The gradual degradation of material exposed to strong radiation, mainly neutrons. After a collision with neutrons, small displacements of atoms occur in the crystal lattice of the material, e.g. the wall of a reactor vessel. Changes in the crystal lattice can lead to changes in the material properties, such as embrittlement and an increased risk of crack formation. In order to guarantee the longevity of the reactor vessel, an alloy is sought which is as resistant as possible to radiation embrittlement. Since the degree of embrittlement of the reactor vessel cannot be measured, irradiated samples of the same material are examined and the neutron flux to which the vessel has been exposed is calculated. The irradiation damage rate is then expressed in terms of displacement per atom, which is the average number of shifts of an atom in the crystal lattice of the material. Radiation embrittlement is also a concern for the vacuum vessel of thermonuclear fusion facilities, where neutron fluxes are expected to be even higher than in nuclear reactors.