Types of nuclear power plants
Over the past decades, a wide range of different types of nuclear power plants have been designed and built around the world, but only a few have reached large-scale commercial operation. The most common reactor types in the world are the pressurised water reactor and the boiling water reactor.
The first reactors for power generation were developed in the early 1950s in the United States. They were of the thermal light water reactor type and were initially used to power submarines. The first full-scale nuclear power plant reactor was started up at Obninsk in the Soviet Union in 1954.
The 1950s and 1960s were also a period of intense development in other countries with high levels of nuclear power, such as England and France. The types of nuclear power plant that became widely commercially available were selected in the 1960s. Since then, the design of reactors has remained basically the same, but safety systems have been considerably strengthened and improved.
According to their physical characteristics, reactors are classified into thermal reactors and fast reactors, according to the energy (speed) of the neutrons that cause fission. Nuclear power reactors are mostly thermal reactors, where fission takes place with pre-hydrogenated or thermal neutrons. Thermal reactors can use up to a few percent of the energy contained in natural uranium. At the prototype stage, there are so-called "utility reactors", which produce more new fuel from natural uranium than they consume.
Thermal reactors are classified according to the substance that slows down the neutrons they produce and the substance that cools the reactor. The most common are light water reactors, of which there are two types. In these reactors, water acts both as a neutron moderator and as a reactor coolant.
Boiling water reactor
In a boiling water reactor (BWR), the reactor core consists of metal-shell fuel rods containing uranium dioxide fuel in the form of ceramic pellets. The rods are assembled in fuel assemblies of 60 to 100 rods. The total number of rods is in the tens of thousands. The reactor core is enclosed in a pressure vessel.
Heat from the fuel rods is transferred to the cooling water flowing through the core, which is heated and partially vaporised. At the top of the pressure vessel, the steam is separated from the water and fed to a steam turbine. The expanding steam rotates the turbine and an electric generator connected to it. The steam cooled in the turbine condenses in the condenser into water, which is pumped back into the reactor pressure vessel.
The power of the reactor is adjusted by means of so-called control rods. They contain a substance that strongly absorbs neutrons, such as boron. When the control rods are pushed into the reactor, they absorb some of the neutrons, thus slowing down the fission chain reaction and reducing the reactor power. The control rods are also used to fast shut down the reactor. The power of the boiling reactor is also adjusted by changing the reactor coolant flow.
TVO's two units (OL1 and OL2) at Olkiluoto are Swedish-built BWRs.
Pressurized water reactor
The most common type of power reactor worldwide is the pressurised water reactor (PWR). The PWR (pressurised water reactor) differs from the BWR in that the cooling water does not evaporate in the reactor. The steam is produced in so-called evaporators, through which the high-pressure water heated in the reactor circulates in thin tubes. The heat is transferred to the low-pressure water in a separate circulation circuit (secondary circuit) around these tubes, which, after vaporisation, is fed to the turbines.
The power of the pressurised water reactor is adjusted by means of control rods and by varying the boron content of the coolant. Fortum's two units at Loviisa are Soviet-built pressurised water reactors, also known as VVERs. TVO's newest OL3 unit at Olkiluoto is a European pressurised water reactor, known as EPR.