V.V. Belyaev, E.N. Volkova Institute for Hydrobiology, National Academy of Sciences of Ukraine, Kiev SIMULATION OF SELF-PURIFICATION PROCESSES OF WATER MASSES FROM RADIOACTIVE SUBSTANCES The rate of self-decontamination of water masses from radionuclides in the most contaminated water bodies in Ukraine is determined. It is shown that the content of 137Cs in the water masses of the exclusion zone water bodies is reduced with halftime of decontamination of 9,6 1,4 years. The mechanism of slowing the rate of decontamination of water masses from radionuclides is considered. It is shown that two years after the radionuclide ingress in the water body there is a significant slowdown in the rate of purification of water masses. Keywords: reservoirs, radionuclides, speed of purification, halftime of purification.