At irradiation temperatures Ti 0.5Tm, where Tm is the absolute melting temperature, irradiation-induced atom displacement damage in steels is unstable, and the flow properties are generally unaffected by irradiation [1,2]. However, austenitic steels and other face-centered-cubic alloys are embrittled at elevated temperatures following or during neutron irradiation in thermal or fast reactors and helium implantation. This embrittlement is manifested as a reduction in tensile ductility [1-4], as shown in Fig. 13.1 for type 316 stainless steel [5]. Creep-rupture life and ductility [4,6-14] and fatigue endurance at low frequencies [12,14-16] are also affected in austenitic steels alloys. The irradiation does not significantly influence the deformation behavior but modifies the fracture process at high temperatures. Thus, the irradiation embrittlement is primarily associated with an increased tendency to intergranular fracture above about 0.5Tm.