Trends in the properties of defects, reflecting the role of Ni and P in the enhanced precipitation of copper during irradiation, have been studied by small angle neutron scattering (SANS) in a set of 8 model RPV steels. The alloys in the set contain different, selected amounts of Cu, Ni and P, while the composition of all the 8 samples concerning the rest of the alloying elements, the ‘RPV steel background’, was invariant throughout the series. The irradiation was done in a test reactor at 290C, at controlled radiation conditions, for two neutron fluences of 5∙1018 and 5∙1019 n/cm2. The SANS data show an approximately quadratic relationship between the precipitated volume fraction fm of damage particles and the nickel content cNi of the sample for the medium-copper cCu = 0.16% alloys, with a nearly zero initial slope of fm; the coefficients in the equation vary substantially with the fluence. A nearly perfect correlation of fm with the increase of microhardness was found for all alloys. The size distribution, as deduced from magnetic scattering, differs systematically from that found by analysis of the nuclear scattering; this difference correlates with both cCu and cNi. For the two low-copper alloys, phosphorus is seen to enhance production of radiation damage centers, whereas for the medium-copper alloys a refinement of the precipitated phase occurs.