HT-9, a 12 percent chromium martensitic stainless steel of interest for use in ducts for fast reactors and as a first-wall material in magnetic fusion reactors, was irradiated in the experimental breeder reactor II to obtain the first evaluation of fracture toughness changes produced by high-temperature/high-fluence irradiation in this class of alloy.

Charpy V-notch, precracked Charpy, and tension specimens were used to evaluate the tensile and fracture behavior of this steel in the as-received condition, after 5000 h of aging at 427 and 538°C and after irradiation to a fluence of 1.1 × 10²² neutron/cm² (E > 0.1 MeV) at 419°C in flowing sodium. The results show some tendency toward temper embrittlement at 538°C and a 108 deg C shift in the ductile-to-brittle transition after irradiation at 419°C. Charpy-V energies on the upper shelf are also depressed by the irradiation but maintain adequate levels for the applications intended. Possible mechanisms for the observed response are discussed.