's PULSTAR Reactor is a 1 MW pool-type
research reactor with 4% enriched, pin-type fuel consisting of UO2 pellets in
zircaloy cladding
Classifications By type of nuclear reaction All commercial power reactors are based on
nuclear fission. They generally use
uranium and its product
plutonium as
nuclear fuel, though a
thorium fuel cycle is also possible. Fission reactors can be divided roughly into two classes, depending on the energy of the neutrons that sustain the fission
chain reaction: •
Thermal-neutron reactors use slowed or
thermal neutrons to keep up the fission of their fuel. Almost all current reactors are of this type. These contain
neutron moderator materials that slow neutrons until their
neutron temperature is
thermalized, that is, until their
kinetic energy approaches the average kinetic energy of the surrounding particles. Thermal neutrons have a far higher
cross section (probability) of fissioning the
fissile nuclei
uranium-235,
plutonium-239, and
plutonium-241, and a relatively lower probability of
neutron capture by
uranium-238 (U-238) compared to the faster neutrons that originally result from fission, allowing use of
low-enriched uranium or even
natural uranium fuel. The moderator is often also the
coolant, usually water under high pressure to increase the
boiling point. These are surrounded by a
reactor vessel, instrumentation to monitor and control the reactor,
radiation shielding, and a
containment building. •
Fast-neutron reactors use
fast neutrons to cause fission in their fuel. They do not have a
neutron moderator, and use less-moderating coolants. Maintaining a chain reaction requires the fuel to be more highly
enriched in
fissile material (about 20% or more) due to the relatively lower probability of fission versus capture by U-238. Fast reactors have the potential to produce less
transuranic waste because all
actinides are fissionable with fast neutrons, but they are more difficult to build and more expensive to operate. Overall, fast reactors are less common than thermal reactors in most applications. Some early power stations were fast reactors, as are some Russian naval propulsion units. Construction of prototypes is continuing (see
fast breeder or
generation IV reactors). In principle,
fusion power could be produced by
nuclear fusion of elements such as the
deuterium isotope of
hydrogen. While an ongoing rich research topic since at least the 1940s, no self-sustaining fusion reactor for any purpose has ever been built.
By moderator material Used by thermal reactors: •
Graphite-moderated reactors • Mostly early reactors such as the Chicago pile, Obninsk am 1, Windscale piles, RBMK, Magnox, and others such as AGR use graphite as a moderator. • Water moderated reactors •
Heavy-water reactors (Used in Canada, India, Argentina, China, Pakistan, Romania and South Korea). •
Light-water-moderated reactors (LWRs). Light-water reactors (the most common type of thermal reactor) use ordinary water to moderate and cool the reactors. During normal operation, pressure is controlled by the amount of steam flowing from the reactor pressure vessel to the turbine. •
Supercritical water reactor (SCWR) • SCWRs are a
Generation IV reactor concept where the reactor is operated at supercritical pressures and water is heated to a supercritical fluid, which never undergoes a transition to steam yet behaves like saturated steam, to power a
steam generator. •
Reduced moderation water reactor [RMWR] which use more highly enriched fuel with the fuel elements set closer together to allow a faster neutron spectrum sometimes called an
Epithermal neutron Spectrum. • Pool-type reactor can refer to unpressurized water cooled
open pool reactors, but not to be confused with
pool type LMFBRs, which are sodium cooled. • Some reactors have been cooled by
heavy water which also served as a moderator. Examples include: • Early
CANDU reactors (later ones use heavy-water moderator but light-water coolant) •
DIDO class research reactors •
Liquid-metal-cooled reactor. Since water is a moderator, it cannot be used as a coolant in a fast reactor. Liquid-metal coolants have included
sodium,
sodium–potassium alloy (NaK), lead,
lead-bismuth eutectic, and in early reactors,
mercury. •
Sodium-cooled fast reactor •
Lead-cooled fast reactor •
Gas-cooled reactors are cooled by a circulating gas. In commercial nuclear power plants carbon dioxide has usually been used, for example in current British AGR nuclear power plants and formerly in a number of first generation British, French, Italian, and Japanese plants.
