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A nuclear reactor, formerly known as an atomic pile, is a device used to initiate and control a fission
nuclear chain reaction 300px, A possible nuclear fission chain reaction: 1) A uranium-235 atom absorbs a neutron">uranium-235.html" ;"title="nuclear fission chain reaction: 1) A uranium-235">nuclear fission chain reaction: 1) A uranium-235 atom absorbs a neutron, ...
or nuclear fusion reactions. Nuclear reactors are used at
nuclear power plant A nuclear power plant (sometimes abbreviated as NPP) is a thermal power station A thermal power station is a power station in which heat energy is converted to electricity. Typically, fuel is used to boil water in a large pressure vessel to ...

nuclear power plant
s for
electricity generation Electricity generation is the process of generating electric power from sources of primary energy. For electric utility, utilities in the electric power industry, it is the stage prior to its Electricity delivery, delivery (Electric power transmiss ...
and in
nuclear marine propulsion Nuclear marine propulsion is propulsion of a ship or submarine with heat provided by a nuclear reactor. The power plant heats water to produce steam for a turbine used to turn the ship's propeller . A propeller is a device with a rotating hub an ...
. Heat from
nuclear fission Nuclear fission is a reaction Reaction may refer to a process or to a response to an action, event, or exposure: Physics and chemistry *Chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic tr ...

nuclear fission
is passed to a
working fluid For fluid power, a working fluid is a gas or liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, or external force ...
(water or gas), which in turn runs through
steam turbine A steam turbine is a machine A machine is any physical system with ordered structural and functional properties. It may represent human-made or naturally occurring device molecular machine that uses Power (physics), power to apply Force, f ...
s. These either drive a ship's
propeller . A propeller is a device with a rotating hub and radiating blades that are set at a pitch to form a helical spiral, that, when rotated, exerts linear thrust upon a working fluid, such as water or air. Propellers are used to pump fluid through a ...

propeller
s or turn
electrical generator In electricity generation Electricity generation is the process of generating electric power from sources of primary energy. For electric utility, utilities in the electric power industry, it is the stage prior to its Electricity delivery, delive ...
s' shafts. Nuclear generated steam in principle can be used for industrial process heat or for
district heating . District heating (also known as heat networks or teleheating) is a system for distributing heat generated in a centralized location through a system of insulated pipes for residential and commercial heating requirements such as space heater, s ...

district heating
. Some reactors are used to produce
isotopes Isotopes are two or more types of atoms that have the same atomic number 300px, The Rutherford–Bohr model of the hydrogen atom () or a hydrogen-like ion (). In this model it is an essential feature that the photon energy (or frequency) of ...
for
medical Medicine is the science Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity, awareness, or understanding of someone or something, such as facts ( descriptive knowledge), skills (proced ...
and
industrial Industrial may also refer to: Industry * Industrial archaeology, the study of the history of the industry * Industrial engineering, engineering dealing with the optimization of complex industrial processes or systems * Industrial loan company, a f ...
use, or for production of
weapons-grade Weapons-grade nuclear material is any fissionable nuclear material that is pure enough to make a nuclear weapon or has properties that make it particularly suitable for nuclear weapons use. Plutonium and uranium in grades normally used in nuclear ...
plutonium Plutonium is a radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material co ...

plutonium
. As of early 2019, the IAEA reports there are 454 nuclear power reactors and 226 nuclear research reactors in operation around the world.


Operation

Just as conventional
thermal power station A thermal power station is a power station in which heat energy is converted to electricity. Typically, fuel is used to boil water in a large pressure vessel to produce high-pressure steam, which drives a steam turbine connected to an electric ...

thermal power station
s generate electricity by harnessing the
thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Thermal energy refers to several distinct physical concepts, such as the internal energy of a system; heat or sensible heat, which are defined as types of energy transfer (as is ...
released from burning
fossil fuels A fossil fuel is a hydrocarbon In organic chemistry, a hydrocarbon is an organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen che ...
, nuclear reactors convert the energy released by controlled
nuclear fission Nuclear fission is a reaction Reaction may refer to a process or to a response to an action, event, or exposure: Physics and chemistry *Chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic tr ...

nuclear fission
into thermal energy for further conversion to mechanical or electrical forms.


Fission

When a large
fissile In nuclear engineering Nuclear engineering is the branch of engineering Engineering is the use of scientific method, scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, veh ...
atomic nucleus The atomic nucleus is the small, dense region consisting of s and s at the center of an , discovered in 1911 by based on the 1909 . After the discovery of the neutron in 1932, models for a nucleus composed of protons and neutrons were quickl ...
such as
uranium-235 Uranium-235 (235U) is an Isotopes of uranium, isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a nuclear chain reaction. It is the only fissile isotope th ...

uranium-235
or
plutonium-239 Plutonium-239 (239Pu, Pu-239) is an isotope Isotopes are variants of a particular chemical element which differ in neutron number, and consequently in nucleon number. All isotopes of a given element have the same number of protons but differe ...

plutonium-239
absorbs a neutron, it may undergo nuclear fission. The heavy nucleus splits into two or more lighter nuclei, (the
fission products Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission In nuclear physics Nuclear physics is the field of physics that studies atomic nuclei and their constituents and interactions. Oth ...
), releasing
kinetic energy In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular ...
,
gamma radiation A gamma ray, or gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration or nuc ...
, and
free neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the atomic nucleus, nuclei of atoms. Since protons and ...

free neutron
s. A portion of these
neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behav ...

neutron
s 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 300px, A possible nuclear fission chain reaction: 1) A uranium-235 atom absorbs a neutron">uranium-235.html" ;"title="nuclear fission chain reaction: 1) A uranium-235">nuclear fission chain reaction: 1) A uranium-235 atom absorbs a neutron, ...
. To control such a nuclear chain reaction,
control rod Control rods are used in nuclear reactors to control the fission rate of uranium Uranium is a chemical element with the Symbol (chemistry), symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic ta ...
s containing
neutron poison In applications such as nuclear reactor A nuclear reactor, formerly known as an atomic pile, is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power pla ...
s and
neutron moderators In nuclear engineering, a neutron moderator is a medium that reduces the speed of Neutron temperature#Fast, fast neutrons, ideally without neutron capture, capturing any, leaving them as Neutron temperature#Thermal, thermal neutrons with only Therma ...
can 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 In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular ...
of fission products is converted to
thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Thermal energy refers to several distinct physical concepts, such as the internal energy of a system; heat or sensible heat, which are defined as types of energy transfer (as is ...
when these nuclei collide with nearby atoms. * The reactor absorbs some of the
gamma rays A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, i ...
produced during fission and converts their energy into heat. * Heat is produced by the
radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...

radioactive decay
of fission products and materials that have been activated by
neutron absorption Neutron capture is a nuclear reaction In nuclear physics Nuclear physics is the field of physics that studies atomic nuclei and their constituents and interactions. Other forms of nuclear matter are also studied. Nuclear physics should not ...
. This decay heat source will remain for some time even after the reactor is shut down. A kilogram of
uranium-235 Uranium-235 (235U) is an Isotopes of uranium, isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a nuclear chain reaction. It is the only fissile isotope th ...

uranium-235
(U-235) converted via nuclear processes releases approximately three million times more energy than a kilogram of coal burned conventionally (7.2 × 1013
joules The joule ( ; symbol: J) is a derived unit of energy In physics, energy is the physical quantity, quantitative physical property, property that must be #Energy transfer, transferred to a physical body, body or physical system to perform W ...
per kilogram of uranium-235 versus 2.4 × 107 joules per kilogram of coal).


