Comparison of the STIP data (high helium) with neutron-irradiation (lower He) data for FMS leads to the following conclusions:

• Up to about 500 appm, excess irradiation hardening and ductility loss in FMS due to He are modest.

• At higher damage levels, due to contributions from bubbles, hardening continues to increase with dpa and He in SPNI but saturates in neutron irradiations.

• At high He levels, bubbles contribute to hardening even more significantly at higher irradiation temperatures.

• Reductions in the uniform and total elongation strains are also similar in neutron and SPN irradiations up to 500 appm, but elastic fracture and increasing brittle IG fracture occur even in tensile tests at higher He levels.

• The effect of He on fast fracture and DBTT shifts is also modest below «500 appm, but at higher
levels it increases rapidly due to both extra hard­ening and reduction in the critical stress for IG fracture that fall below that for cleavage.

• Synergistic hardening and nonhardening embrit­tlement lead to enormous DBTT shifts and an increase in the maximum temperature for signifi­cant DBBT shifts up to irradiation temperatures that may be well in excess of 400 °C.

The effects of He on tensile and fracture properties are less apparent in AuSS. Unpublished results from SPNI also show that He is trapped in small bubbles in NFA in a way that appears to protect GBs from IG embrittlement for the same irradiation conditions that result in severe synergistic He-hardening DBTT shifts.