The effect of nuclear reactions upon the formation of energy spectra of primary knocked-on atoms (PKA) and the rate of primary radiation defect formations under irradiation by protons, neutrons and a-particles is considered. Calculations were performed by using the original technique which correctly accounted the peculiarities of different nuclear reaction channels within the frame of the statistical theory. As an example the considerable differences of separate reaction channels and their influence upon the defect formation processes are shown by irradiation of Cu. The summary contribution of nuclear reaction to the radiation damage of Cu has been analysed. It is shown that it may become a dominant one at neutron energies more than 4 MeV and proton ones more than 27 MeV.