Nuclear reactor

Nuclear reactor

Overview
This article is a subarticle of Nuclear power
Nuclear power
Nuclear power is the use of sustained nuclear fission to generate heat and electricity. Nuclear power plants provide about 6% of the world's energy and 13–14% of the world's electricity, with the U.S., France, and Japan together accounting for about 50% of nuclear generated electricity...

.


A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction
Nuclear chain reaction
A nuclear chain reaction occurs when one nuclear reaction causes an average of one or more nuclear reactions, thus leading to a self-propagating number of these reactions. The specific nuclear reaction may be the fission of heavy isotopes or the fusion of light isotopes...

. Most commonly they are used for generating electricity
Nuclear power
Nuclear power is the use of sustained nuclear fission to generate heat and electricity. Nuclear power plants provide about 6% of the world's energy and 13–14% of the world's electricity, with the U.S., France, and Japan together accounting for about 50% of nuclear generated electricity...

 and for the propulsion of ships
Nuclear marine propulsion
Nuclear marine propulsion is propulsion of a ship by a nuclear reactor. Naval nuclear propulsion is propulsion that specifically refers to naval warships...

. Usually heat from nuclear fission is passed to a working fluid (water or gas), which runs through turbines
Turbine
A turbine is a rotary engine that extracts energy from a fluid flow and converts it into useful work.The simplest turbines have one moving part, a rotor assembly, which is a shaft or drum with blades attached. Moving fluid acts on the blades, or the blades react to the flow, so that they move and...

 that power either ship's propeller
Propeller
A propeller is a type of fan that transmits power by converting rotational motion into thrust. A pressure difference is produced between the forward and rear surfaces of the airfoil-shaped blade, and a fluid is accelerated behind the blade. Propeller dynamics can be modeled by both Bernoulli's...

s or generator
Electrical generator
In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. A generator forces electric charge to flow through an external electrical circuit. It is analogous to a water pump, which causes water to flow...

s. Some produce isotopes for medical
Nuclear medicine
In nuclear medicine procedures, elemental radionuclides are combined with other elements to form chemical compounds, or else combined with existing pharmaceutical compounds, to form radiopharmaceuticals. These radiopharmaceuticals, once administered to the patient, can localize to specific organs...

 and industrial
Industrial radiography
Industrial Radiography is the use of ionizing radiation to view objects in a way that cannot be seen otherwise. It is not to be confused with the use of ionizing radiation to change or modify objects; radiography's purpose is strictly viewing. Industrial radiography has grown out of engineering,...

 use, and some are run only for research
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 for electricity production, heat generation, or maritime propulsion.-Purpose:...

.

Just as conventional power stations
Thermal power station
A thermal power station is a power plant in which the prime mover is steam driven. Water is heated, turns into steam and spins a steam turbine which drives an electrical generator. After it passes through the turbine, the steam is condensed in a condenser and recycled to where it was heated; this...

 generate electricity by harnessing the thermal energy
Thermal energy
Thermal energy is the part of the total internal energy of a thermodynamic system or sample of matter that results in the system's temperature....

 released from burning fossil fuels, nuclear reactors convert the thermal energy released from nuclear fission
Nuclear fission
In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts , often producing free neutrons and photons , and releasing a tremendous amount of energy...

.

When a large fissile
Fissile
In nuclear engineering, a fissile material is one that is capable of sustaining a chain reaction of nuclear fission. By definition, fissile materials can sustain a chain reaction with neutrons of any energy. The predominant neutron energy may be typified by either slow neutrons or fast neutrons...

 atomic nucleus
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...

 such as uranium-235
Uranium-235
- References :* .* DOE Fundamentals handbook: Nuclear Physics and Reactor theory , .* A piece of U-235 the size of a grain of rice can produce energy equal to that contained in three tons of coal or fourteen barrels of oil. -External links:* * * one of the earliest articles on U-235 for the...

 or plutonium-239
Plutonium-239
Plutonium-239 is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 has also been used and is currently the secondary isotope. Plutonium-239 is also one of the three main isotopes demonstrated usable as fuel in...

 absorbs a neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...

, it may undergo nuclear fission.
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Encyclopedia
This article is a subarticle of Nuclear power
Nuclear power
Nuclear power is the use of sustained nuclear fission to generate heat and electricity. Nuclear power plants provide about 6% of the world's energy and 13–14% of the world's electricity, with the U.S., France, and Japan together accounting for about 50% of nuclear generated electricity...

.


A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction
Nuclear chain reaction
A nuclear chain reaction occurs when one nuclear reaction causes an average of one or more nuclear reactions, thus leading to a self-propagating number of these reactions. The specific nuclear reaction may be the fission of heavy isotopes or the fusion of light isotopes...