Cooling

A
nuclear reactor coolant A nuclear reactor coolant is a coolant A coolant is a substance, typically liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied sh ...
— usually water but sometimes a gas or a liquid metal (like liquid sodium or lead) or
molten salt Molten salt is salt Salt is a mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs ...
— 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 A turbine ( or ) (from the Greek , ''tyrbē'', or Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Lat ...

turbines
, like the
pressurized water reactor A pressurized water reactor (PWR) is a type of light-water nuclear reactor A nuclear reactor, formerly known as an atomic pile, is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear react ...

pressurized water reactor
. However, in some reactors the water for the steam turbines is boiled directly by the
reactor core design. A nuclear reactor core is the portion of a nuclear reactor containing the nuclear fuel components where the Nuclear fission, nuclear reactions take place and the heat generation, heat is generated. Typically, the fuel will be low-enriched ...
; for example the
boiling water reactor A boiling water reactor (BWR) is a type of light water nuclear reactor A nuclear reactor, formerly known as an atomic pile, is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reacto ...
.


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 rod Control rods are used in nuclear reactors to control the fission rate of uranium Uranium is a chemical element with the Symbol (chemistry), symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic ta ...
s. Control rods are made of
neutron poison In applications such as nuclear reactor A nuclear reactor, formerly known as an atomic pile, is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power pla ...
s 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 neutronIn nuclear engineering Nuclear engineering is the branch of engineering Engineering is the use of scientific method, scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicle ...
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 neutronIn nuclear engineering Nuclear engineering is the branch of engineering Engineering is the use of scientific method, scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicle ...
s") released immediately upon fission. The fission products which produce delayed neutrons have
half-lives Half-life (symbol ''t''1⁄2) is the time required for a quantity to reduce to half of its initial value. The term is commonly used in nuclear physics Nuclear physics is the field of physics that studies atomic nuclei and their constituents a ...
for their
decay
decay
by
neutron emission Neutron emission is a mode of radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. ...
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 Critical or Critically may refer to: *Critical, or critical but stable, medical state Medical state is a term used to describe a hospital A hospital is a health care institution providing patient treatment with specialized medical and nursing s ...
point. Keeping the reactor in the zone of chain reactivity where delayed neutrons are ''necessary'' to achieve a
critical mass In nuclear engineering Nuclear engineering is the branch of engineering Engineering is the use of scientific method, scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, v ...

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 File:Fukushima I by Digital Globe.jpg, Three of the reactors at Fukushima I nuclear accidents, Fukushima I overheated because the cooling systems failed after a tsunami flooded the power station, causing core meltdowns. This was compounded by hydr ...
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 Prompt may refer to: Computing * Command prompt, characters indicating the computer is ready to accept input * Command Prompt, also known as cmd.exe or cmd, the command-line interpreter in some operating systems * Prompt (natural language) Prompt ...
point. There is a scale for describing criticality in numerical form, in which bare criticality is known as ''zero
dollars Dollar is the name of more than 20 currencies A currency, "in circulation", from la, currens, -entis, literally meaning "running" or "traversing" in the most specific sense is money Image:National-Debt-Gillray.jpeg, In a 1786 James Gi ...
'' and the prompt critical point is ''one dollar'', and other points in the process interpolated in cents. In some reactors, the
coolant A coolant is a substance, typically liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, or external force. Fluids are ...

coolant
also acts as a
neutron moderator In , a neutron moderator is a medium that reduces the speed of , ideally without any, leaving them as with only . These thermal neutrons are immensely more susceptible than fast neutrons to propagate a of or other by colliding with their . ...
. 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 neutron The neutron detection temperature, also called the neutron energy, indicates a free neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a ...
s are more likely than
fast neutron The neutron detection temperature, also called the neutron energy, indicates a free neutron's kinetic energy In physics, the kinetic energy of an object is the energy that it possesses due to its motion (physics), motion. It is defined as th ...
s 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 A scram or SCRAM is an emergency shutdown of a nuclear reactor A nuclear reactor, formerly known as an atomic pile, is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are ...
the reactor in an emergency shut down. These systems insert large amounts of poison (often
boron Boron is a chemical element with the Symbol (chemistry), symbol B and atomic number 5. Produced entirely by cosmic ray spallation and supernovae and not by stellar nucleosynthesis, it is a low-abundance element in the Solar System a ...

boron
in the form of
boric acid Boric acid, also called hydrogen borate, boracic acid, and orthoboric acid is a weak, monobasic Lewis acid A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is ...

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 iodine pit, also called the iodine hole or xenon pit, is a temporary disabling of a due to buildup of short- s in the . The main isotope responsible is , mainly produced by natural of . 135I is a weak , while 135Xe is the strongest known neu ...
. The common
fission product Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium Uranium is a chemical element with the Symbol (chemistry), symbol U and atomic numbe ...
Xenon-135 Xenon-135 (135Xe) is an unstable isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons A proton is a subatomic particle, symbol or , with a positive electric charge Electric charge is the ...