Nitrogen and helium have also been used, helium being considered particularly suitable for high temperature designs. Use of the heat varies, depending on the reactor. Commercial nuclear power plants run the gas through a
heat exchanger to make steam for a steam turbine. Some experimental designs run hot enough that the gas can directly power a gas turbine. •
Molten-salt reactors (MSRs) are cooled by circulating a molten salt, typically a eutectic mixture of fluoride salts, such as
FLiBe. In a typical MSR, the coolant is also used as a matrix in which the fissile material is dissolved. Other eutectic salt combinations used include
"ZrF4" with
"NaF" and
"LiCl" with
"BeCl2". •
Organic nuclear reactors use organic fluids such as biphenyl and terphenyl as coolant rather than water.
By generation • Generation I reactor (early prototypes such as
Shippingport Atomic Power Station, research reactors, non-commercial power producing reactors) •
Generation II reactor (most current
nuclear power plants, 1965–1996) •
Generation III reactor (evolutionary improvements of existing designs, 1996–2016) •
Generation III+ reactor (evolutionary development of Gen III reactors, offering improvements in safety over Gen III reactor designs, 2017–2021) •
Generation IV reactor (technologies still under development; unknown start date, see below) The first mention of "Gen III" was in 2000, in conjunction with the launch of the
Generation IV International Forum (GIF) plans. "Gen IV" was named in 2000, by the
United States Department of Energy (DOE), for developing new plant types.
By type of fuel • Uranium • Plutonium •
Mixed oxide (MOX) fuel • Uranium-plutonium alloy • Transuranium element mix (
neptunium,
plutonium,
americium,
curium) • Thorium
By phase of fuel • Solid fueled • Ceramic • Oxide • Carbide • Nitride • Metal • Fluid fueled •
Aqueous homogeneous reactor •
Molten-salt reactor • Molten metal reactor (e.g.
LAMPRE) •
Gas fueled (theoretical)
By shape of the core • Cubical • Cylindrical • Octagonal • Spherical • Slab • Annulus
By use • Electricity •
Nuclear power plants including
small modular reactors • Propulsion, see
nuclear propulsion •
Nuclear marine propulsion • Various proposed forms of
rocket propulsion • Other uses of heat •
Desalination • Heat for domestic and industrial heating •
Hydrogen production for use in a
hydrogen economy • Production reactors for
transmutation of elements •
Breeder reactors are capable of producing more
fissile material than they consume during the fission chain reaction (by converting
fertile U-238 to Pu-239, or Th-232 to U-233). Thus, a uranium breeder reactor, once running, can be refueled with
natural or even
depleted uranium, and a thorium breeder reactor can be refueled with
thorium; however, an initial stock of fissile material is required. • Creating various
radioactive isotopes, such as
americium for use in
smoke detectors, and cobalt-60, molybdenum-99 and others, used for imaging and medical treatment.,
Argentina) • Production of materials for
nuclear weapons such as
weapons-grade plutonium • Providing a source of
neutron radiation (for example with the pulsed
Godiva device) and
positron radiation (e.g.
neutron activation analysis and
potassium–argon dating) •
Research reactor: Typically reactors used for research and training, materials testing, or the production of radioisotopes for medicine and industry. These are much smaller than power reactors or those propelling ships, and many are on university campuses. There are about 280 such reactors operating, in 56 countries. Some operate with high-enriched uranium fuel, and international efforts are underway to substitute low-enriched fuel.
Current technologies – a PWR •
Pressurized water reactors (PWR) [moderator: high-pressure water; coolant: high-pressure water] :: These reactors use a pressure vessel to contain the nuclear fuel, control rods, moderator, and coolant. The hot radioactive water that leaves the pressure vessel is looped through a steam generator, which in turn heats a secondary (nonradioactive) loop of water to steam that can run turbines. As of 2024, 308 PWRs comprise % of operating power reactors. This is a
thermal neutron reactor design, the newest of which are the Russian
VVER-1200, Japanese
Advanced Pressurized Water Reactor, American
AP1000, Chinese
Hualong Pressurized Reactor and the Franco-German
European Pressurized Reactor. All the
United States Naval reactors are of this type. •
Boiling water reactors (BWR) [moderator: low-pressure water; coolant: low-pressure water] :: A BWR is like a PWR without the steam generator. The lower pressure of its cooling water allows it to boil inside the pressure vessel, producing the steam that runs the turbines. Unlike a PWR, there is no primary and secondary loop. The
thermal efficiency of these reactors can be higher, and they can be simpler, and even potentially more stable and safe. This is a thermal-neutron reactor design, the newest of which are the
Advanced Boiling Water Reactor and the
Economic Simplified Boiling Water Reactor.
