A nuclear reactor is a device used to sustain a controlled fission nuclear chain reaction. They are used for commercial electricity, marine propulsion, weapons production and research. Fissile nuclei (primarily uranium-235 or plutonium-239) absorb single neutrons and split, releasing energy and multiple neutrons, which can induce further fission. Reactors stabilize this, regulating neutron absorbers and moderators in the core. Fuel efficiency is exceptionally high; low-enriched uranium is 120,000 times more energy-dense than coal.
Heat from nuclear fission is passed to a working fluid coolant. In commercial reactors, this drives turbines and electrical generator shafts. Some reactors are used for district heating, and isotope production for medical and industrial use.
After the discovery of fission in 1938, many countries launched military nuclear research programs. Early subcritical experiments probed neutronics. In 1942, the first artificial critical nuclear reactor, Chicago Pile-1, was built by the Metallurgical Laboratory. The first commercial nuclear power station was Calder Hall at Sellafield, England, which started operation in 1956.
Spent fuel can be reprocessed, potentially reducing nuclear waste and recovering reactor-usable fuel. This also poses a nuclear proliferation risk via production of plutonium for nuclear weapons.
Reactor accidents have been caused by combinations of design and operator failure. The 1979 Three Mile Island accident, at INES Level 5, the 1986 Chernobyl disaster and 2011 Fukushima disaster, both at Level 7, all had major effects on the nuclear industry and anti-nuclear movement.
, there are 417 commercial reactors, 226 research reactors, and over 200 marine propulsion reactors in operation globally. Commercial reactors provide 9% of the global electricity supply, compared to 30% from renewables, together comprising low-carbon electricity. Almost 90% of this comes from pressurized and boiling water reactors. Other designs include gas-cooled, fast-spectrum, breeder, heavy-water, molten-salt, and small modular, each of which improves safety, efficiency, cost, fuel type, enrichment, or burnup, and the now-obsolete Light water graphite reactor.
Terminology
During early 1940s nuclear research, the phrase "atomic pile" was used for any assembly involving uranium and attempts at neutron multiplication, including the majority which were subcritical. After Chicago Pile-1 demonstrated a self-sustaining chain reaction, the "reactor" terminology became more common. The phrases "nuclear pile" and "atomic reactor" were also common.
Critical mass experiments, while being far simpler, are sometimes referred to as research reactors, such as the Godiva device.
"Nuclear reactor" is predominantly used to refer to the nuclear fission reactor. It can also refer to a nuclear fusion reactor, of which only net negative power systems have been constructed. Radioisotope thermoelectric generators and radioisotope heater units, while deriving power from nuclear decay reactions, are not referred to as nuclear reactors as they do not induce reactions.
Operation
upright=1.15|thumb|An example of an induced nuclear fission event. A neutron is absorbed by the nucleus of a uranium-235 atom, which in turn splits into fast-moving lighter elements (fission products) and free neutrons. Though both reactors and [[nuclear weapons rely on nuclear chain reactions, the rate of reactions in a reactor is much slower than in a bomb.]]
Just as conventional thermal power stations generate electricity by harnessing the thermal energy released from burning fossil fuels, nuclear reactors convert the energy released by controlled nuclear fission into thermal energy for further conversion to mechanical or electrical forms.
Fission
When a large fissile atomic nucleus such as uranium-235, uranium-233, or plutonium-239 absorbs a neutron, it may undergo nuclear fission. The heavy nucleus splits into two or more lighter nuclei, (the fission products), releasing kinetic energy, gamma radiation, and free neutrons. A portion of these neutrons may be absorbed by other fissile atoms and trigger further fission events, which release more neutrons, and so on. This is known as a nuclear chain reaction.
To control such a nuclear chain reaction, control rods containing neutron poisons and neutron moderators are able to change the portion of neutrons that will go on to cause more fission. Nuclear reactors generally have automatic and manual systems to shut the fission reaction down if monitoring or instrumentation detects unsafe conditions.
Heat generation
The reactor core generates heat in a number of ways:
- The kinetic energy of fission products is converted to thermal energy when these nuclei collide with nearby atoms.
- The reactor absorbs some of the gamma rays produced during fission and converts their energy into heat.