. Most commonly they are used for generating electricity
Nuclear power
Nuclear power is the use of sustained nuclear fission to generate heat and electricity. Nuclear power plants provide about 6% of the world's energy and 13–14% of the world's electricity, with the U.S., France, and Japan together accounting for about 50% of nuclear generated electricity...

 and for the propulsion of ships
Nuclear marine propulsion
Nuclear marine propulsion is propulsion of a ship by a nuclear reactor. Naval nuclear propulsion is propulsion that specifically refers to naval warships...

. Usually heat from nuclear fission is passed to a working fluid (water or gas), which runs through turbines
Turbine
A turbine is a rotary engine that extracts energy from a fluid flow and converts it into useful work.The simplest turbines have one moving part, a rotor assembly, which is a shaft or drum with blades attached. Moving fluid acts on the blades, or the blades react to the flow, so that they move and...

 that power either ship's propeller
Propeller
A propeller is a type of fan that transmits power by converting rotational motion into thrust. A pressure difference is produced between the forward and rear surfaces of the airfoil-shaped blade, and a fluid is accelerated behind the blade. Propeller dynamics can be modeled by both Bernoulli's...

s or generator
Electrical generator
In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. A generator forces electric charge to flow through an external electrical circuit. It is analogous to a water pump, which causes water to flow...

s. Some produce isotopes for medical
Nuclear medicine
In nuclear medicine procedures, elemental radionuclides are combined with other elements to form chemical compounds, or else combined with existing pharmaceutical compounds, to form radiopharmaceuticals. These radiopharmaceuticals, once administered to the patient, can localize to specific organs...

 and industrial
Industrial radiography
Industrial Radiography is the use of ionizing radiation to view objects in a way that cannot be seen otherwise. It is not to be confused with the use of ionizing radiation to change or modify objects; radiography's purpose is strictly viewing. Industrial radiography has grown out of engineering,...

 use, and some are run only for research
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 for electricity production, heat generation, or maritime propulsion.-Purpose:...

.

How it works


Just as conventional power stations
Thermal power station
A thermal power station is a power plant in which the prime mover is steam driven. Water is heated, turns into steam and spins a steam turbine which drives an electrical generator. After it passes through the turbine, the steam is condensed in a condenser and recycled to where it was heated; this...

 generate electricity by harnessing the thermal energy
Thermal energy
Thermal energy is the part of the total internal energy of a thermodynamic system or sample of matter that results in the system's temperature....

 released from burning fossil fuels, nuclear reactors convert the thermal energy released from nuclear fission
Nuclear fission
In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts , often producing free neutrons and photons , and releasing a tremendous amount of energy...

.

Fission


When a large fissile
Fissile
In nuclear engineering, a fissile material is one that is capable of sustaining a chain reaction of nuclear fission. By definition, fissile materials can sustain a chain reaction with neutrons of any energy. The predominant neutron energy may be typified by either slow neutrons or fast neutrons...

 atomic nucleus
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...

 such as uranium-235
Uranium-235
- References :* .* DOE Fundamentals handbook: Nuclear Physics and Reactor theory , .* A piece of U-235 the size of a grain of rice can produce energy equal to that contained in three tons of coal or fourteen barrels of oil. -External links:* * * one of the earliest articles on U-235 for the...

 or plutonium-239
Plutonium-239
Plutonium-239 is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 has also been used and is currently the secondary isotope. Plutonium-239 is also one of the three main isotopes demonstrated usable as fuel in...

 absorbs a neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...

, it may undergo nuclear fission. The heavy nucleus splits into two or more lighter nuclei, releasing kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...

, gamma radiation and free neutrons; collectively known as fission products. A portion of these neutrons may later 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
Nuclear chain reaction
A nuclear chain reaction occurs when one nuclear reaction causes an average of one or more nuclear reactions, thus leading to a self-propagating number of these reactions. The specific nuclear reaction may be the fission of heavy isotopes or the fusion of light isotopes...

.

This nuclear chain reaction
Nuclear chain reaction
A nuclear chain reaction occurs when one nuclear reaction causes an average of one or more nuclear reactions, thus leading to a self-propagating number of these reactions. The specific nuclear reaction may be the fission of heavy isotopes or the fusion of light isotopes...

 can be controlled by using neutron poisons and neutron moderators to change the portion of neutrons that will go on to cause more fissions. Nuclear reactors generally have automatic and manual systems to shut the fission reaction down if unsafe conditions are detected.

Commonly used moderators include regular (light) water (75% of the world's reactors), solid graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...

 (20% of reactors) and heavy water
Heavy water
Heavy water is water highly enriched in the hydrogen isotope deuterium; e.g., heavy water used in CANDU reactors is 99.75% enriched by hydrogen atom-fraction...

