A numerical study of the formation of a radiation‐induced defect cluster center in neutron‐irradiated silicon has been performed by solving a set of semilinear parabolic reaction‐diffusion‐coupled equations. It is found that most of the primary displacement defects [interstitial (I) and vacancy (V)] will be annihilated byI‐Vdirect recombination in an extremely short time. In particular, the formation of four‐vacancy defects is independent of the concentration of sinks and impurities in a sample, and of the energy of the recoil particle relatively. The threshold energy for the formation of a vacancy cluster has also been studied. The results are discussed with the experimental observations.