- Heat is produced by the radioactive decay of fission products and materials that have been activated by neutron absorption. This decay heat source will remain for some time even after the reactor is shut down.
A kilogram of uranium-235 (U-235) converted via nuclear processes releases approximately three million times more energy than a kilogram of coal burned conventionally (7.2 × 10<sup>13</sup> joules per kilogram of uranium-235 versus 2.4 × 10<sup>7</sup> joules per kilogram of coal).<!--see this is more than WP:CALC. See Talk:Nuclear reactor#Energy efficiency?-->
The fission of one kilogram of uranium-235 releases about 19 billion kilocalories, so the energy released by 1 kg of uranium-235 corresponds to that released by burning 2.7 million kg of coal.
Cooling
A nuclear reactor coolant – usually water but sometimes a gas or a liquid metal (like liquid sodium or lead) or molten salt – is circulated past the reactor core to absorb the heat that it generates. The heat is carried away from the reactor and is then used to generate steam. Most reactor systems employ a cooling system that is physically separated from the water that will be boiled to produce pressurized steam for the turbines, like the pressurized water reactor. However, in some reactors the water for the steam turbines is boiled directly by the reactor core; for example the boiling water reactor.
Reactivity control
The rate of fission reactions within a reactor core can be adjusted by controlling the quantity of neutrons that are able to induce further fission events. Nuclear reactors typically employ several methods of neutron control to adjust the reactor's power output. Some of these methods arise naturally from the physics of radioactive decay and are simply accounted for during the reactor's operation, while others are mechanisms engineered into the reactor design for a distinct purpose.
The fastest method for adjusting levels of fission-inducing neutrons in a reactor is via movement of the control rods. Control rods are made of so-called neutron poisons and therefore absorb neutrons. When a control rod is inserted deeper into the reactor, it absorbs more neutrons than the material it displaces – often the moderator. This action results in fewer neutrons available to cause fission and reduces the reactor's power output. Conversely, extracting the control rod will result in an increase in the rate of fission events and an increase in power.
The physics of radioactive decay also affects neutron populations in a reactor. One such process is delayed neutron emission by a number of neutron-rich fission isotopes. These delayed neutrons account for about 0.65% of the total neutrons produced in fission, with the remainder (termed "prompt neutrons") released immediately upon fission. The fission products which produce delayed neutrons have half-lives for their decay by neutron emission that range from milliseconds to as long as several minutes, and so considerable time is required to determine exactly when a reactor reaches the critical point. Keeping the reactor in the zone of chain reactivity where delayed neutrons are necessary to achieve a critical mass state allows mechanical devices or human operators to control a chain reaction in "real time"; otherwise the time between achievement of criticality and nuclear meltdown as a result of an exponential power surge from the normal nuclear chain reaction, would be too short to allow for intervention. This last stage, where delayed neutrons are no longer required to maintain criticality, is known as the prompt critical point. There is a scale for describing criticality in numerical form, in which bare criticality is known as zero dollars and the prompt critical point is one dollar, and other points in the process interpolated in cents.
In some reactors, the coolant also acts as a neutron moderator. A moderator increases the power of the reactor by causing the fast neutrons that are released from fission to lose energy and become thermal neutrons. Thermal neutrons are more likely than fast neutrons to cause fission. If the coolant is a moderator, then temperature changes can affect the density of the coolant/moderator and therefore change power output. A higher temperature coolant would be less dense, and therefore a less effective moderator.
In other reactors, the coolant acts as a poison by absorbing neutrons in the same way that the control rods do. In these reactors, power output can be increased by heating the coolant, which makes it a less dense poison. Nuclear reactors generally have automatic and manual systems to scram the reactor in an emergency shut down. These systems insert large amounts of poison (often boron in the form of boric acid) into the reactor to shut the fission reaction down if unsafe conditions are detected or anticipated.
