[en] This investigation employed laboratory melts of pressure vessel steels (A 533-B or A 302-B base) to probe suspect interactions between copper impurities and manganese, molybdenum, chromium and nickel alloying as influencing elevated temperature, radiation sensitivity development. Radiation sensitivity is judged on the basis of Charpy-V (C/sub v/) notch ductility and yield strength changes by 288/degree/C irradiation to /approximately/2.5 /times/ 10¹⁹ n/cm², E > 1 MeV. The investigation also qualifies the influence of phosphorus content on radiation resistance as a function of copper content and explores suspect contributions of tin and arsenic. The findings demonstrate clearly that important composition interactions exist in radiation sensitivity development. Observed radiation effects are compared to NRC Regulatory Guide 1.99, Revision 2 projections; causes of major outliers (unconservative projections) are discussed. The correlation of C/sub v/ and tension test methods in weighing radiation sensitivity is assessed. 26 refs., 23 figs., 9 tabs