 (5% of reactors). Beryllium
Beryllium
Beryllium is the chemical element with the symbol Be and atomic number 4. It is a divalent element which occurs naturally only in combination with other elements in minerals. Notable gemstones which contain beryllium include beryl and chrysoberyl...

 has also been used in some experimental types, and hydrocarbons have been suggested as another possibility.

Heat generation


The reactor core generates heat in a number of ways:
  • The kinetic energy
    Kinetic energy
    The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...

     of fission products is converted to thermal energy
    Thermal energy
    Thermal energy is the part of the total internal energy of a thermodynamic system or sample of matter that results in the system's temperature....

     when these nuclei collide with nearby atoms.
  • Some of the gamma rays produced during fission are absorbed by the reactor, their energy being converted to heat.
  • Heat produced by the radioactive decay
    Radioactive decay
    Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...

     of fission products and materials that have been activated by neutron absorption. This decay heat source will remain for some time even after the reactor is shut down.


A kilogram of uranium-235
Uranium-235
- References :* .* DOE Fundamentals handbook: Nuclear Physics and Reactor theory , .* A piece of U-235 the size of a grain of rice can produce energy equal to that contained in three tons of coal or fourteen barrels of oil. -External links:* * * one of the earliest articles on U-235 for the...

 (U-235) converted via nuclear processes releases approximately three million times more energy than a kilogram of coal burned conventionally (7.2 × 1013 joules per kilogram of uranium-235 versus 2.4 × 107 joules per kilogram of coal).

Cooling


A nuclear reactor coolant
Nuclear reactor coolant
A nuclear reactor coolant is a coolant in a nuclear reactor used to remove heat from the nuclear reactor core and transfer it to electrical generators and the environment....

 — usually water but sometimes a gas or a liquid metal or molten salt
Molten salt
Molten salt refers to a salt that is in the liquid phase that is normally a solid at standard temperature and pressure . A salt which is normally liquid at STP is usually called a room temperature ionic liquid, although technically molten salts are a class of ionic liquids.-Uses:Molten salts have...

 — is circulated past the reactor core to absorb the heat that it generates. The heat is carried away from the reactor and is then used to generate steam. Most reactor systems employ a cooling system that is physically separated from the water that will be boiled to produce pressurized steam for the turbines, like the pressurized water reactor
Pressurized water reactor
Pressurized water reactors constitute a large majority of all western nuclear power plants and are one of three types of light water reactor , the other types being boiling water reactors and supercritical water reactors...

. But in some reactors the water for the steam turbines is boiled directly by the reactor core, for example the boiling water reactor
Boiling water reactor
The boiling water reactor is a type of light water nuclear reactor used for the generation of electrical power. It is the second most common type of electricity-generating nuclear reactor after the pressurized water reactor , also a type of light water nuclear reactor...

.

Reactivity control



The power output of the reactor is adjusted by controlling how many neutrons are able to create more fissions.

Control rod
Control rod
A control rod is a rod made of chemical elements capable of absorbing many neutrons without fissioning themselves. They are used in nuclear reactors to control the rate of fission of uranium and plutonium...

s that are made of a neutron poison are used to absorb neutrons. Absorbing more neutrons in a control rod means that there are fewer neutrons available to cause fission, so pushing the control rod deeper into the reactor will reduce its power output, and extracting the control rod will increase it.

At the first level of control in all nuclear reactors, a process of delayed neutron
Delayed neutron
In nuclear engineering, a delayed neutron is a neutron emitted after a nuclear fission event by one of the fission products anytime from a few milliseconds to a few minutes later....

 emission by a number of neutron-rich fission isotopes is an important physical process. These delayed neutrons account for about 0.65% of the total neutrons produced in fission, with the remainder (termed "prompt neutrons") released immediately upon fission. The fission products which produce delayed neutrons have half lives for their decay by neutron emission
Neutron emission
Neutron emission is a type of radioactive decay of atoms containing excess neutrons, in which a neutron is simply ejected from the nucleus. Two examples of isotopes which emit neutrons are helium-5 and beryllium-13...

 that range from milliseconds to as long as several minutes. Keeping the reactor in the zone of chain-reactivity where delayed neutrons are necessary to achieve a critical mass state, allows time for mechanical devices or human operators to have time to control a chain reaction in "real time"; otherwise the time between achievement of criticality and nuclear meltdown
Nuclear meltdown
Nuclear meltdown is an informal term for a severe nuclear reactor accident that results in core damage from overheating. The term is not officially defined by the International Atomic Energy Agency or by the U.S. Nuclear Regulatory Commission...

 as a result of an exponential power surge from the normal nuclear chain reaction, would be too short to allow for intervention.

In some reactors, the coolant also acts as a neutron moderator
Neutron moderator
In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....

