Control rods in a pebble-bed-type high-temperature reactor operate at temperatures below 650°C, but in upset conditions short-term excursions up to 850°C can occur. Here, austenitic steels or nickel-base alloys show ductility losses caused by “helium high-temperature embrittlement.” The first of a series of irradiation experiments, followed by postirradiation tensile testing, quantified the losses in ductility of eight alloys (austenitic steels and high-temperature iron- and nickel-base alloys). Relative to the initial values of the rupture elongation, the ductility losses between 600 and 850°C were the same for all alloys with the exception of the strongly precipitation-hardened alloys, which showed more severe embrittlement at 600 to 700°C. The objective of the second experiment was to optimize the microstructure of austenitic steels (1.4981 and 1.4970) by specific thermomechanical treatments to increase the ductility after irradiation. Here again, it was found that all varieties showed nearly the same relative embrittlement behavior. Thus, it can be concluded that maximum ductility after irradiation requires a material with high ductility before irradiation.