A series of 4-way split laboratory melts of low alloy, pressure vessel steel, representing statistical combinations of specific impurity elements and/or alloying elements, are being evaluated after 288°C irradiation to ∼ 2 × 10 19 n/cm 2, E > 1 MeV. The objective is to reveal interactions between elements (or interdependencies) in radiation sensitivity development. The primary base composition for melting was ASTM A 302-B or A 533-B steel; plates from the melt casts were heat treated to simulate 150 mm or thicker plate materials. This report summarizes preirradiation-postirradiation notch ductility determinations for six metls (24 composition variations) produced for the study. The melts were used to test the effects on radiation sensitivity of phosphorus impurities in combination with copper impurities and the significance of high/low levels of nickel, manganese, molybdenum and chromium alloying to steels containing a significant copper content ( ⩾ 0.15 % Cu). Contributions of tin and arsenic impurities were also evaluated. One key finding is an inverse dependence of the phosphorus contribution to radiation sensitivity level, on copper content. Therefore, for the new (improved) steels containing a low copper content, phosphorus level should not be ignored in making projections of radiation sensitivity in service. Other important interactions between elements are reported.