. A moderator increases the power of the reactor by causing the fast neutrons that are released from fission to lose energy and become thermal neutrons. Thermal neutrons are more likely than fast neutrons to cause fission, so more neutron moderation means more power output from the reactors. 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
Scram
A scram or SCRAM is an emergency shutdown of a nuclear reactor – though the term has been extended to cover shutdowns of other complex operations, such as server farms and even large model railroads...

 the reactor in an emergency shut down. These systems insert large amounts of poison (often boron in the form of boric acid) into the reactor to shut the fission reaction down if unsafe conditions are detected or anticipated.

Most types of reactors are sensitive to a process variously known as xenon poisoning, or the iodine pit
Iodine pit
Iodine pit, also called iodine hole and xenon pit, is a temporary disabling of a nuclear reactor due to buildup of short-lived nuclear poisons in the core of a nuclear reactor. The main isotope responsible is xenon-135, mainly produced by natural decay of iodine-135. Iodine-135 is a weak neutron...

. Xenon-135
Xenon-135
Xenon-135 is an unstable isotope of xenon with a half-life of about 9.2 hours. 135Xe is a fission product of uranium and Xe-135 is the most powerful known neutron-absorbing nuclear poison , with a significant effect on nuclear reactor operation...

 is normally produced in the fission process, and acts as a neutron absorbing "neutron poison", which acts to shut the reactor down, but can be controlled in turn within the reactor by keeping neutron and power levels high enough to destroy it as fast as it is produced. The normal fission process also produces iodine-135, which in turn decays with a half life of under seven hours, to new xenon-135. Thus, if the reactor is shut down, iodine-135 in the reactor continues to decay to xenon-135, to the point that the new xenon-135 from this source ("xenon poisoning") makes re-starting the reactor more difficult, for a day or two, than when first shut down (this temporary state is the "iodine pit.") If the reactor has sufficient extra capacity, it can still be re-started before the iodine-135 and xenon-135 decay, but as the extra xenon-135 is "burned off" by transmuting it to xenon-136 (not a neutron poison), within a few hours the reactor may become unstable as a result of such a "xenon burnoff (power) transient," and then rapidly become overheated, unless control rods are reinserted in order 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
Chernobyl disaster
The Chernobyl disaster was a nuclear accident that occurred on 26 April 1986 at the Chernobyl Nuclear Power Plant in Ukraine , which was under the direct jurisdiction of the central authorities in Moscow...

.

Reactors used in nuclear marine propulsion
Nuclear marine propulsion
Nuclear marine propulsion is propulsion of a ship by a nuclear reactor. Naval nuclear propulsion is propulsion that specifically refers to naval warships...

 (especially nuclear submarine
Nuclear submarine
A nuclear submarine is a submarine powered by a nuclear reactor . The performance advantages of nuclear submarines over "conventional" submarines are considerable: nuclear propulsion, being completely independent of air, frees the submarine from the need to surface frequently, as is necessary for...

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 directly into the fuel rods. This allows the reactor to be constructed with a high excess of fissionable material, which is nevertheless made relatively more safe early in the reactor's fuel burn-cycle by the presence of the neutron-absorbing material which is later replaced by naturally 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 energy
Thermal energy is the part of the total internal energy of a thermodynamic system or sample of matter that results in the system's temperature....

 is to use it to boil water to produce pressurized steam which will then drive a steam turbine
Steam turbine
A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into rotary motion. Its modern manifestation was invented by Sir Charles Parsons in 1884....

 that generates electricity.

Early reactors



The neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...

 was discovered in 1932. The concept of a nuclear chain reaction brought about by nuclear reaction
Nuclear reaction
In nuclear physics and nuclear chemistry, a nuclear reaction is semantically considered to be the process in which two nuclei, or else a nucleus of an atom and a subatomic particle from outside the atom, collide to produce products different from the initial particles...

s mediated by neutrons, was first realized shortly thereafter, by Hungarian
Hungary
Hungary , officially the Republic of Hungary , is a landlocked country in Central Europe. It is situated in the Carpathian Basin and is bordered by Slovakia to the north, Ukraine and Romania to the east, Serbia and Croatia to the south, Slovenia to the southwest and Austria to the west. The...

 scientist Leó Szilárd
Leó Szilárd
Leó Szilárd was an Austro-Hungarian physicist and inventor who conceived the nuclear chain reaction in 1933, patented the idea of a nuclear reactor with Enrico Fermi, and in late 1939 wrote the letter for Albert Einstein's signature that resulted in the Manhattan Project that built the atomic bomb...

, in 1933. He filed a patent for his idea of a simple nuclear reactor the following year while working at the Admiralty
Admiralty
The Admiralty was formerly the authority in the Kingdom of England, and later in the United Kingdom, responsible for the command of the Royal Navy...

 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
Lise Meitner
Lise Meitner FRS was an Austrian-born, later Swedish, physicist who worked on radioactivity and nuclear physics. Meitner was part of the team that discovered nuclear fission, an achievement for which her colleague Otto Hahn was awarded the Nobel Prize...