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 There are 37 known isotope Isotopes are variants of a particular chemical element which differ in neutron number, and consequently in nucleon number. All isotopes of a given element have the same number of protons but different numbers of neu ...
, 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 The Chernobyl disaster was a that occurred on 26 April 1986 at the No. 4 in the , near the city of in the north of the in the . It is considered the worst nuclear disaster in history both in cost and casualties. It is one of only two nucle ...
. Reactors used in
nuclear marine propulsion Nuclear marine propulsion is propulsion of a ship or submarine with heat provided by a nuclear reactor. The power plant heats water to produce steam for a turbine used to turn the ship's propeller . A propeller is a device with a rotating hub an ...
(especially
nuclear submarine A nuclear submarine is a submarine powered by a nuclear reactor. The performance advantages of nuclear submarines over "conventional" (typically Marine diesel engine, diesel-electric) submarines are considerable. Nuclear marine propulsion, Nuc ...
s) 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 Thermal radiation in visible light can be seen on this hot metalwork. Thermal energy refers to several distinct physical concepts, such as the internal energy of a system; heat or sensible heat, which are defined as types of energy transfer (as is ...
is to use it to boil water to produce pressurized steam which will then drive a
steam turbine A steam turbine is a machine A machine is any physical system with ordered structural and functional properties. It may represent human-made or naturally occurring device molecular machine that uses Power (physics), power to apply Force, f ...
that turns an
alternator An alternator is an electrical generator In electricity generation Electricity generation is the process of generating electric power from sources of primary energy. For electric utility, utilities in the electric power industry, it is the s ...

alternator
and generates electricity.


Early reactors

The
neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behav ...

neutron
was discovered in 1932 by British physicist
James Chadwick Sir James Chadwick, (20 October 1891 – 24 July 1974) was a British physicist A physicist is a scientist A scientist is a person who conducts scientific research The scientific method is an Empirical evidence, empirical m ...

James Chadwick
. The concept of a nuclear chain reaction brought about by
nuclear reaction In nuclear physics and nuclear chemistry, a nuclear reaction is a process in which two atomic nucleus, nuclei, or a nucleus and an external subatomic particle, collide to produce one or more new nuclides. Thus, a nuclear reaction must cause a t ...
s mediated by neutrons was first realized shortly thereafter, by
HungarianHungarian may refer to: * Hungary, a country in Central Europe * Kingdom of Hungary, state of Hungary, existing between 1000 and 1946 * Hungarians, ethnic groups in Hungary * Hungarian algorithm, a polynomial time algorithm for solving the assignmen ...

Hungarian
scientist
Leó Szilárd Leo Szilard (; hu, Szilárd Leó, pronounced ; born ''Leó Spitz''; February 11, 1898 – May 30, 1964) was a Hungarian-American physicist and inventor. He conceived the nuclear chain reaction in 1933, patented the idea of a nuclear fiss ...
, in 1933. He filed a patent for his idea of a simple reactor the following year while working at the
Admiralty Admiralty usually refers to: * Admiralty (United Kingdom), military department in command of the Royal Navy from 1707 to 1964 *The rank of admiral Admiral is one of the highest ranks in some navy, navies, and in many navies is the highest rank ...
in London. 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
Lise Meitner Elise Meitner ( , ; 7 November 1878 – 27 October 1968) was a leading Austrian-Swedish physicist A physicist is a scientist A scientist is a person who conducts scientific research The scientific method is an Empirical evidence, ...

Lise Meitner
,
Fritz Strassmann Friedrich Wilhelm "Fritz" Strassmann (''german: Fritz Straßmann'', ; 22 February 1902 – 22 April 1980) was a German chemist who, with Otto Hahn in early 1939, identified the element barium Barium is a chemical element with the Symbol (chemis ...
and
Otto Hahn Otto Hahn (; 8 March 1879 – 28 July 1968) was a German chemist A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Latin Medieval Latin was the form of Latin Latin (, or , ) is a classical language ...

Otto Hahn
in 1938 that bombardment of uranium with neutrons (provided by an alpha-on-beryllium fusion reaction, a " neutron howitzer") produced a
barium Barium is a chemical element with the Symbol (chemistry), symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical Reactivity (chemistry), reactivity, ba ...

barium
residue, which they reasoned was created by the fissioning of the uranium nuclei. Subsequent studies in early 1939 (one of them by Szilárd and Fermi) revealed that several neutrons were also released during the fissioning, making available the opportunity for the nuclear
chain reaction A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction, positive feedback Positive feedback (exacerbating feedback, self-reinforcing feedback) is a pro ...
that Szilárd had envisioned six years previously. On 2 August 1939
Albert Einstein Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born theoretical physicist, widely acknowledged to be one of the greatest physicists of all time. Einstein is known for developing the theory of relativity The theo ...

Albert Einstein
signed a letter to President
Franklin D. Roosevelt Franklin Delano Roosevelt (, ; January 30, 1882April 12, 1945), often referred to by his initials FDR, was an American politician who served as the 32nd president of the United States from 1933 until his death in 1945. A member of the De ...

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,
Hitler Adolf Hitler (; 20 April 188930 April 1945) was an Austrian-born German politician who was the dictator of Nazi Germany, Germany from 1933 to 1945. Adolf Hitler's rise to power, He rose to power as the leader of the Nazi Party, becoming Cha ...
's Germany invaded Poland in 1939, starting
World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a global war A world war is "a war War is an intense armed conflict between states State may refer to: Arts, entertainment, and media Literatur ...
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 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 Uranium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

uranium
needed for a
chain reaction A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction, positive feedback Positive feedback (exacerbating feedback, self-reinforcing feedback) is a pro ...
was far lower than had previously been thought. The memorandum was a product of the
MAUD Committee The MAUD Committee was a British scientific working group formed during the Second World War World War II or the Second World War, often abbreviated as WWII or WW2, was a global war that lasted from 1939 to 1945. It involved the vast ...
, which was working on the UK atomic bomb project, known as
Tube Alloys Tube Alloys was the research and development Research and development (R&D, R+D), known in Europe Europe is a continent A continent is any of several large landmasses. Generally identified by convention (norm), convention ra ...
, later to be subsumed within the
Manhattan Project The Manhattan Project was a research and development Research and development (R&D, R+D), known in Europe as research and technological development (RTD), is the set of innovative activities undertaken by corporations or governments in ...
. Eventually, the first artificial nuclear reactor,
Chicago Pile-1 Chicago Pile-1 (CP-1) was the world's first artificial nuclear reactor A nuclear reactor, formerly known as an atomic pile, is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reac ...
, was constructed at the
University of Chicago The University of Chicago (UChicago) is a private Private or privates may refer to: Music * "In Private "In Private" was the third single in a row to be a charting success for United Kingdom, British singer Dusty Springfield, after an abse ...
, by a team led by
Italian Italian may refer to: * Anything of, from, or related to the country and nation of Italy ** Italians, an ethnic group or simply a citizen of the Italian Republic ** Italian language, a Romance language *** Regional Italian, regional variants of the ...
physicist
Enrico Fermi Enrico Fermi (; 29 September 1901 - 28 November 1954) was an Italian (later naturalized American) physicist and the creator of the world's first nuclear reactor, the Chicago Pile-1. He has been called the "architect of the nuclear age" and ...

