An approximate analytic solution of the system of equations describing the kinetics of formation and the parameters of radiation defect clusters in crystals is obtained. A model corresponding to the real parameters of Si is assumed as a basis for calculation and it is shown that two types of secondary radiation defect clusters may be realized corresponding to the congealing and spreading of the initial vacancy clusters. The cluster type depends on the incident energy of the particle which creates the effects and on the physical parameters of the crystal under irradiation. For a congealing cluster most of the initial vacancies react within the original volume of the damage cascade and for a spreading cluster the final concentration of divacancies andA-centres within the original cascade volume is much less than the initial vacancy concentration, i.e. most of the vacancies form divacancies andA-centres dispersed throughout the crystal volume.