3.1 Operating principles of
nuclear power plants
Nuclear power plants produce electricity in the same manner as large condensing power plants using fossil fuels – by heat- ing water into steam and letting the steam rotate a turbogen- erator. The main difference between nuclear power plants and conventional condensing power plants is the method of production of the energy required for heating the water: in nuclear power plants, the heat is produced in a reactor using the energy released by splitting atom nuclei, whereas in con- densing power plants, the water is heated by burning suitable fuel, such as coal, in a boiler.
The plant type considered for this project is the pres- surized water reactor. The operation of a pressurized water reactor is discussed in more detail in section 3.2. In a pres- surized water reactor (Figure 3-1), the high-pressure water exiting the reactor is led into steam generators. In the steam generators, the water flowing in a separate secondary circuit turns into steam, which is then used to rotate a turbine and an electric generator.
In Finland, the fuel used in nuclear power plants is isotope U-235-enriched uranium dioxide (UO2). Enriched uranium
dioxide contains 3–5 percent of isotope U-235, whereas natu- ral uranium only contains less than one percent of the same isotope. The fuel is introduced into the reactor in the form of ceramic pellets placed in hermetically sealed tubes called fuel rods, which are bundled into fuel bundles (Figure 3-2).
Figure 3-1. The operating principle of a pressurized water reactor. Control rods
Feed water pump
Core
Primary circuit pump
Feed water tank
Condenser Fuel bundles Primary circuit Secondary circuit Reactor Pressurizer Cooling water Turbine Steam generator
Containment building shell
Generator Electricity Figure 3-2. Fuel pellets, fuel rods, fuel bundles.
The use of uranium as fuel is based on the heat gen- erated in the splitting, or fission, of atomic nuclei. When neutrons collide with a fissionable atomic nucleus, the latter splits into two lighter nuclei. At the same time, new neutrons, neutrinos, and energy are released. The neutrons released following the splitting of the nucleus can cause new fissions, which enables a chain reaction to start. The fission of U-235 nuclei forms a self-maintaining, controlled chain reaction that enables controlled heat production.
The heat produced in nuclear power plants or other ther- mal power plants (such as coal, oil, or gas plants) cannot be fully converted into electricity. Due to this, part of the heat produced is removed from the power plant using condensers. In the condensers, the low-pressure steam exiting the steam turbines releases energy and turns back into water.
54 3 Technical project description
In Finland, condensers are cooled using cooling water taken directly from a natural water system. The cooling water, the temperature of which rises by 10–12 °C in the process, is then returned back to the water system. In nuclear power plants, more than one-third of the thermal energy generated in the reactor can be converted into electric energy.
Nuclear power plants are best suited as base load plants, which means that they are used continuously at constant power except for a few weeks’ maintenance outages at 12–24-month intervals. Plants are designed for an opera- tional lifetime of at least 60 years.
3.2 Description of the plant type
The most widely used reactor type is the light water reac- tor. The reactors of the nuclear power plants currently in operation in Finland are light water reactors. Light water reactors use regular water to maintain the chain reaction, to cool the reactor, and to transfer heat from the reactor core to the power plant’s process systems. The alternative light water reactor subtypes are the boiling water reactor and the pressurized water reactor. The subtype considered for this project is the pressurized water reactor.
In a pressurized water reactor, the fuel heats the water, but the high pressure (approximately 160 bar) prevents the water from boiling. The temperature of the water inside the reactor reaches a maximum temperature of approximately 330 °C. High-pressure water is led from the reactor to sep- arate steam generators. In the steam generators, the water is distributed into small-diameter heat transfer tubes. Heat transfers from the hot water led from the reactor through the walls of the heat transfer tubes to water flowing in a sep- arate circuit (secondary circuit), which is maintained under lower pressure (60–70 bar). The water in the secondary cir- cuit turns into steam which is then led to the turbine rotat- ing the electric generator (Figure 3-1).
As the reactor system and the secondary circuit are com- pletely separated from each other, the water circulating in the secondary circuit is not radioactive.
In Finland, the currently operating reactors at the Lo - viisa power plant and the reactor of the new power plant unit currently under construction in Olkiluoto are pressur- ized water reactors.
3.2.1 The Rosatom pressurized water reactor
plant
Rosatom’s AES-2006 pressurized water reactor plant (Figure 3-3) is a modern third-generation nuclear power plant, which comes in two different versions. The plant ver- sion chosen by Fennovoima is AES-2006 / V491. Table 1-1 in Chapter 1 contains basic data on the plant.
The AES-2006 plants are based on VVER technology, which has been developed and used for more than 40 years and consequently offers the benefit of long-term opera- tional experience. The plant version considered for Fenno- voima’s project is the latest development step in the VVER plant series. VVER plants have a history of safe operation spanning over 30 years in locations such as Loviisa.
Contracts have been made to build AES-2006 plants in several countries. Additionally, plants of this type are cur- rently under construction in Russia and Belarus. In total, 13 AES-2006 plants are currently under construction or in contract phase. In Russia, the first plant unit of phase II of the Leningrad plant site in Sosnovy Bor is currently under construction. The construction began in 2008. Additionally, two plant units are under construction in both Kaliningrad and Novovoronez.
The target of the safety design of the plant has from the start been to comply with the requirements of IAEA’s safety guidelines and standards, European Utility Require- ments (EUR), and Russia’s own national regulations and requirements. The designing of Fennovoima’s plant will be
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