[en] The paper describes the principles and methodology of up-to-date approach to establishing design limits of fission product (FP) activity in the primary coolant of operating reactor for substantiation of radiation safety (RS) of the NPP with WWER reactors, both operating and being designed. The basic principle which is used in regulation of the level of FP activity in the primary coolant is in the need for a complex account for the technological state of consistently located physical safety barriers on a path of radionuclides extension to environment out of the NPP. In the substantiating of the RS the regulation of the technological state of only one safety barrier, in particular, the number of failed fuel rods and the degree of their depressurization without considering the states of other such barriers is unjustifiable since such an approach may potentially result in the onset of not permissible radiation consequences during the NPP operation. In the substantiation of the RS of the NPP with WWER reactors, both operated and being designed, the conservatism principle consisting in the choice of such input data for the FP source term calculation at which the radiation consequences are the most severe ones, must be used. The most unfavourable radiation consequences of some NPP operation modes may occur not only in the periods of fuel performance with the maximum thermal mechanical loads. Therefore the conservatism used in the analysis of the thermal mechanical and corrosion behavior of fuel rods may be generally insufficient for ensuring a conservative approach in substantiating the RS. For illustration of the approaches used in the RS substantiation some specific examples of calculations of radioactive FP source terms for the normal reactor operation conditions as well as approaches in the analyses of radiation consequences in the defining LOCA design basis accidents, are given. (authors)