The reactor building is a solid structure made mostly of reinforced concrete that holds the nuclear power plant's important components. The building ensures maximum safety during operation and in the event of an accident.
Reactor
The reactor is where the controlled fission of nuclear fuel nuclei takes place, releasing nuclear energy that, as heat, is used to generate electricity. There are many types of reactors; the best known PWR (pressurized water reactor) is characterized by a thick-walled steel pressure vessel.
Steam generator
The steam generator is located in the primary circuit and functions as a heat exchanger. It transfers heat generated in the nuclear reactor into the secondary circuit, where the steam that drives the steam turbine is created.
Pressurizer system
Pressure in the primary circuit is maintained by a pressurizer. The pressurizer is partially filled with water which is heated to the saturation temperature (boiling point) by submerged electrical heaters for the desired pressure.
Main circulation pump
Circulates the primary circuit coolant – the so-called transport of generated heat from the reactor into the steam generators.
Concrete shielding
Also called containment. A reinforced concrete structure which protects the reactor and other components of the primary circuit and acts as a barrier against ionizing radiation.
Cooling tower
A cooling tower is used primarily to cool water used in turbine condensers. In most cases they are squat reinforced concrete chimneys with the shape of a hyperboloid of revolution, up to 150 m high.
Turbine hall
The turbine hall is a large open area containing the equipment needed to generate electricity from steam. The most important of these are turbines, condensers, and electrical generators, though moisture separators or regenerative heaters can also be found here.
Steam turbine
A steam turbine is a rotary device that converts steam energy into mechanical rotational energy belonging to the turbine's spindle. It is used to drive electrical generators in all steam cycles. Larger turbines are made up of several parts, which in turn are made up of runners that gradually increase in size.
Electric generator
An electrical generator is a rotary machine that converts the mechanical energy taken from the turbine rotor to electrical energy. Usually its rotor crates a rotating magnetic field, which induces voltage in the stator windings.
Condenser
The steam from the turbine’s output is condensed inside the condenser. The condenser is a kind of heat-exchanger. The condensed steam is then fed through a system of regeneration heat-exchangers back to the steam generator (the secondary circuit is also closed). Heated cooling water from the condenser is cooled in the cooling tower.
Power output
Net electrical power output is a measure of how much electricity is supplied to the grid. Part of the generated electricity is consumed by the power plant itself for production purposes. Since the power output differs between winter and summer, due to seasonal variation in the cooling efficiency, an average power output or the lowest summer output is used.
The reactor is the nuclear power plant's heat source. Heat is released during the fission chain reaction that takes place in the nuclear fuel inside the reactor.
Steam generator
The steam generator's heat exchanger is the interface between the primary and secondary circuit. Its task is to generate secondary steam using heat from the reactor.
Main circulation pump
Circulates the primary circuit coolant – the so-called transport of generated heat from the reactor into the steam generators.
Pressurizer system
Pressure in the primary circuit is maintained by a pressurizer. The pressurizer is partially filled with water which is heated to the saturation temperature (boiling point) by submerged electrical heaters for the desired pressure.
Fuel loading machine
The fuel loading machine is a nuclear fuel handling piece of equipment designed for loading and unloading operations with nuclear fuel.
Spent fuel pool
Spent fuel withdrawn from the nuclear reactor is stored under the water. The water acts as radioprotective shielding and as a coolant.
Polar crane
The nuclear polar crane can be used to lift the main equipment of the primary circuit (e.g., steam generator, reactor pressure vessel, pressure vessel top cover, pressing unit, basket, and spent fuel containers) during the nuclear power plant construction, operation, and decommissioning.
Heat is generated in a nuclear power plant by the fission of uranium nuclei in the nuclear fuel loaded in the reactor core. Natural or enriched uranium or MOX – a mixture of uranium and plutonium oxides, is usually used to produce nuclear fuel. The PWR reactor core also contains lightweight water as a moderator and at the same time as a coolant. Absorber rods are used to control the course of the fission reaction.
