The fracture toughness of AISI 304 and 316L stainless steels and their welds irradiated to 7 × 10²⁵ neutrons (n)/m² (50 percent of this was at E > 0.1 MeV) was investigated on 10- and 20-mm-thick three-point bend specimens. Measurements of critical values of the J-integral, Jc, and the crack-opening displacement, δc, defined at the onset of stable crack growth, were performed at 20 and 400°C. The electric potential method was adapted to hot-cell operation for the detection of the initiation point. Drastic reductions of fracture toughness occur when combining welding, irradiation, and high temperature. As an example, the value δc = 0.51 mm of base-material nonirradiated specimens tested at 20°C drops to 0.011 mm for welded irradiated specimens tested at 400°C. The consequences on liquid metal fast breeder reactor (LMFBR) structural design are illustrated in the case of the French LMFBR Super Phénix.