Figure 4 Qualitative Representation of Neutron Irradiation Effect on Many Metals

EFFECT DUE TO NEUTRON CAPTURE DOE-HDBK-1017/2-93 Plant Materials Figure 4 Qualitative Representation of Neutron Irradiation Effect on Many Metals For stainless steel exposed to a thermal reactor fluence of 10²¹ neutrons/cm², the tensile properties show some increase in ultimate strength (tensile strength), an almost threefold gain in the yield strength, and a drop of about one third in ductility (elongation), as shown in Table 2. The Nil-Ductility Transition (NDT) temperature, which is the temperature at which a given metal changes from ductile to brittle fracture, is often markedly increased by neutron irradiation. The increase in the NDT temperature is one of the most important effects of irradiation from the standpoint of nuclear power system design. For economic reasons, the large core pressure vessels of large power reactors have been constructed of low carbon steels. The loss of ductility and increase in the NDT temperature of these vessels is a primary concern to reactor designers because of the increased chance of brittle fracture. Brittle fracture of a material is a failure occurring by crystal cleavage and accompanied by essentially no yielding. A brittle fracture of a pressure vessel resembles the shattering of glass. Since such a failure would be disastrous, it is necessary to understand the brittle fracture mechanism. During normal reactor operation, the pressure-vessel steel is subject to increasing fluence of fast neutrons and, as a result, the NDT temperature increases steadily. The NDT temperature is not likely to increase sufficiently to approach the temperature of the steel in the pressure vessel. However, as the reactor is being cooled down, the temperature of the vessel may drop below the NDT value while the reactor vessel is still pressurized. Brittle fracture might then occur. MS-05 Page 40 Rev. 0