Small specimen reuse technique to evaluate fracture toughness of high dose HT9 steel

A small specimen reuse technique has been developed to evaluate static fracture toughness using the tested halves of subsize Charpy impact specimens. The new testing and evaluation procedure consisted of introducing a diamond-saw notch to the miniature bars (∼13 by 3 by 4 mm), precracking using embedded displacement gage signal only, static fracture testing at high temperatures in vacuum, and calculation of J-R curves using a modified normalization method. The HT9 steel has been used as a primary core material for fast fission reactors because of its low radiation-induced swelling and high resistance to irradiation hardening and embrittlement. To maximize the use of existing high dose materials in building mechanical property knowledgebase for the high dose HT9 steel, the fracture resistance (J-R) curves and fracture toughness values were evaluated using the small single-edged bend (SEB) bars in nonirradiated, as-irradiated, and thermally-annealed (at 550 or 650°C) conditions. The original impact specimens were machined from the wall of the ACO-3 duct of fast flux test facility (FFTF) and archive (nonirradiated) material plate. Regardless of vastly different irradiation conditions, no fracture toughness data measured after post-irradiation annealing was lower than 130 MPa√m, indicating that the thermal annealing can be a feasible damage mitigation technique for future fast reactor components.