AbstractThe effects of neutron irradiation, post-irradiation annealing and re-irradiation on the Charpy V-notch impact transition temperature of pressure-vessel steels were investigated. Specimens representing several heats of pressure-vessel steels were irradiated at elevated temperatures to fast (>1 MeV) neutron exposures up to 2×1020nvt. The general observation was that irradiation caused an increase in the transition temperature; however, it was discovered that specimens representing various heats of a given material composition could show a vastly different increase in transition temperature due to irradiation. These results have led to the arbitrary classification of these steels as“sensitive”or“insensitive”heats. Possible correlation of heat to heat sensitivity with microstructure is discussed. Post-irradiation annealing in the temperature range of 650–800 F was found to reduce the effects of irradiation on the transition temperature significantly. However, subsequent re-irradiation of specimens that were post-irradiation annealed at 650 F increased the transition temperature to a level that could not be distinguished from that of specimens that were not annealed prior to re-irradiation. Re-irradiation studies were not conducted on specimens that were post-irradiation annealed at the higher temperatures. No explanation of the re-irradiation behavior after annealing is available.