The concept of displacement per atom (DPA) is not sufficient to account for the irradiation damage in metals. It ignores the distribution of displaced atoms in a cascade and subsequent point defect processes. PKA energy spectrum analysis has been proposed for the understanding of the defect evolution during cascade damage. In the analysis, special attention was paid to the production rate of subcascades as an important factor. DPA rate and the subcascade production rate in iron were compared with five irradiation fields in the KUR, the JMTR and the JOYO. The fast neutron flux was proportional to DPA rate in these irradiation fields. As the threshold energy for subcascade formation increases, the subcascade production rate in the irradiation fields with soft neutron energy spectra decreases more than those with hard neutron energy spectra. The importance of the analysis was discussed and some examples for application were demonstrated.