The effects of three levels of copper content and phosphorus content and two levels of sulfur content on radiation sensitivity and postirradiation heat treatment response were explored for a reactor pressure vessel steel, Type A302-B. Test plates for the investigation were produced from 182-kg (400-lb) laboratory melts. The contributions of individual elements were assessed from Charpy-V (CV) notch ductility changes with 288°C (550°F) irradiation and with a 343°C (650°F), 168-h postirradiation heat treatment. Limited studies of properties recovery by postirradiation 399°C (750°F) heat treatment were also made. Radiation embrittlement sensitivity, as shown by CV transition temperature elevation and CV upper shelf reduction, generally increased with increased copper and phosphorus content and with decreased sulfur content. Certain ranges of phosphorus and copper content were found to be more critical than others. Response to 343°C (650°F) postirradiation heat treatment, as evidenced by transition temperature recovery in degrees Celsius, appeared to be independent of copper, phosphorus, and sulfur content for the ranges investigated. Response to heat treatment also appeared to be independent of the magnitude of the prior transition temperature elevation by irradiation. On the other hand, a dependence of percentage recovery on impurity element content was observed. A dependence of upper shelf recovery on copper content was also found. Six of the eight plate compositions exhibited full upper shelf recovery but only small transition temperature recovery after 343°C (650°F) heat treatment.