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 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 U.S. military developed a number of nuclear reactors for the
Manhattan Project The Manhattan Project was a research and development Research and development (R&D, R+D), known in Europe as research and technological development (RTD), is the set of innovative activities undertaken by corporations or governments in ...
starting in 1943. The primary purpose for the largest reactors (located at the in
Washington Washington commonly refers to: * Washington (state), United States * Washington, D.C., the capital of the United States ** Federal government of the United States (metonym) ** Washington metropolitan area, the metropolitan area centered on Washingt ...
), was the mass production of
plutonium Plutonium is a radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material co ...

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 Experimental Breeder Reactor I (EBR-I) is a decommissioned research reactor Research reactors are nuclear reactors that serve primarily as a neutron source. They are also called non-power reactors, in contrast to power reactors that are used f ...

EBR-I
, which is now a museum near
Arco, Idaho Arco is a city in Butte County, Idaho Idaho () is a U.S. state, state in the Pacific Northwest region of the United States. It borders the state of Montana to the east and northeast, Wyoming to the east, Nevada and Utah to the south, and W ...

Arco, Idaho
. Originally called "Chicago Pile-4", it was carried out under the direction of
Walter Zinn Walter Henry Zinn (December 10, 1906 – February 14, 2000) was an American nuclear physicist who was the first director of the Argonne National Laboratory from 1946 to 1956. He worked at the Manhattan Project's Metallurgical Laboratory during Wor ...
for
Argonne National Laboratory Argonne National Laboratory is a science and engineering research United States Department of Energy National Labs, national laboratory operated by University of Chicago, UChicago Argonne LLC for the United States Department of Energy. The facil ...
. This experimental
LMFBR A breeder reactor is a nuclear reactor that generates more fissile material than it consumes. Breeder reactors achieve this because their neutron economy is high enough to create more fissile fuel than they use, by irradiation of a fertile ma ...
operated by the
U.S. Atomic Energy Commission with AEC chair Lewis Strauss Lewis Lichtenstein Strauss ( "straws"; January 31, 1896January 21, 1974) was an American businessman, philanthropist, and naval officer who served two terms on the U.S. Atomic Energy Commission (AEC), the sec ...

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 Dwight David "Ike" Eisenhower (; October 14, 1890 – March 28, 1969) was an American military officer An officer is a member of an armed forces or uniformed service who holds a position of authority. In its broadest sense, the term " ...
made his famous
Atoms for Peace "Atoms for Peace" was the title of a speech delivered by U.S. President Dwight D. Eisenhower to the UN General Assembly in New York City on December 8, 1953. The United States then launched an "Atoms for Peace" program that supplied equipment ...
speech to the
UN General Assembly The United Nations General Assembly (UNGA or GA; french: link=no, Assemblée générale, AG) is one of the six principal organs of the United Nations The United Nations System consists of the United Nations The United Nations (UN) ...

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 Obninsk Nuclear Power Plant (russian: Обнинская АЭС, Obninskaja AES; ) was built in the "Science City" of Obninsk Obninsk (russian: О́бнинск) is a types of inhabited localities in Russia, city in Kaluga Oblast, Russia, loca ...
, launched on 27 June 1954 in the
Soviet Union The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a that spanned during its existence from 1922 to 1991. It was nominally a of multiple national ; in practice and were highly until its final years. The ...
. It produced around 5 MW (electrical). It was built after the
F-1 (nuclear reactor) The F-1 (from "First Physical Reactor") is a research reactor operated by the Kurchatov Institute in Moscow Moscow (, ; rus, links=no, Москва, r=Moskva, p=mɐˈskva, a=Москва.ogg) is the capital and largest city of Russia ...
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 Army Nuclear Power Program (ANPP) was a program of the United States Army to develop small pressurized water reactor, pressurized water and boiling water reactor, boiling water nuclear reactor, nuclear power reactors to generate electrical and ...
. The Air Force Nuclear Bomber project resulted in the
Molten-Salt Reactor Experiment 404px, MSRE plant diagram: (1) Reactor vessel, (2) Heat exchanger, (3) Fuel pump, (4) Freeze flange, (5) Thermal shield, (6) Coolant pump, (7) Radiator, (8) Coolant drain tank, (9) Fans, (10) Fuel drain tanks, (11) Flush tank, (12) Containment ves ...
. 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 Sellafield is a large multi-function nuclear site close to Seascale Seascale is a village and civil parish on the Irish Sea coast of Cumbria, England, historically within Cumberland. The parish had a population of 1,747 in 2001, increasing s ...
in
Sellafield Sellafield is a large multi-function nuclear site close to Seascale on the coast of Cumbria, England. , activities at the site include nuclear reprocessing, nuclear fuel reprocessing, nuclear waste storage and nuclear decommissioning, and it is ...

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 Camp Century was an Arctic The Arctic ( or ) is a polar region located at the northernmost part of Earth. The Arctic consists of the Arctic Ocean, adjacent seas, and parts of Alaska Alaska (; ale, Alax̂sxax̂; ; ems, Alas'ka ...
from 1960 to 1963.


Reactor types


Classifications


By type of nuclear reaction

All commercial power reactors are based on
nuclear fission Nuclear fission is a reaction Reaction may refer to a process or to a response to an action, event, or exposure: Physics and chemistry *Chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic tr ...

nuclear fission
. They generally use
uranium Uranium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

uranium
and its product
plutonium Plutonium is a radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material co ...

plutonium
as
nuclear fuel Nuclear fuel is material used in nuclear power stations to produce heat to power turbines. Heat is created when nuclear fuel undergoes nuclear fission. Most nuclear fuels contain heavy fissile actinide elements that are capable of Fissile mate ...
, though a
thorium fuel cycle The thorium fuel cycle is a nuclear fuel cycle The nuclear fuel cycle, also called nuclear fuel chain, is the progression of nuclear fuel Nuclear fuel is material used in nuclear power stations to produce heat to power turbines. Heat is cre ...
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 A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction, positive feedback Positive feedback (exacerbating feedback, self-reinforcing feedback) is a pro ...
: * Thermal neutron reactors (the most common type of nuclear reactor) use slowed or
thermal neutron The neutron detection temperature, also called the neutron energy, indicates a free neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a ...
s to keep up the fission of their fuel. Almost all current reactors are of this type. These contain
neutron moderator In , a neutron moderator is a medium that reduces the speed of , ideally without any, leaving them as with only . These thermal neutrons are immensely more susceptible than fast neutrons to propagate a of or other by colliding with their . ...
materials that slow neutrons until their
neutron temperature The neutron detection temperature, also called the neutron energy, indicates a free neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a ...
is ''thermalized'', that is, until their
kinetic energy In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular ...
approaches the average kinetic energy of the surrounding particles. Thermal neutrons have a far higher
cross section Cross section may refer to: * Cross section (geometry), the intersection of a 3-dimensional body with a plane * Cross section (electronics), a common sample preparation technique in electronics * Cross section (geology), the intersection of a 3-dim ...
(probability) of fissioning the
fissile In nuclear engineering Nuclear engineering is the branch of engineering Engineering is the use of scientific method, scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, veh ...
nuclei
uranium-235 Uranium-235 (235U) is an Isotopes of uranium, isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a nuclear chain reaction. It is the only fissile isotope th ...

