The alloys used for fuel and structural components in nuclear reactors suffer intense bombardment by damaging irradiation. For example, there are components in which every atom may be knocked off its lattice site and replaced by another several times during its lifetime in the core. It is not surprising, therefore, that much of the effort of nuclear metallurgists has been devoted to studying the rather esoteric subject of irradiation damage in metals and alloys. In the event, with thermal nuclear reactors at least, it has been relatively common metallurgical and chemical phenomena, such as thermal creep, fatigue, and corrosion, that have caused the majority of the difficulties.