Most types of reactors are sensitive to a process variously known as xenon poisoning, or the iodine pit. The common fission product Xenon-135 produced in the fission process acts as a neutron poison that absorbs neutrons and therefore tends to shut the reactor down. Xenon-135 accumulation can be controlled by keeping power levels high enough to destroy it by neutron absorption as fast as it is produced. Fission also produces iodine-135, which in turn decays (with a half-life of 6.57 hours) to new xenon-135. When the reactor is shut down, iodine-135 continues to decay to xenon-135, making restarting the reactor more difficult for a day or two, as the xenon-135 decays into cesium-135, which is not nearly as poisonous as xenon-135, with a half-life of 9.2 hours. This temporary state is the "iodine pit." If the reactor has sufficient extra reactivity capacity, it can be restarted. As the extra xenon-135 is transmuted to xenon-136, which is much less a neutron poison, within a few hours the reactor experiences a "xenon burnoff (power) transient". Control rods must be further inserted to replace the neutron absorption of the lost xenon-135. Failure to properly follow such a procedure was a key step in the Chernobyl disaster.
Reactors used in nuclear marine propulsion (especially nuclear submarines) often cannot be run at continuous power around the clock in the same way that land-based power reactors are normally run, and in addition often need to have a very long core life without refueling. For this reason many designs use highly enriched uranium but incorporate burnable neutron poison in the fuel rods. This allows the reactor to be constructed with an excess of fissionable material, which is nevertheless made relatively safe early in the reactor's fuel burn cycle by the presence of the neutron-absorbing material which is later replaced by normally produced long-lived neutron poisons (far longer-lived than xenon-135) which gradually accumulate over the fuel load's operating life.
Electrical power generation
The energy released in the fission process generates heat, some of which can be converted into usable energy. A common method of harnessing this thermal energy is to use it to boil water to produce pressurized steam which will then drive a steam turbine that turns an alternator and generates electricity. Some believe nuclear power plants can operate for as long as 80 years or longer with proper maintenance and management. While most components of a nuclear power plant, such as steam generators, are replaced when they reach the end of their useful lifetime, the overall lifetime of the power plant is limited by the life of components that cannot be replaced when aged by wear and neutron embrittlement, such as the reactor pressure vessel. At the end of their planned life span, plants may get an extension of the operating license for some 20 years and in the US even a "subsequent license renewal" (SLR) for an additional 20 years.
Even when a license is extended, it does not guarantee the reactor will continue to operate, particularly in the face of safety concerns or incident. Many reactors are closed long before their license or design life expired and are decommissioned. The costs for replacements or improvements required for continued safe operation may be so high that they are not cost-effective. Or they may be shut down due to technical failure. Other ones have been shut down because the area was contaminated, like Fukushima, Three Mile Island, Sellafield, and Chernobyl. The British branch of the French concern EDF Energy, for example, extended the operating lives of its Advanced Gas-cooled Reactors (AGR) with only between 3 and 10 years. All seven AGR plants were expected to be shut down in 2022 and in decommissioning by 2028. Hinkley Point B was extended from 40 to 46 years, and closed. The same happened with Hunterston B, also after 46 years.
An increasing number of reactors is reaching or crossing their design lifetimes of 30 or 40 years. In 2014, Greenpeace warned that the lifetime extension of ageing nuclear power plants amounts to entering a new era of risk. It estimated the current European nuclear liability coverage in average to be too low by a factor of between 100 and 1,000 to cover the likely costs, while at the same time, the likelihood of a serious accident happening in Europe continues to increase as the reactor fleet grows older.
History
thumb|The [[Chicago Pile-1|Chicago Pile, the first artificial nuclear reactor, built in secrecy at the University of Chicago in 1942 during World War II as part of the US's Manhattan Project]]
thumb|upright=0.75|[[Lise Meitner and Otto Hahn in their laboratory]]
right|thumb|Some of the [[Chicago Pile-1|Chicago Pile Team, including Enrico Fermi (leftmost man front row) and Leó Szilárd (rightmost man middle row)]]
The neutron was discovered in 1932 by British physicist James Chadwick. The concept of a nuclear chain reaction brought about by nuclear reactions mediated by neutrons was first realized shortly thereafter, by Hungarian scientist Leó Szilárd, in 1933. He filed a patent for his idea of a simple reactor the following year while working at the Admiralty in London, England. However, Szilárd's idea did not incorporate the idea of nuclear fission as a neutron source, since that process was not yet discovered. Szilárd's ideas for nuclear reactors using neutron-mediated nuclear chain reactions in light elements proved unworkable.
