Primary damage due to displacement cascades (10e100 keV) has been assessed in Fee1%Mne1%Ni-0.5% Cu and its binary alloys by molecular dynamics (MD), using a recent interatomic potential, specially developed to address features of the FeeMneNieCu system in the dilute limit. The latter system represents the model matrix for reactor pressure vessel steels. The applied potential reproduces major interaction features of the solutes with point defects in the binary, ternary and uaternary dilute alloys. As compared to pure Fe, the addition of one type of a solute or all solutes together does not change the major characteristics of primary damage. However, the chemical structure of the self-interstitial defects is strongly sensitive to the presence and distribution of Mn and Cu in the matrix. 20 keV cascades were also studied in the FeeNieMneCu matrix containing 〈100〉 dislocation loops (with density of 1024 m3 and size 2 nm). Two solute distributions were nvestigated, namely: a random one and one obtained by Metropolis Monte Carlo simulations from our previous work. The presence of the loops did not affect the defect production efficiency but slightly reduced the fraction of isolated self-interstitials and vacancies. The cascade event led to the transformation of the loops into ½〈111〉 glissile configurations with a success rate of 10% in the matrix with random solute distribution, while all the pre-created loops remain stable if the alloy's distribution was applied using the Monte-Carlo method. This suggests that solute segregation to loops “stabilizes” the pre-existing loops against transformation or migration induced by collision cascades.