Fuel assembly
PWR fuel assembly usually contains 179–264 rods; the fuel assembly diameter is approximately 20 cm. The rods are structurally supported not to be mutually displaced by the reactor core's extreme thermal and strain stresses. Some assemblies have fewer rods; control and regulating rods are inserted into the resulting gaps. The total fuel, about 121–193 fuel assemblies, is called a fuel load in the reactor core. In total, there are 18 million pellets in the reactor core. The PWR fuel assemblies have a square cross-section; the fuel for the Russian VVER reactors has a hexagonal cross-section.
Control rod drive mechanism
A mechanism on the top of the reactor vessel cover capable of inserting or withdrawing a control rod at a slow, controlled rate, as well as providing a rapid insertion in the event of an abnormal situation.
Control rods
Control rods (also called regulating rods) made of steel alloyed with boron and containing cadmium or hafnium, are inserted in between the fuel bundles in a nuclear reactor. The absorber concentration is decreased by pulling the rods out of the reactor core, causing the reactor power to increase. Inserting more control rods causes the reaction to be inhibited and the power output decreases. In some types of reactors, the upper part of the fuel bundles is made of absorbing material; the depth of the fuel insertion into the reactor core controls the reactor power.
Neutron reflector
The reactor core is surrounded by a neutron reflector (reactor core baffle) that reflects back neutrons that would otherwise escape. These neutrons are again available for chain reaction.
Reactor pressure vessel
The reactor vessel body is the most significant component of the primary circuit. It contains the fuel assemblies, coolant, and fittings to support a coolant flow, and support structures. It is usually cylindrical and is open at the top to allow the fuel to be loaded.
Cooling water inlet
Water than has expended some of its energy in the steam generator is returned to the reactor via the cold branch by the main circulation pump. After passing through the coupling in the pressure vessel, the water is directed down along the wall of the vessel, at the bottom it makes a turn and enters the core in a uniform manner.
Hot water outlet
The outlet coupler is where the hot branch of the primary pipes is connected to the reactor's pressure vessel. After being heated as it passes through the reactor core, the water then proceeds through the outlet piping to the steam generator. The number of inlet and outlet openings in the reactor vessel is the same as the number of branches in the primary circuit.
Did you know?
A PWR has about 150-250 fuel assemblies, each assembly containing 200 to 300 fuel rods. It has about 80–100 tons of uranium in its reactor core.
A hermetically sealed tube made of zirconium and niobium alloy, and filled with fuel pellets is called a fuel rod. It is usually 4–5 meters long. Each rod contains 350–400 pellets. A single rod contains about 1.5 kg of UO₂. The cladding prevents any direct contact between the cooling water and the fuel, while it has to be transparent to the neutrons. The narrow gap between the pellet and the casing is usually filled with helium for better heat removal. This gap also provides space for possible pellet expansion caused by heat and gases produced during fission.
Fuel pellet
A Cylindrical form of nuclear fuel made of sintered uranium oxide UO₂. The fresh fuel is enriched with uranium ²³⁵U isotope to about 4%–5%. The PWR fuel assembly is composed of uranium cylindrical pellets, 1 cm long and about 8 mm in diameter.
Cladding
Fuel pellets are placed in a narrow metal tube made of pure zirconium alloyed with titanium, niobium, and chromium. The tube is hermetically sealed on both ends and the empty space between the fuel and the tube wall is filled with helium. The cladding protects the fuel from corrosion and prevents the release of fission products into the primary circuit.
Rod cluster control
Control rods holder that allows the control rods to be moved inside the fuel assembly all at once.
