Types of nuclear power plants
Nuclear power is a type of energy that humans have harnessed for over seventy years. It is a highly efficient and reliable electricity source widely used worldwide.
Nuclear power plants come in different types, each with unique operating characteristics. Here are the major types of nuclear power plants:
- Pressurised water reactor (PWR)
- Boiling-water nuclear reactor (BWR)
- Light water graphite-moderated reactor (LWGR)
- Advanced gas-cooled reactor (AGR)
- Pressurised heavy water reactor (PHWR)
- Fast neutron reactor (FNR)
In this article, we will discuss how nuclear power plants work and go into more depth on how each nuclear reactor works.
How does a nuclear power plant work?
A nuclear power plant is a facility that harnesses the energy released by nuclear reactions to generate electricity. The process involves several steps:
1. Nuclear reaction
In the nuclear reactor core, uranium atoms are split apart in a process called nuclear fission. This releases a tremendous amount of energy in the form of heat.
2. Heat transfer
The heat generated by the nuclear reactions is transferred to a coolant, such as water, which circulates through the reactor core. The coolant absorbs the heat and carries it away from the reactor to a heat exchanger.
3. Steam generation
The heat exchanger converts the heat from the coolant into steam. This steam is typically produced at very high temperatures and pressures.
4. Turbine and generator
The high-pressure steam is directed to a turbine connected to a generator. As the steam flows through the turbine, it causes the turbine blades to spin, generating mechanical energy.
5. Electrical power
The generator converts the mechanical energy from the turbine into electrical energy, which is then sent to a transformer and distributed to the electrical grid.
6. Cooling
The steam is then cooled back into water and returned to the heat exchanger to be heated again in a continuous cycle.
Nuclear power plants require careful monitoring and maintenance to ensure their safe and efficient operation. They must also address the challenge of safely managing the nuclear waste produced by the fission process. However, they offer a reliable and relatively clean source of electricity with a low carbon footprint compared to traditional fossil fuel power plants.
Types of nuclear power plants
There are many types of nuclear power plants, the most common of which are the pressurised water reactor (PWR) and boiling water nuclear reactor (BWR).
Pressurised water reactor (PWR)
Pressurised water reactors (PWR) take up almost 70% of the global reactor fleet, though not in the UK.
Sanmen Nuclear Power Station in China is an example of a new generation of pressurised water reactors.
They are light-water nuclear reactors that use water as their primary coolant. This water is pressurised and pumped into the reactor core, where it is heated by the energy released by the fission of atoms.
The hot and pressurised water is directed towards a steam generator, usually a type of shell and tube heat exchanger with a U-tube design, where it transmits thermal energy to a secondary system with lower-pressure water where steam is generated. The steam then drives turbines, which spin an electric generator.
The pressure in the primary coolant loop prevents water from boiling within the reactor and uses ordinary coolant and neutron moderators. Typically, these reactors are equipped with two to four steam generators mounted vertically.
Pressurised water reactors (PWRs) were initially developed to function as nuclear propulsion systems for nuclear submarines.
Advantages of pressurised water reactor (PWR)
- Easy to operate and stable as less power is produced when the heat increases
- High power density
- Isolation of radioactive materials from the main steam system
Disadvantages of pressurised water reactor (PWR)
- The construction of the PWR is costly
- Reactors cannot be re-fuelled while operating, and the reactors will need to be shut down to be refuelled
Boiling-water nuclear reactor (BWR)
Boiling-water nuclear reactors (BWR) are the second most common reactor type globally, making up 15% of the global fleet. A BWR is a light-water nuclear reactor used to generate electricity. It uses nuclear fission to heat water and produce steam, which drives a turbine to generate electricity.
In a BWR, the nuclear fuel is arranged in fuel rods within the reactor core. Water circulates through the core, where the nuclear reaction boils it, producing steam. The steam turns a turbine, which drives a generator to produce electricity, and the steam is condensed back into water and returned to the core to be heated again.
Advantages of boiling-water nuclear reactors
- BWRs require less water and use a simpler design than PWR
- Lower construction and maintenance costs
- The efficiency of the system is high
Disadvantages of boiling-water nuclear reactors
- Possible chance of radioactive contamination of the steam turbines
- It cannot meet sudden changes in load on the plant
- The system requires extensive safety devices against radioactive radiation, which are costly
Light water graphite-moderated reactor (LWGR)
A light water graphite-moderated reactor (LWGR) uses carbon as light water as a coolant and graphite as a moderator. The fuel in an LWGR is typically enriched uranium dioxide, which is contained in fuel rods arranged in a regular pattern within the reactor’s core.
The function of the graphite moderator is to slow down the neutrons produced during nuclear fission so that they can more easily collide with other uranium atoms and sustain the nuclear chain reaction. The coolant, usually ordinary water, removes the heat generated by the fission reactions and carries it away from the reactor core to drive steam turbines that generate electricity.
Advanced gas-cooled reactor (AGR)
Advanced gas-cooled reactors (AGR) are designed and operated in the United Kingdom, and almost all the active nuclear reactors in the UK use this design.
An example is the Torness nuclear power plant about 30 miles from Edinburgh.
Photo: By Taras Young – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=48238064)
AGR uses graphite as the neutron moderator and carbon dioxide as the coolant. The fuel in an AGR is typically enriched uranium dioxide, which is contained in fuel rods arranged in a regular pattern within the reactor’s core.
As in a light water-moderated reactor, the graphite moderator allows the neutrons to collide with other uranium atoms more easily by slowing them down, keeping the nuclear chain reaction going. The coolant, which in an AGR is carbon dioxide gas, removes the heat from the fission reactions and carries it away from the reactor core to drive steam turbines that generate electricity.
Pressurised heavy water reactor (PHWR)
A pressurised heavy water reactor (PHWR) is a reactor that uses heavy water as the coolant and the moderator. Heavy water is a form of water that contains a higher proportion of the isotope deuterium than normal water, which makes it more effective at slowing down neutrons and facilitating nuclear reactions. The heavy water coolant is kept under pressure to avoid boiling, allowing it to reach a higher temperature without forming steam bubbles.
The fuel found in PHWRs is typically natural uranium dioxide, contained in fuel bundles arranged in a regular pattern within the reactor’s core. The heavy water moderator slows down the neutrons produced during nuclear fission so that they can more easily collide with other uranium atoms and sustain the nuclear chain reaction. In addition, the coolant, which is also heavy water, removes the heat generated by the fission reactions and carries it away from the reactor core to drive steam turbines that generate electricity.
Fast neutron reactor (FNR)
A Fast Neutron Reactor (FNR) is a nuclear reactor that uses fast neutrons to sustain a nuclear chain reaction. Unlike conventional reactors, which use slow neutrons, FNRs operate at very high temperatures and with a high neutron flux. This allows them to use a broader range of fuel types and achieve higher efficiency in converting nuclear energy into electricity.
FNRs can use a variety of fuels, including plutonium, uranium-233, and thorium, and are capable of producing more fuel than they consume, making them a potentially sustainable source of nuclear energy. The coolant used in FNRs is typically liquid sodium, which has excellent heat transfer properties and can operate at high temperatures without boiling.
FNRs are still in the development stage, and there are currently only a few operational reactors of this type in the world. FNRs have the potential to offer a safer and more sustainable alternative to conventional nuclear reactors, but they also present technical and safety challenges that must be carefully addressed.
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