Abstract. In the article the analysis of the modernization of the condensing unit. Thermal calculations and design of the condensing unit of the Kalininskaya nuclear power plant was conducted. Extension of service life of the operating NPP power units is one of the most important tendencies of the modern stage of nuclear power engineering development and the most efficient funds investment for preserving generating capacities. The data indicate when modernizing the condensing unit, its cooling area was increased for about 4,000 m 2 due to increasing number of tubes. In addition, tube geometrical dimensions were changed; the wall thickness was reduced from 1 and 2 mm to 0.7 and 1.25 mm respectively.