uranium-235
,
plutonium-239 Plutonium-239 (239Pu, Pu-239) is an isotope Isotopes are variants of a particular chemical element which differ in neutron number, and consequently in nucleon number. All isotopes of a given element have the same number of protons but differe ...

plutonium-239
, and
plutonium-241 Plutonium-241 (241Pu, Pu-241) is an isotope Isotopes are variants of a particular chemical element which differ in neutron number, and consequently in nucleon number. All isotopes of a given element have the same number of protons but different ...

plutonium-241
, and a relatively lower probability of
neutron capture Neutron capture is a nuclear reaction In nuclear physics Nuclear physics is the field of physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion a ...
by
uranium-238 Uranium-238 (238U or U-238) is the most common isotope Isotopes are two or more types of atoms that have the same atomic number 300px, The Rutherford–Bohr model of the hydrogen atom () or a hydrogen-like ion (). In this model it is an ...
(U-238) compared to the faster neutrons that originally result from fission, allowing use of
low-enriched uranium Enriched uranium is a type of uranium Uranium is a chemical element with the Symbol (chemistry), symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 ...
or even
natural uranium Natural uranium (NU, Unat) refers to uranium Uranium is a chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and ...
fuel. The moderator is often also the
coolant A coolant is a substance, typically liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, or external force. Fluids are ...

coolant
, usually water under high pressure to increase the
boiling point The boiling point of a substance is the temperature at which the vapor pressure 280px, The ''pistol test tube'' experiment. The tube contains alcohol and is closed with a piece of cork. By heating the alcohol, the vapors fill in the space, inc ...
. These are surrounded by a
reactor vessel A reactor pressure vessel (RPV) in a nuclear power plant is the pressure vessel containing the nuclear reactor coolant, core shroud, and the Nuclear reactor core, reactor core. Classification of nuclear power reactors Russian Soviet era RBMK r ...
, instrumentation to monitor and control the reactor,
radiation shielding Radiation protection, also known as radiological protection, is defined by the International Atomic Energy Agency The International Atomic Energy Agency (IAEA) is an international organization ''International Organization'' is a quarterly pe ...
, and a
containment building A containment building, in its most common usage, is a reinforced steel, concrete or lead Lead is a chemical element with the Symbol (chemistry), symbol Pb (from the Latin ) and atomic number 82. It is a heavy metals, heavy metal that is dens ...
. *
Fast neutron reactor A fast-neutron reactor (FNR) or simply a fast reactor is a category of nuclear reactor in which the fission chain reaction A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take plac ...
s use
fast neutron The neutron detection temperature, also called the neutron energy, indicates a free neutron's kinetic energy In physics, the kinetic energy of an object is the energy that it possesses due to its motion (physics), motion. It is defined as th ...
s to cause fission in their fuel. They do not have a
neutron moderator In , a neutron moderator is a medium that reduces the speed of , ideally without any, leaving them as with only . These thermal neutrons are immensely more susceptible than fast neutrons to propagate a of or other by colliding with their . ...
, and use less-moderating coolants. Maintaining a chain reaction requires the fuel to be more highly enriched in
fissile In nuclear engineering Nuclear engineering is the branch of engineering Engineering is the use of scientific method, scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, veh ...
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 The transuranium elements (also known as transuranic elements) are the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure subst ...
waste because all
actinides The actinoid ( IUPAC nomenclature, also called actinide ) series encompasses the 15 metallic chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, a ...
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 A breeder reactor is a nuclear reactor that generates more fissile material than it consumes. Breeder reactors achieve this because their neutron economy is high enough to create more fissile fuel than they use, by irradiation of a fertile ma ...
or
generation IV reactors Generation IV reactors (Gen IV) are a set of nuclear reactor designs currently being researched for commercial applications by the Generation IV International Forum. They are motivated by a variety of goals including improved safety, sustainabi ...
). 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 * Water moderated reactors **Heavy-water reactors (Used in Canada, India, Argentina, China, Pakistan, Romania and South Korea). ** Light-water reactor, Light-water-moderated reactors (LWRs). Light-water reactors (the most common type of thermal reactor) use ordinary water to moderate and cool the reactors. Because the light hydrogen isotope is a slight neutron poison these reactors need artificially enriched fuels. When at operating temperature, if the temperature of the water increases, its density drops, and fewer neutrons passing through it are slowed enough to trigger further reactions. That negative feedback stabilizes the reaction rate. Graphite and heavy-water reactors tend to be more thoroughly thermalized than light water reactors. Due to the extra thermalization, and the absence of the light hydrogen poisoning effects these types can use
natural uranium Natural uranium (NU, Unat) refers to uranium Uranium is a chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and ...
/unenriched fuel. * Light-element-moderated reactors. ** Molten salt reactors (MSRs) are moderated by light elements such as lithium or beryllium, which are constituents of the coolant/fuel matrix salts Lithium fluoride , "LiF" and Beryllium fluoride , "BeF2", Lithium chloride, "LiCh" and Beryllium chloride, "BeCh2" and other light element containing salts can all cause a moderating effect. ** Liquid metal cooled reactors, such as those whose coolant is a mixture of lead and bismuth, may use BeO as a moderator. * Organic nuclear reactor, Organically moderated reactors (OMR) use biphenyl and terphenyl as moderator and coolant.


