Nuclear reactors are categorized by their underlying nuclear reaction, the coolant they employ, and their design development maturity (Generation type).
1) Type of Nuclear Reactor Based on Nuclear Reaction:
Fission – All commercial power nuclear reactors are Fission reactors. Fuel used as Uranium or Plutonium. Thorium Cycle is also possible. These can be further sub-divided as Thermal Neutron Reactors and Fast Neutron Reactors
Fusion - Fusion power produced by nuclear fusion of elements such as the deuterium isotope of hydrogen which is still in experimental stages.
2) Type of Nuclear Reactor Based on Type of Coolant
- Water Cooled Reactors:
- Light water-cooled Reactors - used in Western Countries e.g. Pressurised Water Reactor (PWR), Boiling Water Reactors (BWR) and Supercritical Water Reactor
- Heavy water-cooled Reactors - used in India, Argentina, China, South Korea e.g. Pressurised Heavy Water Reactor
- Liquid Metal Cooled Reactors
- Gas Cooled Reactors
- Molten Salt Reactors
3) Type of Nuclear Reactor Based on design upgradation (Generation type)
- Generation I - early prototypes design
- Generation II - most current nuclear power plants, 1965–1996)
- Generation III - evolutionary improvements of existing designs, 1996–2016
- Generation III+ - evolutionary development of Gen III reactors, offering improvements in safety over Gen III reactor designs, 2017–2021
- Generation IV - technologies still under development; start possibly by 2030.
Some of the major reactors deployed across the globe is detailed out in the below part.
Pressurized water reactors (PWRs)
Pressurized Water Reactors keep water under high pressure, so it does not boil. That hot water flows through pipes to a steam generator, where it heats a separate water loop to make steam for turbines. Inside the reactor vessel, fuel rods hold uranium-235. Control rods move between the fuel rods to control how fast the atoms split and how much power is generated. The primary hot water stays inside its loop, so most radioactivity stays in the reactor. About 70 percent of reactors around the world are PWRs because they use well‐tested technology and run reliably.
Boiling water reactors (BWRs)
Boiling Water Reactors turn water into steam right inside the big reactor tank, so they do not need extra steam generators. Demineralized water flows through the fuel rods and gets hot until it becomes a mix of water and steam. This mix goes to a separator that takes out water droplets, leaving only dry steam to spin the turbine. Control rods move upward from the bottom of the vessel into the core to adjust how quickly atoms split and how much power is produced. Their simple design means fewer pipes, but the turbine must use steam that is a little bit radioactive.
Pressurized heavy water reactors (PHWRs)
Pressurized Heavy Water Reactors use heavy water (D₂O) instead of normal water as both moderator and coolant. This lets them run on natural uranium without enriching it first. The neutron economy provided by heavy water's minimal neutron absorption enables a sustained fission reaction, even with natural uranium. Canada’s CANDU reactors and India’s PHWRs have high uptime due to online fueling capability in the design and can also use recycled fuel. Since they need only unprocessed uranium, PHWRs make good use of domestic uranium deposits.
High-temperature gas-cooled reactors (HTGRs)
High-temperature gas-cooled reactors circulate helium gas through a core moderated by graphite at about 700 °C to over 900 °C. Their fuel uses TRISO-coated particles - tiny kernels wrapped in several ceramic layers, packed into graphite pebbles or prismatic blocks. High outlet temperatures boost thermal efficiency to nearly 45 percent and enable industrial uses like hydrogen production or processing heat. Helium is chemically inert and never becomes radioactive. Passive systems rely on natural convection to remove decay heat, so even without active power, the reactor can cool itself safely.
Sodium-cooled fast reactors (SFRs)
Sodium-cooled fast reactors do not use a moderator. They rely on fast neutrons to split atoms. Liquid sodium flows through the core at about 500 °C under near-atmospheric pressure to carry away heat. Because sodium is an excellent heat conductor, these reactors run efficiently and support high power density. Fast neutrons can split uranium-238 and plutonium-239, allowing SFRs to set up for breeding to create more fuel than they use. This lets them consume old plutonium from used fuel or run on mixed-oxide fuel, helping to close the nuclear fuel cycle.
Molten salt reactors (MSRs)
Molten Salt Reactors use a mixture of fluoride or chloride salts as both fuel solvent and coolant. They run at atmospheric pressure and high temperatures, typically 600–700 °C. Dissolving fuel directly into the salt removes the need for solid fuel assemblies and allows continuous online reprocessing to remove waste and add fissile material. These reactors operate at low pressure, feature strong negative temperature feedback, and do not require complex coolant pumps, reducing the chance of a loss-of-coolant accident. Developed at Oak Ridge in the 1950s, MSRs are still in experimental and prototype phases.
Small modular reactors (SMRs)
Small Modular Reactors usually produce up to 300 MWe and are built in factories for easier, faster deployment. They use proven designs like small versions of pressurized water reactors or newer concepts such as high-temperature gas or fast reactors. Standardized parts and simple safety systems reduce onsite construction time and costs. Their compact size lets them work on smaller grids or in remote areas. Many SMR designs include features that make them more resistant to earthquakes and can isolate seismic shocks. Because they are smaller and modular, utilities can add capacity in steps rather than all at once.
SMRs offer numerous safety benefits like:
- Passive heat removal, i.e., heat is dissipated from the reactor without operator or control system actions
- Below grade installation of the reactor pool and spent fuel storage for enhanced resistance to seismic events and improved security
- Integrated and simplified design with reduced componentry to increase accident-free operation
- Convection cooling in which vessel and componentry facilitate natural convection cooling of the core and vessel
- Reduced on-site inventory and increased security
Economic benefits of SMRs include:
- Modular design and factory fabrication - reduced capital costs & construction time, reduced financial risk and financing costs
- Smaller core and smaller overall size - reduced footprint
- Smaller power modules - increased flexibility to add capacity and increase grid compatibility
- Carbon free power & ability to pair with renewables, such as wind and solar
- Industrial applications for heat and / or process steam
- Less refueling frequency
Micro Reactors
- An unprecedented development trend emerged on very small SMRs called Micro reactors designed to generate electrical power of typically up to 10 MWe. The range of electrical output is between 1.5 to 5 MW(e), the adopted enrichment is between 4% up to 19.75%, with long fuel cycle between 36 months up to 20 years.
- They are from different types of coolant, including HTGRs and designs that use heat pipes for heat transport.
- Several designs are undertaking licensing activities in Canada and the United States for planned near-term deployment.
- Microreactors serve future niche electricity and district heat markets in remote regions, mining, industries, and fisheries that for decades have been served by diesel power plants.