The principle of electricity generation
A nuclear power plant differs from a conventional steam power plant primarily in the way it produces heat. Instead of conventional fuel (e.g. coal, natural gas or peat), a nuclear power plant uses uranium as a source of energy. The use of uranium as fuel is based on the fission reaction of the atomic nuclei. The reaction releases a large amount of energy.
The nuclear reactor produces heat when the uranium nuclei inside the fuel assemblies split when hit by a neutron. The split produces new free neutrons that can split more uranium nuclei, and this enables a chain reaction. Electricity production in a nuclear power plant is based on the utilisation of the thermal energy generated by a controlled chain reaction.
The cooling water flowing through the reactor core is heated to approximately 300 degrees Celsius. However, the water does not boil due to the high pressure (123 bar) in the pressurised water reactor. The thermal energy is transferred in separate steam generators from the primary circuit water to the secondary circuit water, which evaporates. The high-pressure steam is led to the turbines. The expansion of the steam rotates the turbine, which rotates the generator converting the kinetic energy into electricity.
Primary circuit
The Loviisa power plant units' reactor coolant pumps circulate the 300-degree water into the steam generators and back to the reactor. The Loviisa power plant units contain a total of six of these circuits working in parallel. Together they form the primary circuit of the reactor.
Secondary circuit
In the steam generators, the water of the primary circuit heats the water of the separate secondary circuit. The water starts to boil, because the pressure in the secondary circuit (44 bar) is much lower than in the primary circuit. The generated steam is led to the turbines. The turbine rotates the generator, which converts the kinetic energy into electricity.
The electricity is routed through the main transformer to the national grid at a voltage of 400 kilovolts.
Cooling water system
The third separate circuit is the cooling water system. After the turbine, the low-pressure steam is led to the condensers. There the steam is condensed with cold sea water back into water, which is then pumped through the preheaters into the steam generators.
When both power plant units are operational, it takes on average about 40 cubic metres of sea water per second to cool the steam. The water used in the cooling is pumped back into the sea. It is 10 degrees warmer but its consistency is unchanged.
Over one third (34.5%) of the heat created in the reactor is converted into electricity. The rest is transmitted to the cooling water in the condensers and from there to the sea, just like in other condensing power plants generating electricity.