By coolant

* Water cooled reactor. These constitute the great majority of operational nuclear reactors: as of 2014, 93% of the world's nuclear reactors are water cooled, providing about 95% of the world's total nuclear generation capacity. ** Pressurized water reactor (PWR) Pressurized water reactors constitute the large majority of all Western nuclear power plants. *** A primary characteristic of PWRs is a pressurizer, a specialized pressure vessel. Most commercial PWRs and naval reactors use pressurizers. During normal operation, a pressurizer is partially filled with water, and a steam bubble is maintained above it by heating the water with submerged heaters. During normal operation, the pressurizer is connected to the primary reactor pressure vessel (RPV) and the pressurizer "bubble" provides an expansion space for changes in water volume in the reactor. This arrangement also provides a means of pressure control for the reactor by increasing or decreasing the steam pressure in the pressurizer using the pressurizer heaters. *** Pressurized heavy water reactors are a subset of pressurized water reactors, sharing the use of a pressurized, isolated heat transport loop, but using heavy water as coolant and moderator for the greater neutron economies it offers. ** Boiling water reactor (BWR) *** BWRs are characterized by boiling water around the fuel rods in the lower portion of a primary reactor pressure vessel. A boiling water reactor uses 235U, enriched as uranium dioxide, as its fuel. The fuel is assembled into rods housed in a steel vessel that is submerged in water. The nuclear fission causes the water to boil, generating steam. This steam flows through pipes into turbines. The turbines are driven by the steam, and this process generates electricity. 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 (boiler), steam generator. ** Reduced moderation water reactor [RWMR] 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 (nuclear reactor), 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, NaK, lead, lead-bismuth eutectic, and in early reactors, mercury (element), 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, & Japanese plants. Nitrogen and helium have also been used, helium being considered particularly suitable for high temperature designs. Utilization 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 Zirconium tetrafluoride, "ZrF4" with Sodium Fluoride, "NaF" and Lithium chloride, "LiCh" with Beryllium chloride, "BeCh2" * 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 plant A nuclear power plant (sometimes abbreviated as NPP) is a thermal power station A thermal power station is a power station in which heat energy is converted to electricity. Typically, fuel is used to boil water in a large pressure vessel to ...

nuclear power plant
s, 1965–1996) * Generation III reactor (evolutionary improvements of existing designs, 1996–2016) *Generation III reactor#Generation III+ reactors, 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, possibly 2030) In 2003, the French Commissariat à l'Énergie Atomique (CEA) was the first to refer to "Gen II" types in ''Nucleonics Week''. 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 phase of fuel

* Solid fueled * Fluid fueled ** Aqueous homogeneous reactor ** Molten salt reactor * Gaseous fission reactor, 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 Nuclear transmutation, transmutation of elements ** Breeder reactors are capable of producing more fissile material than they consume during the fission chain reaction (by converting Fertile material, fertile U-238 to Pu-239, or Th-232 to U-233). Thus, a uranium breeder reactor, once running, can be refueled with natural uranium, natural or even depleted uranium, and a thorium breeder reactor can be refueled with thorium; however, an initial stock of fissile material is required. ; see "Fuel Cycles and Sustainability" ** Creating various radiation, radioactive isotopes, such as americium for use in smoke detectors, and cobalt-60, molybdenum-99 and others, used for imaging and medical treatment. ** Production of materials for nuclear weapons such as
weapons-grade Weapons-grade nuclear material is any fissionable nuclear material that is pure enough to make a nuclear weapon or has properties that make it particularly suitable for nuclear weapons use. Plutonium and uranium in grades normally used in nuclear ...
plutonium Plutonium is a radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material co ...

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

* 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. They represent the majority (around 80%) of current reactors. This is a
thermal neutron The neutron detection temperature, also called the neutron energy, indicates a free neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a ...
reactor design, the newest of which are the Russian VVER-1200, Japanese Advanced Pressurized Water Reactor, American AP1000, Chinese Hualong One, 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. * Pressurised heavy water reactor, 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 Isotope separation, 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 Uranium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

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. : * Reaktor Bolshoy Moschnosti Kanalniy (High Power Channel Reactor) (RBMK) [moderator: graphite; coolant: high-pressure water] :: A Soviet design, RBMKs are in some respects similar to CANDU in that they are refuelable during power operation and employ a pressure tube design instead of a PWR-style pressure vessel. However, unlike CANDU they are very unstable and large, making
containment building A containment building, in its most common usage, is a reinforced steel, concrete or lead Lead is a chemical element with the Symbol (chemistry), symbol Pb (from the Latin ) and atomic number 82. It is a heavy metals, heavy metal that is dens ...
s 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 The Chernobyl disaster was a that occurred on 26 April 1986 at the No. 4 in the , near the city of in the north of the in the . It is considered the worst nuclear disaster in history both in cost and casualties. It is one of only two nucle ...
. Their main attraction is their use of light water and unenriched uranium. As of 2022, 8 remain open, mostly due to safety improvements and help from international safety agencies such as the DOE. 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 The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a that spanned during its existence from 1922 to 1991. It was nominally a of multiple national ; in practice and were highly until its final years. The ...
. * Gas-cooled reactor (GCR) and advanced gas-cooled reactor (AGR) [moderator: graphite; coolant: carbon dioxide] :: These designs an 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 large volume of reactor core. * Breeder reactor, Liquid metal Fast breeder reactor#Fast breeder reactor, 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 Neutron capture is a nuclear reaction In nuclear physics Nuclear physics is the field of physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion a ...
. 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 The neutron detection temperature, also called the neutron energy, indicates a free neutron's kinetic energy In physics, the kinetic energy of an object is the energy that it possesses due to its motion (physics), motion. It is defined as th ...
, not thermal neutron designs. These reactors come in two types: :::Lead-cooled fast reactor, 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 fast reactor, Sodium-cooled :::: Most LMFBRs are of this type. The TOPAZ nuclear reactor, TOPAZ, BN-350 and BN-600 in USSR; Superphénix in France; and Enrico Fermi Nuclear Generating Station, 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 Nuclear Power Plant, Monju reactor in Japan suffered a sodium leak in 1995 and could not be Monju Nuclear Power Plant#2010 Restart, restarted until May 2010. The
EBR-I Experimental Breeder Reactor I (EBR-I) is a decommissioned research reactor Research reactors are nuclear reactors that serve primarily as a neutron source. They are also called non-power reactors, in contrast to power reactors that are used f ...

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 prototype was the AVR reactor, AVR and 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 404px, MSRE plant diagram: (1) Reactor vessel, (2) Heat exchanger, (3) Fuel pump, (4) Freeze flange, (5) Thermal shield, (6) Coolant pump, (7) Radiator, (8) Coolant drain tank, (9) Fans, (10) Fuel drain tanks, (11) Flush tank, (12) Containment ves ...
, 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 pressurized water reactor, PWR, boiling water reactor, BWR and Pressurised Heavy Water Reactor, PHWR designs above, 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 still 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 Bhabha Atomic Research Centre, 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 currently being researched. These designs are generally not expected to be available for commercial construction 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 Nuclear thermal rocket#Liquid core, 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. * Gaseous fission reactor, Gas-core reactor. A closed loop version of the Nuclear lightbulb, 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 Gas core reactor rocket, 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 IFMIF, 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 IFMIF, 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 breeder reactor, blanket of fertile material, like Uranium-238, U-238 or thorium, Th-232 and Nuclear transmutation, 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 The actinoid ( IUPAC nomenclature, also called actinide ) series encompasses the 15 metallic chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, a ...
, but significant scientific and technical obstacles remain. Several fusion reactors have been built, but reactors have never been able to release more energy than the amount of energy used in the process. 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

