A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. Heat from nuclear fission is passed to a working fluid, which in turn runs through steam turbines. These either drive a ship's propellers or turn electrical generators' shafts. Nuclear generated steam in principle can be used for industrial process heat or for district heating. Some reactors are used to produce isotopes for medical and industrial use, or for production of weapons-grade plutonium. As of 2022, the International Atomic Energy Agency reports there are 422 nuclear power reactors and 223 nuclear research reactors in operation around the world.
A pressurized water reactor (PWR) is a type of light-water nuclear reactor. PWRs constitute the large majority of the world's nuclear power plants. In a PWR, the primary coolant (water) is pumped under high pressure to the reactor core where it is heated by the energy released by the fission of atoms. The heated, high pressure water then flows to a steam generator, where it transfers its thermal energy to lower pressure water of a secondary system where steam is generated. The steam then drives turbines, which spin an electric generator. In contrast to a boiling water reactor (BWR), pressure in the primary coolant loop prevents the water from boiling within the reactor. All light-water reactors use ordinary water as both coolant and neutron moderator. Most use anywhere from two to four vertically mounted steam generators; VVER reactors use horizontal steam generators.
A breeder reactor is a nuclear reactor that generates more fissile material than it consumes. These reactors can be fueled with more-commonly available isotopes of uranium and thorium, such as uranium-238 and thorium-232, as opposed to the rare uranium-235 which is used in conventional reactors. These materials are called fertile materials since they can be bred into fuel by these breeder reactors.
A fast-neutron reactor (FNR) or fast-spectrum reactor or simply a fast reactor is a category of nuclear reactor in which the fission chain reaction is sustained by fast neutrons, as opposed to slow thermal neutrons used in thermal-neutron reactors. Such a fast reactor needs no neutron moderator, but requires fuel that is relatively rich in fissile material when compared to that required for a thermal-neutron reactor. Around 20 land based fast reactors have been built, accumulating over 400 reactor years of operation globally. The largest of this was the Superphénix Sodium cooled fast reactor in France that was designed to deliver 1,242 MWe. Fast reactors have been intensely studied since the 1950s, as they provide certain decisive advantages over the existing fleet of water cooled and water moderated reactors. These are:
The integral fast reactor is a design for a nuclear reactor using fast neutrons and no neutron moderator. IFR would breed more fuel and is distinguished by a nuclear fuel cycle that uses reprocessing via electrorefining at the reactor site.
A molten salt reactor (MSR) is a class of nuclear fission reactor in which the primary nuclear reactor coolant and/or the fuel is a mixture of molten salt with a fissionable material.
Nuclear fuel is material used in nuclear power stations to produce heat to power turbines. Heat is created when nuclear fuel undergoes nuclear fission.
Generation IVreactors are nuclear reactor design technologies that are envisioned as successors of generation III reactors. The Generation IV International Forum (GIF) – an international organization that coordinates the development of generation IV reactors – specifically selected six reactor technologies as candidates for generation IV reactors. The designs target improved safety, sustainability, efficiency, and cost. The first commercial Gen IV plants are not expected before 2040–2050, although the World Nuclear Association in 2015 suggested that some might enter commercial operation before 2030.
The lead-cooled fast reactor is a nuclear reactor design that features a fast neutron spectrum and molten lead or lead-bismuth eutectic coolant. Molten lead or lead-bismuth eutectic can be used as the primary coolant because especially lead, and to a lesser degree bismuth have low neutron absorption and relatively low melting points. Neutrons are slowed less by interaction with these heavy nuclei and therefore, help make this type of reactor a fast-neutron reactor. In simple terms, if a neutron hits a particle with a similar mass, it tends to lose kinetic energy. In contrast, if it hits a much heavier atom such as lead, the neutron will "bounce off" without losing this energy. The coolant does, however, serve as a neutron reflector, returning some escaping neutrons to the core. Fuel designs being explored for this reactor scheme include fertile uranium as a metal, metal oxide or metal nitride. Smaller capacity lead-cooled fast reactors can be cooled by natural convection, while larger designs use forced circulation in normal power operation, but will employ natural circulation emergency cooling. No operator interference is required, nor pumping of any kind to cool the residual heat of the reactor after shutdown. The reactor outlet coolant temperature is typically in the range of 500 to 600 °C, possibly ranging over 800 °C with advanced materials for later designs. Temperatures higher than 800 °C are theoretically high enough to support thermochemical production of hydrogen through the sulfur-iodine cycle, although this has not been demonstrated.
