Light water reactors are simple in their basic characteristics and inherently stable. Their natural safety characteristics, which are based on physical feedback loops that moderate the increase of power output, together with the basic design of the reactors make the uncontrollable increase of reactor power output impossible. Light water reactors are very suitable for energy production, and they have proven safe and reliable, which explains their popularity. For these reasons, the further development and construction of new nuclear power plants has mainly focused on light water reactor technology.
Boiling water reactor
In a boiling water reactor (BWR), the energy released in the fission reaction heats the fuel, which in turn heats the coolant flowing through the core so that the water boils in the reactor, thus generating steam with a temperature of about 300°C and a pressure of 70 bar (about 70 times the normal atmospheric pressure). The high-pressure saturated steam is conducted via the steam separators and team dryer located in the reactor pressure vessel to the turbine, which is rotated by the expanding team. An electric generator mounted on the same shaft as the turbine produces electricity.
After the turbine, the steam is conducted into sea water cooled condensers, where it condenses back to water. In a boiling water reactor plant, the wa ter is pumped back into the reactor pressure vessel. The sea water used for cooling is returned to the sea, somewhat warmer. The basic operating principle of a boiling water reactor plant is shown below.
Pressurised water reactor
Compared with a boiling water reactor, the pressure in a pressurised water reactor (PWR) is notably higher, typically 120-16 bar. The high pressure prevents the water from boiling inside the reactor pressure vessel as it runs through the reactor core and is heated by the energy released in the fission reactions. A pressurised water reactor plant has two separate circulation systems: the primary coolant circuit, which circulates the water pumped through the reactor core, and the secondary coolant circuit, where the steam is generated for the turbine.
Energy is transferred from the reactor with the pressurised water heated up to 300-330°C to separate steam generators, where the energy is transferred to the secondary coolant circuit water, evaporating it. The generated steam (260-295°C and 45-78 bar) flows to the turbine. The primary circuit water cooled in the steam generators is pumped back into the reactor pressure vessel.
After the turbine, the steam is conducted to the condensers, where it condenses, cooled by cold sea water, into water. In a pressurised water reactor plant, the water is pumped back from the condensers to the steam generators. The sea water used for cooling is returned somewhat warmer to the sea. The basic operating principle of a pressurised water reactor plant is shown below.
Loviisa power plant has two VVER pressurised water reactors with capacities of 498 MW net and 500 MW net.