, Fritz Strassman and Otto Hahn
Otto Hahn
Otto Hahn FRS was a German chemist and Nobel laureate, a pioneer in the fields of radioactivity and radiochemistry. He is regarded as "the father of nuclear chemistry". Hahn was a courageous opposer of Jewish persecution by the Nazis and after World War II he became a passionate campaigner...

 in 1938 that bombardment of uranium with neutrons (provided by an alpha-on-beryllium fusion reaction, a "neutron howitzer
Neutron howitzer
The name "Neutron Howitzer" was given to a neutron source that was developed after it was discovered in the 1930s that alpha radiation that strikes the Beryllium nucleus would release neutrons...

") produced a barium
Barium
Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in Group 2, a soft silvery metallic alkaline earth metal. Barium is never found in nature in its pure form due to its reactivity with air. Its oxide is historically known as baryta but it reacts with...

 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
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 leads to a self-amplifying chain of events....

 that Szilárd had envisioned six years previously.

On August 2, 1939 Albert Einstein
Albert Einstein
Albert Einstein was a German-born theoretical physicist who developed the theory of general relativity, effecting a revolution in physics. For this achievement, Einstein is often regarded as the father of modern physics and one of the most prolific intellects in human history...

 signed a letter to President Franklin D. Roosevelt
Franklin D. Roosevelt
Franklin Delano Roosevelt , also known by his initials, FDR, was the 32nd President of the United States and a central figure in world events during the mid-20th century, leading the United States during a time of worldwide economic crisis and world war...

 (written by Szilard) 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-Szilard letter
Einstein-Szilárd letter
The Einstein–Szilárd letter was a letter sent to the United States President Franklin D. Roosevelt on August 2, 1939, that was signed by Albert Einstein but largely written by Leó Szilárd in consultation with fellow Hungarian physicists Edward Teller and Eugene Wigner. The letter suggested that the...

 to alert the U.S. government.

Shortly after, Hitler's Germany invaded Poland in 1939, starting World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

 in Europe. The U.S. was not yet officially at war, but in October, when the Einstein-Szilard letter was delivered to Roosevelt, he 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
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...

 needed for a chain reaction
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 leads to a self-amplifying chain of events....

 was far lower than had previously been thought. The memorandum was a product of the MAUD Committee
MAUD Committee
The MAUD Committee was the beginning of the British atomic bomb project, before the United Kingdom joined forces with the United States in the Manhattan Project.-Frisch & Peierls:...

, which was working on the UK atomic bomb project, known as Tube Alloys
Tube Alloys
Tube Alloys was the code-name for the British nuclear weapon directorate during World War II, when the development of nuclear weapons was kept at such a high level of secrecy that it had to be referred to by code even in the highest circles of government...

, later to be subsumed within the Manhattan Project
Manhattan Project
The Manhattan Project was a research and development program, led by the United States with participation from the United Kingdom and Canada, that produced the first atomic bomb during World War II. From 1942 to 1946, the project was under the direction of Major General Leslie Groves of the US Army...

.

Eventually, the first artificial nuclear reactor, Chicago Pile-1
Chicago Pile-1
Chicago Pile-1 was the world's first man-made nuclear reactor. CP-1 was built on a rackets court, under the abandoned west stands of the original Alonzo Stagg Field stadium, at the University of Chicago. The first self-sustaining nuclear chain reaction was initiated in CP-1 on December 2, 1942...

, was constructed at the University of Chicago
University of Chicago
The University of Chicago is a private research university in Chicago, Illinois, USA. It was founded by the American Baptist Education Society with a donation from oil magnate and philanthropist John D. Rockefeller and incorporated in 1890...

, by a team led by Enrico Fermi
Enrico Fermi
Enrico Fermi was an Italian-born, naturalized American physicist particularly known for his work on the development of the first nuclear reactor, Chicago Pile-1, and for his contributions to the development of quantum theory, nuclear and particle physics, and statistical mechanics...

, in late 1942. By this time, the program had been pressured for a year by U.S. entry into the war. The Chicago Pile achieved criticality
Critical mass
A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The...

 on December 2, 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
Manhattan Project
The Manhattan Project was a research and development program, led by the United States with participation from the United Kingdom and Canada, that produced the first atomic bomb during World War II. From 1942 to 1946, the project was under the direction of Major General Leslie Groves of the US Army...

 starting in 1943. The primary purpose for the largest reactors (located at the Hanford Site
Hanford Site
The Hanford Site is a mostly decommissioned nuclear production complex on the Columbia River in the U.S. state of Washington, operated by the United States federal government. The site has been known by many names, including Hanford Works, Hanford Engineer Works or HEW, Hanford Nuclear Reservation...

 in Washington state), was the mass production of plutonium
Plutonium
Plutonium is a transuranic radioactive chemical element with the chemical symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation...

 for nuclear weapons. Fermi and Szilard applied for a patent on reactors on 19 December 1944. Its issuance was delayed for 10 years because of wartime secrecy.

