F82H (in Japan). These have the following additives to iron [4]:

These steels have only short-lived radioactivity and, unlike fission products, are nonvolatile and can be re-used after storage for 50-100 years. The amount of swelling under neutron bombardment is much smaller than for ordinary stainless steel. Swelling and embrittlement come from helium and hydrogen bubbles trapped in the steel. There are experimental oxide dispersion strengthened (ODS) steels which have nanoparticles of Y2O3 that can trap helium and hydrogen, strengthen the material, and reduce creep. Though much has to be done to manufacture these materials with low impurity levels, to study their welding properties, and to test their limits in temperature and radiation resistance in full­time operation, structural materials are not one of the worrisome problems in fusion technology.

Figure 9.8 shows the predicted radioactivity of Eurofer and SiC in a fusion reactor after 25 years of full-power operation. Note that the scales are logarithmic, so that each vertical division represents a factor of 10, and each horizontal division a factor of 100. After 100 years, the radioactivity has decayed by a factor of almost 1,000,000. This material is solid and will not leak out of its containers. The main danger from radioactivity comes from tritium, which decays in 12 years and will be considered in detail later. Note that even this small amount of radioactivity compared with fission is caused by the fact that the D-T reaction emits energetic neutrons. In second-generation fusion reactors using advanced fuels there will be almost no radioactivity.