Qinshan Nuclear Power Plant •
Pressurized Heavy-Water Reactor (PHWR) [moderator: high-pressure heavy water; coolant: high-pressure heavy water] :: A Canadian design (known as
CANDU), very similar to PWRs but using
heavy water. While heavy water is significantly more expensive than ordinary water, it has greater
neutron economy (creates a higher number of thermal neutrons), allowing the reactor to operate without
fuel enrichment facilities. Instead of using a single large pressure vessel as in a PWR, the fuel is contained in hundreds of pressure tubes. These reactors are fueled with natural
uranium and are thermal-neutron reactor designs. PHWRs can be refueled while at full power, (
online refueling) which makes them very efficient in their use of uranium (it allows for precise flux control in the core). CANDU PHWRs have been built in Canada,
Argentina, China,
India,
Pakistan,
Romania, and
South Korea. India also operates a number of PHWRs, often termed 'CANDU derivatives', built after the Government of Canada halted nuclear dealings with India following the 1974
Smiling Buddha nuclear weapon test. : – a RBMK type (closed 2009) • Reaktor Bolshoy Moschnosti Kanalniy (High Power Channel Reactor) (
RBMK) (also known as a Light-Water Graphite-moderated Reactor—LWGR) [moderator: graphite; coolant: high-pressure water] :: A Soviet design, RBMKs are in some respects similar to CANDU in that they can be refueled during power operation and employ a pressure tube design instead of a PWR-style pressure vessel. However, unlike CANDU they are unstable and large, making
containment buildings for them expensive. A series of critical safety flaws have also been identified with the RBMK design, though some of these were corrected following the
Chernobyl disaster. Their main attraction is their use of light water and unenriched uranium. As of 2024, 7 remain open, mostly due to safety improvements and help from international safety agencies such as the U.S. Department of Energy. Despite these safety improvements, RBMK reactors are still considered one of the most dangerous reactor designs in use. RBMK reactors were deployed only in the former
Soviet Union.
Sizewell A nuclear power station – an AGR •
Gas-cooled reactor (GCR) and
advanced gas-cooled reactor (AGR) [moderator: graphite; coolant: carbon dioxide] :: These designs have a high thermal efficiency compared with PWRs due to higher operating temperatures. There are a number of operating reactors of this design, mostly in the United Kingdom, where the concept was developed. Older designs (i.e.
Magnox stations) are either shut down or will be in the near future. However, the AGRs have an anticipated life of a further 10 to 20 years. This is a thermal-neutron reactor design. Decommissioning costs can be high due to the large volume of the reactor core. •
Liquid-metal fast-breeder reactor (LMFBR) [moderator: none; coolant: liquid metal] :: This totally unmoderated reactor design produces more fuel than it consumes. They are said to "breed" fuel, because they produce fissionable fuel during operation because of
neutron capture. These reactors can function much like a PWR in terms of efficiency, and do not require much high-pressure containment, as the liquid metal does not need to be kept at high pressure, even at very high temperatures. These reactors are
fast neutron, not thermal neutron designs. These reactors come in two types: , closed in 1998, was one of the few FBRs :::
Lead-cooled :::: Using lead as the liquid metal provides excellent radiation shielding, and allows for operation at very high temperatures. Also, lead is (mostly) transparent to neutrons, so fewer neutrons are lost in the coolant, and the coolant does not become radioactive. Unlike sodium, lead is mostly inert, so there is less risk of explosion or accident, but such large quantities of lead may be problematic from toxicology and disposal points of view. Often a reactor of this type would use a
lead-bismuth eutectic mixture. In this case, the bismuth would present some minor radiation problems, as it is not quite as transparent to neutrons, and can be transmuted to a radioactive isotope more readily than lead. The Russian
Alfa class submarine uses a lead-bismuth-cooled fast reactor as its main power plant. :::
Sodium-cooled :::: Most LMFBRs are of this type. The
TOPAZ,
BN-350 and
BN-600 in USSR;
Superphénix in France; and
Fermi-I in the United States were reactors of this type. The sodium is relatively easy to obtain and work with, and it also manages to actually prevent corrosion on the various reactor parts immersed in it. However, sodium explodes violently when exposed to water, so care must be taken, but such explosions would not be more violent than (for example) a leak of superheated fluid from a pressurized-water reactor. The
Monju reactor in Japan suffered a sodium leak in 1995 and could not be
restarted until May 2010. The
EBR-I, the first reactor to have a core meltdown, in 1955, was also a sodium-cooled reactor. •
Pebble-bed reactors (PBR) [moderator: graphite; coolant: helium] :: These use fuel molded into ceramic balls, and then circulate gas through the balls. The result is an efficient, low-maintenance, very safe reactor with inexpensive, standardized fuel. The prototypes were the