"World's first nuclear power plant" is the claim made by signs at the site of the EBR-I, which is now a museum near Arco, Idaho
Arco, Idaho
Arco is a city in Butte County, Idaho, United States. The population was 995 at the 2010 census. The city is the county seat of Butte County.Craters of the Moon National Monument is located along U.S. Route 20, southwest of the city. The Idaho National Laboratory is located east of Arco...

. This experimental LMFBR operated by the U.S. Atomic Energy Commission produced 0.8 kW in a test on December 20, 1951 and 100 kW (electrical) the following day, having a design output of 200 kW (electrical).

Besides the military uses of nuclear reactors, there were political reasons to pursue civilian use of atomic energy. U.S. President Dwight Eisenhower made his famous Atoms for Peace
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....

 speech to the UN General Assembly on December 8, 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
Obninsk Nuclear Power Station, , was built in the "Science City" of Obninsk, about 110 km southwest of Moscow. It was the first civilian nuclear power station in the world...

, launched on June 27, 1954 in the Soviet Union
Soviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....

. It produced around 5 MW (electrical).

After World War II, the U.S. military sought other uses for nuclear reactor technology. Research by the Army and the Air Force never came to fruition; however, the U.S. Navy succeeded when they steamed the USS Nautilus
USS Nautilus (SSN-571)
USS Nautilus is the world's first operational nuclear-powered submarine. She was the first vessel to complete a submerged transit beneath the North Pole on August 3, 1958...

 (SSN-571) on nuclear power January 17, 1955.

The first commercial nuclear power station, Calder Hall in Sellafield
Sellafield
Sellafield is a nuclear reprocessing site, close to the village of Seascale on the coast of the Irish Sea in Cumbria, England. The site is served by Sellafield railway station. Sellafield is an off-shoot from the original nuclear reactor site at Windscale which is currently undergoing...

, England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...

 was opened in 1956 with an initial capacity of 50 MW (later 200 MW).

The first portable nuclear reactor "Alco PM-2A" used to generate electrical power (2 MW) for Camp Century from 1960.

Components


The key components common to most types of nuclear power plants are:
  • Nuclear fuel
    Nuclear fuel
    Nuclear fuel is a material that can be 'consumed' by fission or fusion to derive nuclear energy. Nuclear fuels are the most dense sources of energy available...

  • Nuclear reactor core
    Nuclear reactor core
    A nuclear reactor core is the portion of a nuclear reactor containing the nuclear fuel components where the nuclear reactions take place.- Description :...

  • Neutron moderator
    Neutron moderator
    In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....

  • Neutron poison
  • Coolant
    Coolant
    A coolant is a fluid which flows through a device to prevent its overheating, transferring the heat produced by the device to other devices that use or dissipate it. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, and chemically inert, neither causing nor...

     (often the Neutron Moderator and the Coolant are the same, usually both purified water)
  • Control rod
    Control rod
    A control rod is a rod made of chemical elements capable of absorbing many neutrons without fissioning themselves. They are used in nuclear reactors to control the rate of fission of uranium and plutonium...

    s
  • Reactor vessel
    Reactor vessel
    In a nuclear power plant, the reactor vessel is a pressure vessel containing the Nuclear reactor coolant and reactor core.Not all power reactors have a reactor vessel. Power reactors are generally classified by the type of coolant rather than by the configuration of the reactor vessel used to...

  • Boiler feedwater pump
    Boiler feedwater pump
    A boiler feedwater pump is a specific type of pump used to pump feedwater into a steam boiler. The water may be freshly supplied or returning condensate produced as a result of the condensation of the steam produced by the boiler...

  • Steam generators
    Steam generator (nuclear power)
    Steam generators are heat exchangers used to convert water into steam from heat produced in a nuclear reactor core. They are used in pressurized water reactors between the primary and secondary coolant loops....

     (not in BWRs)
  • Steam turbine
    Steam turbine
    A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into rotary motion. Its modern manifestation was invented by Sir Charles Parsons in 1884....

  • Electrical generator
    Electrical generator
    In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. A generator forces electric charge to flow through an external electrical circuit. It is analogous to a water pump, which causes water to flow...

  • Condenser
    Condenser (steam turbine)
    A surface condenser is a commonly used term for a water-cooled shell and tube heat exchanger installed on the exhaust steam from a steam turbine in thermal power stations. These condensers are heat exchangers which convert steam from its gaseous to its liquid state at a pressure below atmospheric...