Regulating rod / Control rod
Control rods (also called regulating rods) made of steel alloyed with boron and containing cadmium or hafnium, are inserted in between the fuel bundles in a nuclear reactor. The absorber concentration is decreased by pulling the rods out of the reactor core, causing the reactor power to increase. Inserting more control rods causes the reaction to be inhibited and the power output decreases. In some types of reactors, the upper part of the fuel bundles is made of absorbing material; the depth of the fuel insertion into the reactor core controls the reactor power.
Support lattice
It is a part of the load-bearing structure of a fuel assembly composed of lattices, a single central channel used for measurements, and 18 guide thimbles for clusters. The lattices support the fuel rods.
The circular feed pipe containing a number of bent outlet tubes is located in the upper part of the steam generator. It brings feedwater into the space between the wall of the pressure vessel and the wall of the pipe bundle.
Input collector (hot leg)
A cylindrical chamber dividing the heated primary circuit coolant into individual heat-exchange tubes.
Output collector (cold leg)
Coolant, releasing some of its energy in the steam generator, is returned via the output collector, back to the nuclear reactor.
Heat-exchange tube bundle
Steam is generated when feedwater comes into contact with the surfaces of hot tube bundles (thousands of small diameter tubes) conducting the primary coolant.
Primary moisture separator
Primary separation of small water droplets from generated steam is usually performed in vertical steam generators by centrifugal cyclone separators.
Secondary moisture separator
In secondary louvre separators, the steam is dried further. They are usually made of corrugated sheet metal, creating zig-zag channels that let steam pass while capturing water droplets.
Steam outlet
Typically saturated steam from the PWR steam generator has a temperature of approximately 275 °C (527 °F). From the steam generator steam is fed to a steam turbine (secondary circuit).
Did you know?
Each steam generator can measure up to 21 m (70 feet) in height and weigh up to 800 tons. Each steam generator can contain from 3,000 to 16,000 heat-exchange tubes.
A steam turbine is a rotary device that converts steam energy into mechanical rotational energy belonging to the turbine's spindle. It is used to drive electrical generators in all steam cycles. Larger turbines are made up of several parts, which in turn are made up of runners that gradually increase in size.
Electric generator
An electrical generator is a rotary machine that converts the mechanical energy taken from the turbine rotor to electrical energy. Usually its rotor crates a rotating magnetic field, which induces voltage in the stator windings.
Condenser
The steam from the turbine’s output is condensed inside the condenser. The condenser is a kind of heat-exchanger. The condensed steam is then fed through a system of regeneration heat-exchangers back to the steam generator (the secondary circuit is also closed). Heated cooling water from the condenser is cooled in the cooling tower.
Power output
Net electrical power output is a measure of how much electricity is supplied to the grid.
Part of the generated electricity is consumed by the power plant itself for production purposes. Since the power output differs between winter and summer, due to seasonal variation in the cooling efficiency, an average power output or the lowest summer output is used.
Steam enters the turbine with a certain internal heat energy, which gradually changes in the individual sections of the turbine by expansion to the kinetic energy of the rotating turbine shaft.
Low pressure section
The impeller diameters of the last low-pressure sections are limited by the maximum centrifugal force acting on the blades and thus their length. If there is more steam in the low-pressure area of the turbine, it is necessary to divide it into two, sometimes three low-pressure parts. For multi-section steam turbines, the impeller diameter of each subsequent section is larger, because the gradual expansion of the steam also increases its volume.
Turbine rotor
The rotor of a steam turbine consists of a central spindle and several runners mounted on the spindle. The energy of the steam in the turbine causes the rotor to rotate, transferring mechanical energy to an electrical generator.
Runner blades
The runner blades have a complex shape, and their manufacture is subject to stringent requirements. They are usually either cast or precision milled. Due to the changing parameters of steam, the blades of each runner are larger then the previous ones. In the case of low-pressure sections their length would exceed strain limits, hence the steam is split into several smaller parallel low-pressure sections.
Guide vanes
Wheels with immobile guide vanes are attached to the turbine casing. These vanes direct the steam to the runner blades. Every runner on the rotor has its own stationary wheel with guide vanes.