AVR and the
THTR-300 in Germany, which produced up to 308MW of electricity between 1985 and 1989 until it was shut down after experiencing a series of incidents and technical difficulties. The
HTR-10 is operating in China, where the
HTR-PM is being developed. The HTR-PM is expected to be the first generation IV reactor to enter operation. •
Molten-salt reactors (MSR) [moderator: graphite, or none for fast spectrum MSRs; coolant: molten salt mixture] ::These dissolve the fuels in
fluoride or
chloride salts, or use such salts for coolant. MSRs potentially have many safety features, including the absence of high pressures or highly flammable components in the core. They were initially designed for aircraft propulsion due to their high efficiency and high power density. One prototype, the
Molten-Salt Reactor Experiment, was built to confirm the feasibility of the
Liquid fluoride thorium reactor, a thermal spectrum reactor which would breed fissile uranium-233 fuel from thorium. •
Aqueous homogeneous reactor (AHR) [moderator: high-pressure light or heavy water; coolant: high-pressure light or heavy water] :: These reactors use as fuel soluble nuclear salts (usually
uranium sulfate or
uranium nitrate) dissolved in water and mixed with the coolant and the moderator. As of April 2006, only five AHRs were in operation.
Future and developing technologies Advanced reactors More than a dozen advanced reactor designs are in various stages of development. Some are evolutionary from the
PWR,
BWR and
PHWR designs above, and some are more radical departures. The former include the
advanced boiling water reactor (ABWR), two of which are now operating with others under construction, and the planned
passively safe Economic Simplified Boiling Water Reactor (ESBWR) and
AP1000 units (see
Nuclear Power 2010 Program). • The
integral fast reactor (IFR) was built, tested and evaluated during the 1980s and then retired under the Clinton administration in the 1990s due to nuclear non-proliferation policies of the administration. Recycling spent fuel is the core of its design and it therefore produces only a fraction of the waste of current reactors. • The
pebble-bed reactor, a
high-temperature gas-cooled reactor (HTGCR), is designed so high temperatures reduce power output by
Doppler broadening of the fuel's neutron cross-section. It uses ceramic fuels so its safe operating temperatures exceed the power-reduction temperature range. Most designs are cooled by inert helium. Helium is not subject to steam explosions, resists neutron absorption leading to radioactivity, and does not dissolve contaminants that can become radioactive. Typical designs have more layers (up to 7) of passive containment than light water reactors (usually 3). A unique feature that may aid safety is that the fuel balls actually form the core's mechanism, and are replaced one by one as they age. The design of the fuel makes fuel reprocessing expensive. • The
small, sealed, transportable, autonomous reactor (SSTAR) is being primarily researched and developed in the US, intended as a fast-breeder reactor that is passively safe and could be remotely shut down in case the suspicion arises that it is being tampered with. • The
Clean and Environmentally Safe Advanced Reactor (CAESAR) is a nuclear reactor concept that uses steam as a moderator – this design is in development. • The
reduced moderation water reactor builds upon the
Advanced boiling water reactor ABWR) that is presently in use. It is not a complete fast reactor instead using mostly
epithermal neutrons, which are between thermal and fast neutrons in speed. • The
hydrogen-moderated self-regulating nuclear power module (HPM) is a reactor design emanating from the
Los Alamos National Laboratory that uses
uranium hydride as fuel. •
Subcritical reactors are designed to be safer and more stable, but pose a number of engineering and economic difficulties. One example is the
energy amplifier. • Thorium-based reactors – It is possible to convert Thorium-232 into U-233 in reactors specially designed for the purpose. In this way, thorium, which is four times more abundant than uranium, can be used to breed U-233 nuclear fuel. U-233 is also believed to have favourable nuclear properties as compared to traditionally used U-235, including better neutron economy and lower production of long lived transuranic waste. •
Advanced heavy-water reactor (AHWR) – A proposed heavy-water-moderated nuclear power reactor that will be the next generation design of the PHWR type. Under development in the
Bhabha Atomic Research Centre (BARC), India. •
KAMINI – A unique reactor using Uranium-233 isotope for fuel. Built in India by
BARC and Indira Gandhi Center for Atomic Research (
IGCAR). • India is also planning to build fast-breeder reactors using the thorium – Uranium-233 fuel cycle. The FBTR (Fast Breeder Test Reactor) in operation at
Kalpakkam (India) uses Plutonium as a fuel and liquid sodium as a coolant. • China, which has control of the
Cerro Impacto deposit, has a reactor and hopes to replace
coal energy with nuclear energy. Rolls-Royce aims to sell nuclear reactors for the production of
synfuel for aircraft.
