Experimental determination of the effect of helium on the fracture toughness of steel

A fundamental issue with the use of structural steels for fusion applications is the effect of helium on mechanical properties. This concern has been difficult to address due to the limited neutron energies, hence limited helium production, of the fission reactor facilities used to simulate the effects of irradiation on properties such as fracture toughness. This paper will compare results from identical pre-cracked DCT fracture toughness samples irradiated using spallation and fission neutrons at ≃60-90 °C. Materials studied were 304L and 316L stainless steel. The spallation neutron irradiated specimens were irradiated over a dose range up to ≃10 dpa with a helium and hydrogen-to-dpa ratio of ≈60 and 400, resp. Fracture toughness was seen to rapidly decreased from a value of ≃250 to ≃150 MPa m^1/2 by the 1 dpa level. Following fission irradiation in the 1-1.5 dpa range, fracture toughness results were indistinguishable from those irradiated in the spallation neutron. The helium and hydrogen concentration for the fission neutron irradiated materials was ≃6 appm and ≃30 appm, respectively. It is concluded that, within the measurement uncertainty, increasing the helium and hydrogen concentration by more than an order of magnitude has had little influence on the fracture toughness of 304L and 316L stainless steel irradiated to similar displacement doses of a few dpa in the temperature range of 60-90 °C.