In order to understand the characteristic defect clustering processes under hydrogen ion irradiation with fusion relevant energy and the underlining defect interactions controlling the processes, accumulation of defects and defect-hydrogen complexes are calculated numerically using the homogeneous reaction kinetics and rate equations. These calculations do not consider the nucleation and growth of hydrogen bubbles. The present calculations yield a very high concentration (order of 10 - 2 ) of vacancy-hydrogen (VH) complexes and a high growth rate and relatively low concentration of interstitial loops (IL). These results are in disagreement with the experimental results. Within the framework of the present calculations, the experimental results could be explained, however, by postulating very low reaction efficiencies of interstitials with VH complexes and interstitial loops. The validity of these postulates and impacts of the simplified assumptions are discussed.