Inspiration for a new type of reactor using uranium came from the discovery by Otto Hahn, Lise Meitner, and Fritz Strassmann in 1938 that bombardment of uranium with neutrons (provided by an alpha-on-beryllium fusion reaction, a "neutron howitzer") produced a barium residue, which they reasoned was created by fission of the uranium nuclei. In their second publication on nuclear fission in February 1939, Hahn and Strassmann predicted the existence and liberation of additional neutrons during the fission process, opening the possibility of a nuclear chain reaction. Subsequent studies in early 1939 (one of them by Szilárd and Fermi), revealed that several neutrons were indeed released during fission, making available the opportunity for the nuclear chain reaction that Szilárd had envisioned six years previously.
On 2 August 1939, Albert Einstein signed a letter to President Franklin D. Roosevelt (written by Szilárd) suggesting that the discovery of uranium's fission could lead to the development of "extremely powerful bombs of a new type", giving impetus to the study of reactors and fission. Szilárd and Einstein knew each other well and had worked together years previously, but Einstein had never thought about this possibility for nuclear energy until Szilard reported it to him, at the beginning of his quest to produce the Einstein-Szilárd letter to alert the U.S. government.
Shortly after, Nazi Germany invaded Poland in 1939, starting World War II in Europe. The U.S. was not yet officially at war, but in October, when the Einstein-Szilárd letter was delivered to him, Roosevelt commented that the purpose of doing the research was to make sure "the Nazis don't blow us up." The U.S. nuclear project followed, although with some delay as there remained skepticism (some of it from Enrico Fermi) and also little action from the small number of officials in the government who were initially charged with moving the project forward.
The following year, the U.S. Government received the Frisch–Peierls memorandum from the UK, which stated that the amount of uranium needed for a chain reaction was far lower than had previously been thought. The memorandum was a product of the MAUD Committee, which was working on the UK atomic bomb project, known as Tube Alloys, later to be subsumed within the Manhattan Project.
Eventually, the first artificial nuclear reactor, Chicago Pile-1, was constructed at the University of Chicago, by a team led by Italian physicist Enrico Fermi, in late 1942. By this time, the program had been pressured for a year by U.S. entry into the war. The Chicago Pile achieved criticality on 2 December 1942 at 3:25 PM. The reactor support structure was made of wood, which supported a pile (hence the name) of graphite blocks, embedded in which was natural uranium oxide 'pseudospheres' or 'briquettes'.
Soon after the Chicago Pile, the Metallurgical Laboratory developed a number of nuclear reactors for the Manhattan Project starting in 1943. The primary purpose for the largest reactors (located at the Hanford Site in Washington), was the mass production of plutonium for nuclear weapons. Fermi and Szilard applied for a patent on reactors on 19 December 1944. Its issuance was delayed for 10 years because of wartime secrecy.
"World's first nuclear power plant" is the claim made by signs at the site of the EBR-I, which is now a museum near Arco, Idaho. Originally called "Chicago Pile-4", it was carried out under the direction of Walter Zinn for Argonne National Laboratory. This experimental LMFBR operated by the U.S. Atomic Energy Commission produced 0.8 kW in a test on 20 December 1951 and 100 kW (electrical) the following day, having a design output of 200 kW (electrical).
Besides the military uses of nuclear reactors, there were political reasons to pursue civilian use of atomic energy. U.S. President Dwight Eisenhower made his famous Atoms for Peace speech to the UN General Assembly on 8 December 1953. This diplomacy led to the dissemination of reactor technology to U.S. institutions and worldwide.
The first nuclear power plant built for civil purposes was the AM-1 Obninsk Nuclear Power Plant, launched on 27 June 1954 in the Soviet Union. It produced around 5 MW (electrical). It was built after the F-1 (nuclear reactor) which was the first reactor to go critical in Europe, and was also built by the Soviet Union.
After World War II, the U.S. military sought other uses for nuclear reactor technology. Research by the Army led to the power stations for Camp Century, Greenland and McMurdo Station, Antarctica Army Nuclear Power Program. The Air Force Nuclear Bomber project resulted in the Molten-Salt Reactor Experiment. The U.S. Navy succeeded when they steamed the USS Nautilus (SSN-571) on nuclear power 17 January 1955.
The first commercial nuclear power station, Calder Hall in Sellafield, England was opened in 1956 with an initial capacity of 50 MW (later 200 MW).