Generation IV reactors Generation IV reactors are a set of theoretical nuclear reactor designs. These are generally not expected to be available for commercial use before 2040–2050, although the World Nuclear Association suggested that some might enter commercial operation before 2030. Current reactors in operation around the world are generally considered second- or third-generation systems, with the first-generation systems having been retired some time ago. Research into these reactor types was officially started by the Generation IV International Forum (GIF) based on eight technology goals. The primary goals being to improve nuclear safety, improve proliferation resistance, minimize waste and natural resource utilization, and to decrease the cost to build and run such plants. •
Gas-cooled fast reactor •
Lead-cooled fast reactor •
Molten-salt reactor •
Sodium-cooled fast reactor •
Supercritical water reactor •
Very-high-temperature reactor Generation V+ reactors Generation V reactors are designs which are theoretically possible, but which are not being actively considered or researched at present. Though some generation V reactors could potentially be built with current or near term technology, they trigger little interest for reasons of economics, practicality, or safety. • Liquid-core reactor. A closed loop
liquid-core nuclear reactor, where the fissile material is molten uranium or uranium solution cooled by a working gas pumped in through holes in the base of the containment vessel. •
Gas-core reactor. A closed loop version of the
nuclear lightbulb rocket, where the fissile material is gaseous uranium hexafluoride contained in a fused silica vessel. A working gas (such as hydrogen) would flow around this vessel and absorb the UV light produced by the reaction. This reactor design could also function
as a rocket engine, as featured in Harry Harrison's 1976 science-fiction novel
Skyfall. In theory, using UF6 as a working fuel directly (rather than as a stage to one, as is done now) would mean lower processing costs, and very small reactors. In practice, running a reactor at such high power densities would probably produce unmanageable
neutron flux, weakening most
reactor materials, and therefore as the flux would be similar to that expected in fusion reactors, it would require similar materials to those selected by the
International Fusion Materials Irradiation Facility. • Gas core EM reactor. As in the gas core reactor, but with
photovoltaic arrays converting the
UV light directly to electricity. This approach is similar to the experimentally proved
photoelectric effect that would convert the X-rays generated from
aneutronic fusion into electricity, by passing the high energy photons through an array of conducting foils to transfer some of their energy to electrons, the energy of the photon is captured electrostatically, similar to a
capacitor. Since X-rays can go through far greater material thickness than electrons, many hundreds or thousands of layers are needed to absorb the X-rays. •
Fission fragment reactor. A fission fragment reactor is a nuclear reactor that generates electricity by decelerating an ion beam of fission byproducts instead of using nuclear reactions to generate heat. By doing so, it bypasses the
Carnot cycle and can achieve efficiencies of up to 90% instead of 40–45% attainable by efficient turbine-driven thermal reactors. The fission fragment ion beam would be passed through a
magnetohydrodynamic generator to produce electricity. •
Hybrid nuclear fusion. Would use the neutrons emitted by fusion to fission a
blanket of
fertile material, like
U-238 or
Th-232 and
transmute other reactor's
spent nuclear fuel/nuclear waste into relatively more benign isotopes.
Fusion reactors Controlled
nuclear fusion could in principle be used in
fusion power plants to produce power without the complexities of handling
actinides, but significant scientific and technical obstacles remain. Despite research having started in the 1950s, no commercial fusion reactor is expected before 2050. The
ITER project is currently leading the effort to harness fusion power. ==Nuclear fuel cycle==