Materials in nuclear reactors are subjected to conditions and environments that are new in scope and magnitude, and the designer must consider a number of unique factors such as neutron cross-section, radiation heating, and unusual corrosive media. In addition, as numerous recent papers (1–12) have pointed out, neutron irradiation is capable of causing substantial changes in the engineering properties of metals. Because metals are generally less sensitive to radiation effects than other classes of engineering materials, only the metals in the high flux parts of a nuclear reactor will be exposed to enough radiation to cause property changes of significance. Therefore if special ASTM standards are ever written to take into account the effects of irradiation, they need only apply to a very small part of a nuclear power plant.