How does a nuclear power plant work?
A nuclear power plant is an industrial site that generates electricity from nuclear power, released in the form of thermal energy through a nuclear fission chain reaction inside the vessel of a nuclear reactor.
The main component of a nuclear power plant is the nuclear reactor, which contains the nuclear fuel (usually uranium) and has systems that make it possible to start, sustain and stop the nuclear reaction in a controlled manner.
The way a nuclear power plant works is similar to a conventional thermal plant, where thermal energy is obtained through the combustion of fossil fuels. In a nuclear reactor, however, this energy is obtained via the nuclear fission chain reactions of the uranium atoms from the nuclear fuel.
A nuclear power plant is an industrial site that generates electricity from the thermal energy produced by nuclear fission chain reactions in the vessel of a nuclear reactor
The liberated thermal energy is used to heat water at high pressure and high temperature until it becomes steam. This steam spins a turbine connected to a generator which transforms the mechanic energy of the turbine spins into electric energy.
There are different types of nuclear reactors, but two outstanding designs are present in over 80% of the almost 450 operative units in the world:
- Pressurized Water Reactor (PWR)
- Boiling Water Reactor (BWR)
How a PWR reactor works
It is important to keep in mind that in nuclear fission the nuclei of heavy atoms are bombarded with neutrons and then decompose into smaller and lighter nuclei. When this happens they release the energy that binds the neutrons and protons that compose them, and then they emit two or three neutrons. These can produce more fissions as they interact with new heavy nuclei, which will then emit new neutrons and so on, so that the reaction sustains itself. This multiplying effect is known as nuclear fission chain reaction.
The way a nuclear power plant works can be simplified into five stages:
- The uranium fission takes place inside the nuclear reactor. It releases a large amount of energy that heats the coolant water circulating at a very high pressure. This water is transported via the primary circuit to a heat exchanger (steam generator) that produces water steam.
- This steam is transported to the generator-turbine set via a secondary circuit.
- Once there, the vanes in the turbine move the alternator and the mechanic energy is transformed into electricity.
- When the water steam goes through the turbine it is sent to a condenser to cool off and become liquid water again.
The water is then transported to the new steam generator to become steam again inside a closed circuit.
Main components
It was previously indicated that a nuclear reactor is a site that can initiate, sustain and stop nuclear fission chain reactions in a controlled way, with the adequate means to extract the generated heat.
The central component of a nuclear power plant is the reactor, the site that houses the nuclear fuel
Its main components are:
- Fuel. The material, usually enriched uranium dioxide, where the fission reactions take place. It is used simultaneously as a source of energy and of neutrons to sustain the chain reaction. It is presented in a solid state in the form of cylindrical pills encapsulated into metallic rods a few meters long.
- Moderator. Water that slows down the fast neutrons generated by the fission, which leads to new fissions and the sustainment of the chain reaction.
- Cooling water. The same water that induces the fissions as a moderator, which now serves to extract the heat generated by the fission reaction from the uranium in the fuel.
- Control rods. The control elements in the reactor. They act as neutron absorbers. These rods are made of indium-cadmium or boron carbide and make it possible to constantly control the neutron population whilst keeping the reactor stable; they also make it possible to stop the reaction whenever necessary.
- Shielding. It prevents the leak of radiations and neutrons from inside the reactor to the outside. Usually shielding is made up of concrete, steel or lead.
- Safety elements. All nuclear power plants have multiple safety systems to prevent the leak of radioactivity to the outside. These systems include the containment building.
The components of a nuclear power plant
A component of the primary coolant circuit. It balances the liquid stage and the steam stage at conditions of saturation in order to control its pressure.
A steel vessel that houses the nuclear reactor, the main component of the nuclear power plant, where the chain reaction nuclear fission takes place. The nucleus is composed of the fuel elements.
The material where the fission reactions take place. It is usually enriched uranium dioxide. It is used simultaneously as a source of energy and of neutrons in order to sustain the chain reaction. It is presented in a solid state in the form of cylindrical encapsulated pills, inside metal rods about 4 meters long.
These are the reactor’s control elements, which serve as neutron absorbents. They are bars made of indium-cadmium or boron carbide and make it possible to constantly control the population of neutrons and the reactor’s reactivity. This way it is critical during its operation and sub-critical during outages.
Heat exchangers where the primary circuit coolant water, which circulates inside the inverted U-shaped tubes, gives all its energy to the secondary circuit and becomes water steam.
It houses the reactor’s coolant system as well as several auxiliary systems. It acts as shielding during normal operation and prevents the leakage of polluting products to the exterior. Its functional responsibility, along with other safeguard elements, is to avoid releasing fission elements to the atmosphere in the event of an accident.
This is where the water steam from the steam generators is collected, and where the vanes transform its energy into mechanical rotation energy. There are various sections for the expansion of steam. The axis is firmly attached to the alternator axis.
A system that produces electricity by converting the turbine’s mechanical rotation energy into medium-power, high-intensity electric energy.
This system increases the tension of the electricity produced in the alternator to minimize losses in its transport to the consumption points.
Water taken from a river, reservoir or the sea and used to liquify the water steam in the condenser. It can be returned to its source (open cycle) or reused in the cooling tower (closed circuit).
A site that makes it possible to send out to the atmosphere part of the residual heat produced in the generation of electricity; the atmosphere acts as a cold focus. It is used to cool down the water that circulates through the condenser and is part of the plant’s auxiliary cooling circuit.
Heat exchanger composed by a set of tubes, inside which the coolant water circulates. The water steam that enters the condenser from the turbine is then liquified. This conversion produces a vacuum that improves the operation of the turbine.