Thermal reactors generally depend on refined and enriched uranium. Some nuclear reactors can operate with a mixture of plutonium and uranium (see MOX). The process by which uranium ore is mined, processed, enriched, used, possibly nuclear reprocessing, reprocessed and disposed of is known as the nuclear fuel cycle. Under 1% of the uranium found in nature is the easily fissionable U-235 isotope and as a result most reactor designs require enriched fuel. Enrichment involves increasing the percentage of U-235 and is usually done by means of gaseous diffusion or gas centrifuge. The enriched result is then converted into uranium dioxide powder, which is pressed and fired into pellet form. These pellets are stacked into tubes which are then sealed and called Nuclear fuel, fuel rods. Many of these fuel rods are used in each nuclear reactor. Most BWR and PWR commercial reactors use uranium enriched to about 4% U-235, and some commercial reactors with a high neutron economy do not require the fuel to be enriched at all (that is, they can use natural uranium). According to the International Atomic Energy Agency there are at least 100 research reactors in the world fueled by highly enriched (weapons-grade/90% enrichment) uranium. Theft risk of this fuel (potentially used in the production of a nuclear weapon) has led to campaigns advocating conversion of this type of reactor to low-enrichment uranium (which poses less threat of proliferation). Fissile U-235 and non-fissile but fissionable and Fertile material, fertile U-238 are both used in the fission process. U-235 is fissionable by thermal (i.e. slow-moving) neutrons. A thermal neutron is one which is moving about the same speed as the atoms around it. Since all atoms vibrate proportionally to their absolute temperature, a thermal neutron has the best opportunity to fission U-235 when it is moving at this same vibrational speed. On the other hand, U-238 is more likely to capture a neutron when the neutron is moving very fast. This U-239 atom will soon decay into plutonium-239, which is another fuel. Pu-239 is a viable fuel and must be accounted for even when a highly enriched uranium fuel is used. Plutonium fissions will dominate the U-235 fissions in some reactors, especially after the initial loading of U-235 is spent. Plutonium is fissionable with both fast and thermal neutrons, which make it ideal for either nuclear reactors or nuclear bombs. Most reactor designs in existence are thermal reactors and typically use water as a neutron moderator (moderator means that it slows down the neutron to a thermal speed) and as a coolant. But in a fast breeder reactor, some other kind of coolant is used which will not moderate or slow the neutrons down much. This enables fast neutrons to dominate, which can effectively be used to constantly replenish the fuel supply. By merely placing cheap unenriched uranium into such a core, the non-fissionable U-238 will be turned into Pu-239, "breeding" fuel. In
thorium fuel cycle The thorium fuel cycle is a nuclear fuel cycle The nuclear fuel cycle, also called nuclear fuel chain, is the progression of nuclear fuel Nuclear fuel is material used in nuclear power stations to produce heat to power turbines. Heat is cre ...
thorium-232 absorbs a
neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behav ...

neutron
in either a fast or thermal reactor. The thorium-233 beta decays to protactinium-233 and then to uranium-233, which in turn is used as fuel. Hence, like
uranium-238 Uranium-238 (238U or U-238) is the most common isotope Isotopes are two or more types of atoms that have the same atomic number 300px, The Rutherford–Bohr model of the hydrogen atom () or a hydrogen-like ion (). In this model it is an ...
, thorium-232 is a fertile material.


Fueling of nuclear reactors

The amount of energy in the reservoir of
nuclear fuel Nuclear fuel is material used in nuclear power stations to produce heat to power turbines. Heat is created when nuclear fuel undergoes nuclear fission. Most nuclear fuels contain heavy fissile actinide elements that are capable of Fissile mate ...
is frequently expressed in terms of "full-power days," which is the number of 24-hour periods (days) a reactor is scheduled for operation at full power output for the generation of heat energy. The number of full-power days in a reactor's operating cycle (between refueling outage times) is related to the amount of
fissile In nuclear engineering Nuclear engineering is the branch of engineering Engineering is the use of scientific method, scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, veh ...
uranium-235 Uranium-235 (235U) is an Isotopes of uranium, isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a nuclear chain reaction. It is the only fissile isotope th ...

uranium-235
(U-235) contained in the fuel assemblies at the beginning of the cycle. A higher percentage of U-235 in the core at the beginning of a cycle will permit the reactor to be run for a greater number of full-power days. At the end of the operating cycle, the fuel in some of the assemblies is "spent", having spent 4 to 6 years in the reactor producing power. This spent fuel is discharged and replaced with new (fresh) fuel assemblies. Though considered "spent," these fuel assemblies contain a large quantity of fuel. In practice it is economics that determines the lifetime of nuclear fuel in a reactor. Long before all possible fission has taken place, the reactor is unable to maintain 100%, full output power, and therefore, income for the utility lowers as plant output power lowers. Most nuclear plants operate at a very low profit margin due to operating overhead, mainly regulatory costs, so operating below 100% power is not economically viable for very long. The fraction of the reactor's fuel core replaced during refueling is typically one-third, but depends on how long the plant operates between refueling. Plants typically operate on 18 month refueling cycles, or 24 month refueling cycles. This means that 1 refueling, replacing only one-third of the fuel, can keep a nuclear reactor at full power for nearly 2 years. The disposition and storage of this spent fuel is one of the most challenging aspects of the operation of a commercial nuclear power plant. This nuclear waste is highly radioactive and its toxicity presents a danger for thousands of years. After being discharged from the reactor, spent nuclear fuel is transferred to the on-site spent fuel pool. The spent fuel pool is a large pool of water that provides cooling and shielding of the spent nuclear fuel. Once the energy has decayed somewhat (approximately 5 years), the fuel can be transferred from the fuel pool to dry shielded casks, that can be safely stored for thousands of years. After loading into dry shielded casks, the casks are stored on-site in a specially guarded facility in impervious concrete bunkers. On-site fuel storage facilities are designed to withstand the impact of commercial airliners, with little to no damage to the spent fuel. An average on-site fuel storage facility can hold 30 years of spent fuel in a space smaller that a football field. Not all reactors need to be shut down for refueling; for example, pebble bed reactors, RBMK, RBMK reactors, molten salt reactors, Magnox, Advanced gas-cooled reactor, AGR and CANDU reactors allow fuel to be shifted through the reactor while it is running. In a CANDU reactor, this also allows individual fuel elements to be situated within the reactor core that are best suited to the amount of U-235 in the fuel element. The amount of energy extracted from nuclear fuel is called its burnup, which is expressed in terms of the heat energy produced per initial unit of fuel weight. Burn up is commonly expressed as megawatt days thermal per metric ton of initial heavy metal.