  • Cooling tower
    Cooling tower
    Cooling towers are heat removal devices used to transfer process waste heat to the atmosphere. Cooling towers may either use the evaporation of water to remove process heat and cool the working fluid to near the wet-bulb air temperature or in the case of closed circuit dry cooling towers rely...

     (not always required)
  • Radwaste System (a section of the plant handling radioactive waste
    Radioactive waste
    Radioactive wastes are wastes that contain radioactive material. Radioactive wastes are usually by-products of nuclear power generation and other applications of nuclear fission or nuclear technology, such as research and medicine...

    )
  • Refueling Floor
  • Spent fuel pool
    Spent fuel pool
    Spent fuel pools are storage pools for spent fuel from nuclear reactors. They are typically 40 or more feet deep, with the bottom 14 feet equipped with storage racks designed to hold fuel assemblies removed from the reactor. A reactor's pool is specially designed for the reactor in which the...

  • Nuclear safety systems
    Nuclear safety systems
    The three primary objectives of nuclear reactor safety systems as defined by the Nuclear Regulatory Commission are to shut down the reactor, maintain it in a shutdown condition, and prevent the release of radioactive material during events and accidents...

    • Reactor Protective System
      Reactor Protective System
      A reactor protection system is a set of nuclear safety components in a nuclear power plant designed to safely shutdown the reactor and prevent the release of radioactive materials. The System can "trip" automatically , or it can be tripped by the operators. Trips occurs when the parameters meet...

       (RPS)
    • Emergency Diesel Generators
      Emergency power system
      Emergency power systems are a type of system, which may include lighting, generators, fuel cells and other apparatus, to provide backup power resources in a crisis or when regular systems fail. They find uses in a wide variety of settings from residential homes to hospitals, scientific...

    • Emergency Core Cooling Systems (ECCS)
    • Standby Liquid Control System (emergency boron
      Boron
      Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...

       injection, in BWRs only)
  • Essential service water system (ESWS)
  • Containment building
    Containment building
    A containment building, in its most common usage, is a steel or reinforced concrete structure enclosing a nuclear reactor. It is designed, in any emergency, to contain the escape of radiation to a maximum pressure in the range of 60 to 200 psi...

  • Control room
    Control room
    A control room is a room serving as an operations centre where a facility or service can be monitored and controlled. Examples include:*in television production, the master control is the technical hub of a broadcast operation common among most over-the-air television stations, television networks...

  • Emergency Operations Facility
  • Nuclear training facility (usually contains a Control Room simulator)

Reactor types



Classifications


Nuclear Reactors are classified by several methods; a brief outline of these classification methods is provided.

Classification by type of nuclear reaction

  • Nuclear fission
    Nuclear fission
    In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts , often producing free neutrons and photons , and releasing a tremendous amount of energy...

    . All commercial power reactors are based on nuclear fission. They generally use uranium
    Uranium
    Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...

     and its product plutonium
    Plutonium
    Plutonium is a transuranic radioactive chemical element with the chemical symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation...

     as nuclear fuel
    Nuclear fuel
    Nuclear fuel is a material that can be 'consumed' by fission or fusion to derive nuclear energy. Nuclear fuels are the most dense sources of energy available...

    , though a thorium fuel cycle
    Thorium fuel cycle
    The thorium fuel cycle is a nuclear fuel cycle that uses the naturally abundant isotope of thorium, , as the fertile material. In the reactor, is transmuted into the fissile artificial uranium isotope which is the nuclear fuel. Unlike natural uranium, natural thorium contains only trace amounts...

     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
    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 leads to a self-amplifying chain of events....

    :
    • Thermal reactor
      Thermal reactor
      A thermal reactor is a nuclear reactor that uses slow or thermal neutrons. Most power reactors are of this type. These type of reactors use a neutron moderator to slow neutrons until they approach the average kinetic energy of the surrounding particles, that is, to reduce the speed of the neutrons...

      s use slowed or thermal neutrons. Almost all current reactors are of this type. These contain neutron moderator
      Neutron moderator
      In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....

       materials that slow neutrons until their neutron temperature
      Neutron temperature
      The neutron detection temperature, also called the neutron energy, indicates a free neutron's kinetic energy, usually given in electron volts. The term temperature is used, since hot, thermal and cold neutrons are moderated in a medium with a certain temperature. The neutron energy distribution is...

       is thermalized, that is, until their kinetic energy
      Kinetic energy
      The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...

       approaches the average kinetic energy of the surrounding particles. Thermal neutrons have a far higher cross section
      Nuclear cross section
      The nuclear cross section of a nucleus is used to characterize the probability that a nuclear reaction will occur. The concept of a nuclear cross section can be quantified physically in terms of "characteristic area" where a larger area means a larger probability of interaction...

