A nuclear power plant is a thermal power plant, in which a nuclear reactor is used to generate large amounts of heat. This heat is used to generate steam (directly or via steam generator) which drives a steam turbine connected to a generator which produces electricity.
Steam turbine is a common feature of all thermal power plants. Steam Turbine was invented in 1884 by Sir Charles Parsons, whose first model was connected to a dynamo that generated 7.5 kW (10 hp) of electricity. Exceptional feature of the nuclear power plant is the nuclear reactor and its safety and auxiliary systems.
Nuclear power plants may take many hours, if not days, to startup or to change their power output. Modern power plants can and do operate as load following power plants and alter their output to meet varying demands. But base load operation is the most economical and technically simple mode of operation. It is primarily due to the fact, they require a long period of time to heat up the nuclear steam supply system and the turbine-generator to operating temperature. From this point of view power plant generally are divided into two basic categories:
Special link: 3D model of NPP – sketchfab.com
As you can see the nuclear power plant consist of two main buildings:
- Containment building (houses Nuclear Reactor)
- Turbine building (houses Turbo Generator)
The containment building is the key building of the nuclear island. It is an air-tight building, which houses a nuclear reactor and its pressurizer, reactor coolant pumps, steam generators, and other equipment or piping that might otherwise release fission products to the atmosphere in the event of an accident. Such buildings are usually made of steel-reinforced concrete.
The turbine building is the key building of the conventional (turbine) island. The turbine building houses a turbine, generator, condenser and other equipment, which is used for conversion thermal energy from pressurized steam to mechanical work used for drive the generator.
Also a cooling tower can be part of the nuclear power plant, but it is not necessary. Many nuclear power plants (coastal nuclear power plants) do not cool the cooling water via cooling towers.
Components of a nuclear power plant
The key components common to most nuclear power plants are:
- Nuclear Reactor. A nuclear reactor is a key device of nuclear power plants. Main purpose of the nuclear reactor is to initiate and control a sustained nuclear chain reaction.
- Steam Generators. Steam generators are heat exchangers used to convert feedwater into steam from heat produced in a nuclear reactor core. They are used in pressurized water reactors (PWR) between the primary and secondary coolant loops.
- Pressurizer. Pressure in the primary circuit is maintained by a pressurizer, a separate vessel that is connected to the primary circuit (hot leg) and partially filled with water which is heated to the saturation temperature (boiling point) for the desired pressure by submerged electrical heaters. Temperature in the pressurizer can be maintained at 345 °C (653 °F), which gives a subcooling margin (the difference between the pressurizer temperature and the highest temperature in the reactor core) of 30 °C.
- Reactor Coolant Pumps. Reactor coolant pumps are used to pump primary coolant around the primary circuit. These pumps are powerful, they can consume up to 6 MW each and they can be used for heating the primary coolant before a reactor start-up.
- Safety Systems. According to the U.S. Nuclear Regulatory Commission primary objectives of nuclear reactor safety systems are to shut down the reactor, maintain it in a shutdown condition and prevent the release of radioactive material. Reactor safety systems consist of:
- Reactor Protection Systems
- Essential service water system
- Emergency core cooling systems
- Emergency power systems
- Containment systems
- Steam Turbine. A steam turbine is a device that extracts thermal energy from pressurized steam and uses it to do mechanical work on a rotating output shaft.
- Generator. A generator is a device that converts mechanical energy of the steam turbine to electrical energy.
- Condenser. A condenser is a heat exchanger used to condense steam from last stage of turbine.
- Condensate-Feedwater System. Condensate-Feedwater Systems have two major functions. To supply adequate high quality water (condensate) to the steam generator and to heat the water (condensate) to a temperature close to saturation.
Manufacturing of Reactor – Youtube
How does Nuclear Power Plant work?
- Nuclear reactors in these power plants are “only” used to generate heat. This heat is used to generate steam which drives a steam turbine connected to a generator which produces electricity.
- Typical reactor nominal thermal power is about 3400MW. The heat is produced by fission in a nuclear reactor and passes into the primary cooling water.
- For PWRs, the coolant (water) is heated in the reactor core from about 290°C (554°F) to approximately 325°C (617°F) as the water flows through the core.
- Hot coolant is then pumped via main coolant pumps into steam generators.
- In steam generators, this heat is transferred through the walls of these tubes to the lower pressure secondary coolant located on the secondary side of the exchanger where the coolant evaporates to pressurized steam (saturated steam 280°C; 536°F; 6,5 MPa).
- This pressurized steam is then routed into the steam turbine, in which steam expands from pressures about 6 MPa to pressures about 0.008 MPa. Steam turbines in western nuclear power plants are among the largest steam turbines ever.
- Steam turbine is connected to the main generator which generates electricity.
Most of nuclear power plants operates a single-shaft turbine-generator that consists of one multi-stage HP turbine and three parallel multi-stage LP turbines, a main generator and an exciter. HP Turbine is usually double-flow impulse turbine (or reaction type) with about 10 stages with shrouded blades and produces about 30-40% of the gross power output of the power plant unit. LP turbines are usually double-flow reaction turbines with about 5-8 stages (with shrouded blades and with free-standing blades of last 3 stages). LP turbines produce approximately 60-70% of the gross power output of the power plant unit. Each turbine rotor is mounted on two bearings, i.e. there are double bearings between each turbine module.
Thermal Efficiency of Nuclear Power Plant
In modern nuclear power plants the overall thermal efficiency is about one-third (33%), so 3000 MWth of thermal power from the fission reaction is needed to generate 1000 MWe of electrical power. The reason lies in relatively low steam temperature (6 MPa; 275.6°C). Higher efficiencies can be attained by increasing the temperature of the steam. But this requires an increase in pressures inside boilers or steam generators. However, metallurgical considerations place an upper limits on such pressures. In comparison to other energy sources the thermal efficiency of 33% is not much. But it must be noted that nuclear power plants are much more complex than fossil fuel power plants and it is much easier to burn fossil fuel ,than to generate energy from nuclear fuel. Sub-critical fossil fuel power plants, that are operated under critical pressure (i.e. lower than 22.1 MPa), can achieve 36–40% efficiency.
The Database on Nuclear Power Reactors by IAEA
The Power Reactor Information System (PRIS), developed and maintained by the IAEA for over four decades, is a comprehensive database focusing on nuclear power plants worldwide. PRIS contains information on power reactors in operation, under construction or those being decommissioned.
The database covers:
- Reactor specification data (status, location, operator, owner, suppliers, milestone dates) and technical design characteristics.
- Performance data including energy production and energy loss data, outage and operational event information.
See also: PRIS by IAEA