A sodium-cooled fast reactor is a fast neutron reactor cooled by liquid sodium.
The Hallam Nuclear Power Facility (HNPF) in Nebraska was a 75 MWe sodium-cooled graphite-moderated nuclear power plant built by Atomics International and operated by Consumers Public Power District of Nebraska. It was built in tandem with and co-located with a conventional coal-fired power station, the Sheldon Power Station. The facility featured a shared turbo generator that could accept steam from either heat source, and a shared control room.
In nuclear power technology, online refuelling is a technique for changing the fuel of a nuclear reactor while the reactor is critical. This allows the reactor to continue to generate electricity during routine refuelling, and therefore improve the availability and profitability of the plant.
The liquid fluoride thorium reactor is a type of molten salt reactor. LFTRs use the thorium fuel cycle with a fluoride-based molten (liquid) salt for fuel. In a typical design, the liquid is pumped between a critical core and an external heat exchanger where the heat is transferred to a nonradioactive secondary salt. The secondary salt then transfers its heat to a steam turbine or closed-cycle gas turbine.
A traveling-wave reactor (TWR) is a proposed type of nuclear fission reactor that can convert fertile material into usable fuel through nuclear transmutation, in tandem with the burnup of fissile material. TWRs differ from other kinds of fast-neutron and breeder reactors in their ability to use fuel efficiently without uranium enrichment or reprocessing, instead directly using depleted uranium, natural uranium, thorium, spent fuel removed from light water reactors, or some combination of these materials. The concept is still in the development stage and no TWRs have ever been built.
The Energy Multiplier Module is a nuclear fission power reactor under development by General Atomics. It is a fast-neutron version of the Gas Turbine Modular Helium Reactor (GT-MHR) and is capable of converting spent nuclear fuel into electricity and industrial process heat.
Thorium-based nuclear power generation is fueled primarily by the nuclear fission of the isotope uranium-233 produced from the fertile element thorium. A thorium fuel cycle can offer several potential advantages over a uranium fuel cycle—including the much greater abundance of thorium found on Earth, superior physical and nuclear fuel properties, and reduced nuclear waste production. One advantage of thorium fuel is its low weaponization potential; it is difficult to weaponize the uranium-233/232 and plutonium-238 isotopes that are largely consumed in thorium reactors.
The Thorcon nuclear reactor is a design of a molten salt reactor with a graphite moderator, proposed by the US-based Thorcon company. These nuclear reactors are designed as part of a floating power plant, to be manufactured on an assembly line in a shipyard, and to be delivered via barge to any ocean or major waterway shoreline. The reactors are to be delivered as a sealed unit and never opened on site. All reactor maintenance and fuel processing is done at an off-site location. As of 2022, no reactor of this type has been built. A proposal to build a prototype in Indonesia has been submitted to the IAEA.
The Stable Salt Reactor (SSR) is a nuclear reactor design under development by Moltex Energy Canada Inc. and its subsidiary Moltex Energy USA LLC, based in Canada, the United States, and the United Kingdom, as well as MoltexFLEX Ltd., based in the United Kingdom.
A nuclear microreactor is a plug-and-play type of nuclear reactor which can be easily assembled and transported by road, rail or air. Microreactors are 100 to 1,000 times smaller than conventional nuclear reactors, and when compared with small modular reactors (SMRs), their capacity is between 1 and 20 megawatts whereas SMRs comes in the range from 20 to 300 megawatts. Due to their size, they can be deployed to locations such as isolated military bases or communities affected by natural disasters. It can operate as part of the grid, independent of the grid, or as part of a small grid for electricity generation and heat treatment. They are designed to provide resilient, non-carbon emitting, and independent power in challenging environments. The nuclear fuel source for the majority of the designs is "High-Assay Low-Enriched Uranium", or HALEU.