The first portable nuclear reactor "Alco PM-2A" was used to generate electrical power (2 MW) for Camp Century from 1960 to 1963.
Table by date
{| class="wikitable sortable mw-collapsible"
|+Reactors constructed before 1950
!Name
!Alternate names
!Country
!Location
!Moderator
!Criticality date
|-
|Chicago Pile-1
|CP-1
|
|University of Chicago, Illinois
|Graphite
|2 December 1942
|-
|Chicago Pile-2
|CP-2
|
|Site A, Illinois
|Graphite
|20 March 1943
|-
|Oak Ridge Graphite Reactor
|X-10, Clinton Pile
|
|Clinton Laboratories, Tennessee
|Graphite
|4 November 1943
|
|
|Hanford Site, Washington
|Graphite
|March 1944
|-
|Los Alamos LOPO Reactor
|LOPO
|
|Los Alamos Laboratory, New Mexico
|Light water
|9 May 1944
|-
|B Reactor
|
|
|Hanford Site, Washington
|Graphite
|26 September 1944
|-
|Los Alamos Water Boiler
|HYPO
|
|Los Alamos Laboratory, New Mexico
|Light water
|December 1944
|-
|D Reactor
|
|
|Hanford Site, Washington
|Graphite
|December 1944
|-
|Dragon
|
|
|Los Alamos Laboratory, New Mexico
|None (fast)
|20 January 1945
|-
|F Reactor
|
|
|Hanford Site, Washington
|Graphite
|February 1945
|-
|Los Alamos Fast Reactor
|Clementine
|
|Los Alamos Laboratory, New Mexico
|None (fast)
|19 November 1946
|-
|F-1
|
|
|Laboratory No. 2, Moscow
|Graphite
|25 December 1946
|-
|National Research Experimental
|NRX
|
|Chalk River Laboratories, Ontario
|Heavy water
|22 July 1947
|-
|Graphite Low Energy Experimental Pile
|GLEEP
|
|Atomic Energy Research Establishment, Oxfordshire
|Graphite
|15 August 1947
|-
|Reactor A
|
|
|Mayak Production Association, Chelyabinsk Oblast
|Graphite
|10 June 1948
|-
|British Experimental Pile Operation
|BEPO
|
|Atomic Energy Research Establishment, Oxfordshire
|Graphite
|3 July 1948
|-
|Eau Lourde-1 (Heavy Water-1)
|EL-1, Zoé
|
|Fort de Châtillon, Paris
|Heavy water
|15 December 1948
|-
|Physical Boiler on Fast Neutrons
|FKBN
|
|Design Bureau No. 11, Sarov
|None (fast)
|1 February 1949
|-
|TVR
|TVR
|
|Laboratory No. 3, Moscow
|Heavy water
|April 1949
|-
| colspan="5" |RDS-1, first Soviet nuclear test
|29 August 1949
|-
|H Reactor
|
|
|Hanford Site, Washington
|Graphite
|October 1949
|Shippingport Atomic Power Station
|
|-
|
|ZEEP
|
|Nuclear Power Demonstration
|
|-
|
|F-1
|
|Obninsk Nuclear Power Plant
|
|-
|
|GLEEP
|
|Calder Hall nuclear power station
|
|-
|
|EL-1 (Zoé)
|
|Marcoule Nuclear Site
|
|-
|
|JEEP
|
|None constructed
|n/a
|-
|
|R1
|
|Ågesta Nuclear Plant
|
|-
|
|
|
|
|
|-
|
|Apsara
|
|Tarapur Atomic Power Station
|
|-
|
|
|27 August 1957
|Tōkai Nuclear Power Plant
|25 July 1966
|-
|
|
|
|Kahl Nuclear Power Plant
|
|-
|
|
|16 December 1957
|Rheinsberg Nuclear Power Plant
|6 May 1966
|-
|
|HWRR
|
|Qinshan Nuclear Power Plant
|
|-
|
|
|
|Latina Nuclear Power Plant
|
|}
thumb|Primary coolant system showing [[reactor pressure vessel (red), steam generators (purple), pressurizer (blue), and pumps (green) in the three coolant loop Hualong One pressurized water reactor design]]