AbstractThe recoil energy spectrum of the irradiation from presently proposed intense high energy neutron sources for the development of fusion reactor materials deviates appreciably from that in the fusion environment. A wide variation of representative elements of defect structure evolution are quoted from the experimental results and analyses of irradiation with various energetic particles; electrons, ions and neutrons in fission reactors and fusion neutrons. After all the discussion on the influence of primary recoil energy spectrum, a strong warning is sent concerning the establishment of a neutron source without serious consideration of the problems arising from the difference in the recoil energy spectrum from that in the fusion environment. The establishment of a medium sized neutron source with a nuetron energy spectrum closest to that in the fusion environment is strongly recommended.