The paper presents an analysis of the the radiotoxicity of the fission products of the cores of thermal (VVER, RBMK) and fast (FN) neutron reactors, a model of fuel elements with an inert matrix. It is shown that the exclusion of 238U from the fuel composition makes it possible to reduce the radioactivity of the fuel composition for thermal neutron reactors by three orders of magnitude; for fast neutron reactors by 1-2 orders of magnitude compared to traditional fuel compositions containing 238U. Allows you to reduce the long-term radioactivity created by actinides by more than 2 orders of magnitude. Russia's experience in the use of highly enriched fuel in research reactors (for example, IVV-2), Topaz nuclear power plant (90% 235U) confirms the results of the analysis to reduce the radioactivity of core and spent nuclear fuel.