Bearings
The entire long turbine rotor is supported by several radial sliding bearings. To eliminate axial forces, a thrust bearing is installed on the spindle. Turbine bearings have their own circulatory system for cooling and lubricating oil.
Turbine casing
A turbine's stator casing is usually cast from steel, and for low-pressure parts can also be a welded structure. It is sectioned horizontally and its shape follows the shape of the rotor. A sectioned casing permits convenient installation of guide vanes and precise rotor placement.
The electric generator works on the principle of electromagnetic induction - the rotating magnetic field formed by the rotor coils generates an alternating electric voltage in the fixed stator coils.
Stator
In current power systems, a three-phase power line is used, so all power electric generators usually have at least three pairs of stator coils, one for each section.
Power output
Power electric generators usually have at least three pairs of stator coils, one for each phase. The resulting electrical current is then led via three encapsulated conductors to a transformer.
Did you know?
As of December 31, 2020, all operational pressurized water reactors (PWR) worldwide together achieved a net electrical power output of 287.1 gigawatts. PWR's are the most common type of nuclear reactors used in nuclear power plants, with 302 in operation at that time. By comparison, boiling water reactors were the second most used reactor type, with a net electrical power of 64.1 gigawatts.
Hot water is distributed to the spray nozzles by piping or by concrete flumes that ensures an even distribution of warm water to all spray nozzles.
Cold water
Cooled water is collected in the round pool at the tower base and fed by pumps back to the turbines condenser.
Hot water
Cooling water heated in steam generator condensers is transported via pipes to the cooling tower. There, it is transported upward through riser channels and sent to all spray nozzles by a distribution system.
Fill
The fill of a colling tower is a large, usually plastic structure with many vertical channels. Heated water is sprayed on their walls and runs down them as cold air flows up the channels. Over a longer period of time the thin film of water allows for efficient heat transfer, mainly through evaporation.
Supports
The hyperboloid concrete shell of a cooling tower is stands on a series of supporting concrete struts. Large amounts of cold outside air can be sucked through the gaps between these diagonal columns.
Hot water spray system
The incoming warm water is distributed through spray nozzles installed inside the tower at roughly 10% of its total height. The spray nozzles spray the warm water evenly over the entire fill. Cooling starts as soon as the water exits the nozzles, so it is necessary to maximize the interaction between the hottest water and the coldest air. A nozzle that generates very small droplets will have a greater cooling effect than a nozzle that makes big, fat droplets.
Drift eliminators
The air rising from the cooling tower carries small water droplets. To minimize water loss in the exhaust air it has to pass through a system of baffle-like devices called drift eliminators. The direction of air travel is altered by the eliminators foils or blades. This causes water droplets entrained in the airstream to collect on the eliminator foil surfaces. The droplets agglomerate and they rapidly gain sufficient size and mass to overcome the discharge airstream velocity and drain back into the cooling tower.
Tower shell
Cooling towers with a natural air draft have the shape of a rotational hyperboloid. This shape requires the least amount of concrete to be needed for such a big structure, gives the cooling tower an ability to withstand the force of cross-winds, and accelerates the air flow through the tower, which increases the tower’s cooling capacity. The cooling towers are very large structures, with bases as wide as 100 m and up to 200 m tall.
Natural Air Draft
The natural draft is created by the density difference between moist warm air inside the tower and cold dry air outside the tower. Moist warm air has a lower density than drier air, and so rises up, and the moist air’s buoyancy produces a current of air through the tower. At the wide base, there is enough space for a cooling system which sucks large amounts of cold air from outside. At the middle of the towers’ height the air speed increases and its density decreases. After expansion from a wider opening, the air cools quickly and is violently mixed with outside cold air.
Did you know?
The hyperboloid cooling tower was patented by Frederik van Iterson and Gerard Kuypers in 1918.
