This paper investigates the dependence of the concentration of point defects in the EK64 steel on the characteristics of neutron irradiation in the framework of the model of migration of point defects. The results obtained were used to analyze the data on the experimental determination of the characteristics of radiation porosity in the samples of the EK164 steel irradiated using various dose–temperature parameters. The values of the critical diameters of the vacancy nuclei of pores have been calculated. It has been shown that, with an increase in the integral area of the pore surface, the concentration of point defects decreases and the size of the critical diameter increases. This leads to a multi-modal distribution of pore sizes. The obtained numerical estimates of the peak values of the concentration of small pores correspond satisfactorily to the experimental pore-size distributions.