The effect of variable content of residual elements on the sensitivity of pressure-vessel steels to embrittlement from irradiation at 550°F was examined. Results indicate that phosphorus and copper can contribute significantly to the 550°F-irradiation embrittlement sensitivity of Type A302-B steel. The results also show that vanadium may have a slight adverse effect and that sulfur is neutral, although the latter serves to decrease the full shear energy absorption level of the steel. Nitrogen variations from ∼ 0.008 to 0.015% in aluminum-deoxidized steel have no significant effect, while the addition of aluminum to Ni-Cr-Mo steel with a given nitrogen content may slightly promote irradiation embrittlement. The program results demonstrate that apparent insensitivity to 55°F-irradiation embrittlement can be consistently achieved with laboratory heats of a nominal A302-B steel composition by maintaining the total residual element contents at a low level. Investigations of radiation-embrittlement sensitivity of weldments, aimed at the development of low-sensitivity weld fillers for joining Ni-Cr-Mo steel, again point to copper as a dominating factor in determining irradiation-embrittlement sensitivity, further verifying the results obtained in the A302-B steel investigation. Two experimental weld wire compositions with low copper contents (<0.1%) are shown to possess resistance to 550°F-irradiation embrittlement equal, or superior, to that of A543 base plate.