AbstractThe post-irradiation weldability of neutron-irradiated type 316 stainless steel has been examined. The tungsten inert-gas arc welded joints were made of type 316 stainless steel used as wrapper tube in the JOYO fast reactor at 668–683 K to a maximum dose and helium content of about 22 dpa and 9 appm respectively. Large loss of tensile ductility was observed in welded joints made of the irradiated steels, being accompanied with intergranular brittle fracture in the heat affected zone at ambient temperature (about 293 K) and at 773 K, although ductile fracture of unirradiated specimens was observed in the weld metal region. The brittle fracture is closely related to helium bubble formation on grain boundaries during the welding.