Embrittlement of pressure vessel steels and welds during irradiation at service temperatures around 290°C is known to be related to the presence of residual elements such as copper but the mechanism of embrittlement has not been clear. Pressure vessel steels and welds with high and low copper content and binary iron alloys with 0.3 at. pct of Cu, V, Ni, and P and 0.1 at. pct C were irradiated to fluences of 2.5 x 1019 and 4.5 x 1020 n per sq cm > 1 MeV and compression tested to determine the radiation strengthening. Alloys containing copper had more rapid and greater radiation strengthening. It is concluded that the embrittlement is due to this radiation strengthening which in turn results from a higher density of defect aggregates. Vacancy trapping by the copper atoms which modifies the kinetics of defect aggregate nucleation is shown to be the most likely cause for the differences in microstructure.