Level of radioactive contamination in primary coolant of nuclear reactors must be kept as low as possible, therefore essential conditions is the leakage-free state of the fuel rods. To monitor the leakage defects in VVER reactors, the radioactivity of fission products must be measured in the primary loops. From radioactivity data it is possible to infer the fuel assemblies' hermetic or time-dependent leakage state. A new, time-dependent leakage model was developed for operating conditions of VVER nuclear reactors. This leakage model is able to calculate isotopes’ radioactivity inside the fuel and in the primary coolant originating from nuclear fission and neutron-activated reactions. The core of the leakage model consists of several systems of differential equations describing dominant nuclear processes on time-dependent number of radioactive isotope nuclei within fuel assemblies and primary coolant. The model considers the effect of technological parameters on the activity of radioisotopes in the primary coolant.