In the pressurized water reactors, fuel in the form of fuel assemblies is placed in reactor pressure vessel to which chemically treated water flows. The water flows through channels in the fuel assemblies and removes the heat that is produced during fission reaction. Water coming from the reactor has a temperature of about 297 °C (VVER reactor type); it is then led through the hot arm of the primary piping into heat exchanger - steam generator. In steam generator, water flows through a bunch of pipes and delivers the heat to water from secondary circuit and has a temperature of 222 °C. When cooled down, primary circuit water is led back to reactor core. Secondary circuit water is evaporated in steam generator and the steam is led via steam collector to the blades of a turbine. The turbine shaft turns a generator that generates electric energy.