Steels are important structural materials in modern nuclear-power systems. Com­mon alloy systems include ferritic materials which have high resistance to radiation induced swelling, nickel-based alloys for high-temperature applications, and au­stenitic (stainless) steels which typically offer superior corrosion resistance. Typical applications are piping, pressure vessels, heat exchangers, steam generators, and general structural components. Some reactor vessels are made from a ferritic shell which is then clad with stainless steel for corrosion resistance. Structural integrity issues which have been investigated by neutron methods include stress corrosion cracking, weld and cladding cracking, and loss of creep strength and embrittlement due to exposure to temperature and radiation.

Hydrogen can be absorbed into metals, particularly at high temperatures. This interstitial hydrogen can significantly affect the strength and ductility of the material, leading to reduced structural integrity. Hydrogen flows along gradients of interstitial spacing, particularly the variations caused by temperature and stress. Thus it will flow to hot areas associated with welding, and to stress concentrations associated with crack tips. As a strong incoherent scatterer of neutrons, the hydrogen distribution in metals can be investigated with radiography and prompt — gamma neutron activation analysis.

A new class of steels, the oxide-dispersion strengthened steels, have recently been developed specifically for improved radiation tolerance. This material contains a fine dispersion of nano-sized, precipitate-like features which improve high-tem­perature creep properties and act as sinks for transmutation-produced helium, provide better void-swelling resistance and promote recombination of irradiation — induced point defects. Small-angle neutron scattering has proved to be a particularly useful technique for bulk characterisation of the nano-cluster distribution, volume fraction, shape, interface and size.