Nuclear safety

Nuclear safety covers the actions taken to prevent nuclear and radiation accidents and incidents or to limit their consequences. The nuclear power industry has improved the safety and performance of reactors, and has proposed new, safer (but generally untested) reactor designs but there is no guarantee that the reactors will be designed, built and operated correctly. Mistakes do occur and the designers of reactors at Timeline of the Fukushima nuclear accidents, Fukushima in Japan did not anticipate that a tsunami generated by an earthquake would disable the backup systems that were supposed to stabilize the reactor after the earthquake, despite multiple warnings by the NRG and the Japanese nuclear safety administration. According to UBS AG, the Fukushima I nuclear accidents have cast doubt on whether even an advanced economy like Japan can master nuclear safety. Catastrophic scenarios involving terrorist attacks are also conceivable. An interdisciplinary team from MIT has estimated that given the expected growth of nuclear power from 2005 to 2055, at least four serious nuclear accidents would be expected in that period.


Nuclear accidents

Serious, though rare, Nuclear and radiation accidents and incidents, nuclear and radiation accidents have occurred. These include the SL-1 accident (1961), the Three Mile Island accident (1979),
Chernobyl disaster The Chernobyl disaster was a that occurred on 26 April 1986 at the No. 4 in the , near the city of in the north of the in the . It is considered the worst nuclear disaster in history both in cost and casualties. It is one of only two nucle ...
(1986), and the Fukushima Daiichi nuclear disaster (2011). Nuclear-powered submarine mishaps include the Soviet submarine K-19, K-19 reactor accident (1961),Strengthening the Safety of Radiation Sources
p. 14.
the Soviet submarine K-27, K-27 reactor accident (1968), and the Soviet submarine K-431, K-431 reactor accident (1985).The Worst Nuclear Disasters
''Time''.
Nuclear reactors have been launched into Earth orbit at least 34 times. A number of incidents connected with the unmanned nuclear-reactor-powered Soviet RORSAT radar satellite program resulted in spent nuclear fuel reentering the Earth's atmosphere from orbit.


Natural nuclear reactors

Almost two billion years ago a series of self-sustaining nuclear fission "reactors" self-assembled in the area now known as Oklo in Gabon, West Africa. The conditions at that place and time allowed a natural nuclear fission reactor, natural nuclear fission to occur with circumstances that are similar to the conditions in a constructed nuclear reactor. Fifteen fossil natural fission reactors have so far been found in three separate ore deposits at the Oklo uranium mine in Gabon. First discovered in 1972 by French physicist Francis Perrin (physicist), Francis Perrin, they are collectively known as the Natural nuclear fission reactor, Oklo Fossil Reactors. Self-sustaining
nuclear fission Nuclear fission is a reaction Reaction may refer to a process or to a response to an action, event, or exposure: Physics and chemistry *Chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic tr ...

nuclear fission
reactions took place in these reactors approximately 1.5 billion years ago, and ran for a few hundred thousand years, averaging 100 kW of power output during that time. The concept of a natural nuclear reactor was theorized as early as 1956 by Paul Kuroda at the University of Arkansas. Such reactors can no longer form on Earth in its present geologic period. Radioactive decay of formerly more abundant uranium-235 over the time span of hundreds of millions of years has reduced the proportion of this naturally occurring fissile isotope to below the amount required to sustain a chain reaction with only plain water as a moderator. The natural nuclear reactors formed when a uranium-rich mineral deposit became inundated with groundwater that acted as a neutron moderator, and a strong chain reaction took place. The water moderator would boil away as the reaction increased, slowing it back down again and preventing a meltdown. The fission reaction was sustained for hundreds of thousands of years, cycling on the order of hours to a few days. These natural reactors are extensively studied by scientists interested in geologic radioactive waste disposal. They offer a case study of how radioactive isotopes migrate through the Earth's crust. This is a significant area of controversy as opponents of geologic waste disposal fear that isotopes from stored waste could end up in water supplies or be carried into the environment.


Emissions

Nuclear reactors produce tritium as part of normal operations, which is eventually released into the environment in trace quantities. As an isotope of hydrogen, tritium (T) frequently binds to oxygen and forms tritiated water, T2O. This molecule is chemically identical to water, H2O and so is both colorless and odorless, however the additional neutrons in the hydrogen nuclei cause the tritium to undergo beta decay with a half-life of 12.3 years. Despite being measurable, the tritium released by nuclear power plants is minimal. The United States Nuclear Regulatory Commission, NRC estimates that a person drinking water for one year out of a well contaminated by what they would consider to be a significant tritiated water spill would receive a radiation dose of 0.3 millirem. For comparison, this is an order of magnitude less than the 4 millirem a person receives on a round trip flight from Washington, D.C. to Los Angeles, a consequence of less atmospheric protection against highly energetic cosmic rays at high altitudes. The amounts of strontium-90 released from nuclear power plants under normal operations is so low as to be undetectable above natural background radiation. Detectable strontium-90 in ground water and the general environment can be traced to weapons testing that occurred during the mid-20th century (accounting for 99% of the Strontium-90 in the environment) and the Chernobyl accident (accounting for the remaining 1%).


See also

* List of nuclear reactors * List of small modular reactor designs * List of United States Naval reactors * Neutron transport * Nuclear power by country * Nuclear power in space * One Less Nuclear Power Plant * Radioisotope thermoelectric generator * Safety engineering * Sayonara Nuclear Power Plants * Small modular reactor * Thorium-based nuclear power * Traveling wave reactor, Traveling-wave reactor (TWR) * ''World Nuclear Industry Status Report''


References


External links


The Database on Nuclear Power Reactors – IAEA

Uranium Conference adds discussion of Japan accident

A Debate: Is Nuclear Power The Solution to Global Warming?

Union of Concerned Scientists, Concerns re: US nuclear reactor program

Freeview Video 'Nuclear Power Plants — What's the Problem' A Royal Institution Lecture by John Collier by the Vega Science Trust.

Nuclear Energy Institute — How it Works: Electric Power Generation

Annotated bibliography of nuclear reactor technology from the Alsos Digital Library
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ソヴィエト連邦における宇宙用原子炉の開発とその実用
{{DEFAULTSORT:Nuclear Reactor Technology Energy conversion Nuclear technology Power station technology Nuclear reactors, Pressure vessels Nuclear research reactors, Nuclear research reactors Nuclear power reactor types, Nuclear power reactor types Neutron sources