       (probability) of fissioning the fissile
      Fissile
      In nuclear engineering, a fissile material is one that is capable of sustaining a chain reaction of nuclear fission. By definition, fissile materials can sustain a chain reaction with neutrons of any energy. The predominant neutron energy may be typified by either slow neutrons or fast neutrons...

       nuclei uranium-235
      Uranium-235
      - References :* .* DOE Fundamentals handbook: Nuclear Physics and Reactor theory , .* A piece of U-235 the size of a grain of rice can produce energy equal to that contained in three tons of coal or fourteen barrels of oil. -External links:* * * one of the earliest articles on U-235 for the...

      , plutonium-239
      Plutonium-239
      Plutonium-239 is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 has also been used and is currently the secondary isotope. Plutonium-239 is also one of the three main isotopes demonstrated usable as fuel in...

      , and plutonium-241
      Plutonium-241
      Plutonium-241 is an isotope of plutonium formed when plutonium-240 captures a neutron. Like Pu-239 but unlike 240Pu, 241Pu is fissile, with a neutron absorption cross section about 1/3 greater than 239Pu, and a similar probability of fissioning on neutron absorption, around 73%. In the non-fission...

      , and a relatively lower probability of neutron capture
      Neutron capture
      Neutron capture is a kind of nuclear reaction in which an atomic nucleus collides with one or more neutrons and they merge to form a heavier nucleus. Since neutrons have no electric charge they can enter a nucleus more easily than positively charged protons, which are repelled...

       by uranium-238
      Uranium-238
      Uranium-238 is the most common isotope of uranium found in nature. It is not fissile, but is a fertile material: it can capture a slow neutron and after two beta decays become fissile plutonium-239...

       (U-238) compared to the faster neutrons that originally result from fission, allowing use of low-enriched uranium or even natural uranium
      Natural uranium
      Natural uranium refers to refined uranium with the same isotopic ratio as found in nature. It contains 0.7 % uranium-235, 99.3 % uranium-238, and a trace of uranium-234 by weight. In terms of the amount of radioactivity, approximately 2.2 % comes from uranium-235, 48.6 % uranium-238, and 49.2 %...

       fuel. The moderator is often also the coolant
      Coolant
      A coolant is a fluid which flows through a device to prevent its overheating, transferring the heat produced by the device to other devices that use or dissipate it. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, and chemically inert, neither causing nor...

      , usually water under high pressure to increase the boiling point
      Boiling point
      The boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....

      . These are surrounded by a reactor vessel
      Reactor vessel
      In a nuclear power plant, the reactor vessel is a pressure vessel containing the Nuclear reactor coolant and reactor core.Not all power reactors have a reactor vessel. Power reactors are generally classified by the type of coolant rather than by the configuration of the reactor vessel used to...

      , instrumentation to monitor and control the reactor, radiation shielding, and a containment building
      Containment building
      A containment building, in its most common usage, is a steel or reinforced concrete structure enclosing a nuclear reactor. It is designed, in any emergency, to contain the escape of radiation to a maximum pressure in the range of 60 to 200 psi...

      .
    • Fast neutron reactor
      Fast neutron reactor
      A fast neutron reactor or simply a fast reactor is a category of nuclear reactor in which the fission chain reaction is sustained by fast neutrons...

      s use fast neutrons to cause fission in their fuel. They do not have a neutron moderator
      Neutron moderator
      In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....

      , and use less-moderating coolants. Maintaining a chain reaction requires the fuel to be more highly enriched
      Isotope separation
      Isotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes, for example separating natural uranium into enriched uranium and depleted uranium. This is a crucial process in the manufacture of uranium fuel for nuclear power stations, and is...

       in fissile
      Fissile
      In nuclear engineering, a fissile material is one that is capable of sustaining a chain reaction of nuclear fission. By definition, fissile materials can sustain a chain reaction with neutrons of any energy. The predominant neutron energy may be typified by either slow neutrons or fast neutrons...

       material (about 20% or more) due to the relatively lower probability of fission versus capture by U-238. Fast reactors have the potential to produce less transuranic waste because all actinides are fissionable with fast neutrons, but they are more difficult to build and more expensive to operate. Overall, fast reactors are less common than thermal reactors in most applications. Some early power stations were fast reactors, as are some Russian naval propulsion units. Construction of prototypes is continuing (see fast breeder or generation IV reactors).
  • Nuclear fusion
    Nuclear fusion
    Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...

    . Fusion power
    Fusion power
    Fusion power is the power generated by nuclear fusion processes. In fusion reactions two light atomic nuclei fuse together to form a heavier nucleus . In doing so they release a comparatively large amount of energy arising from the binding energy due to the strong nuclear force which is manifested...

     is an experimental technology, generally with hydrogen
    Hydrogen
    Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

     as fuel. While not suitable for power production, Farnsworth-Hirsch fusors are used to produce neutron radiation
    Neutron radiation
    Neutron radiation is a kind of ionizing radiation which consists of free neutrons. A result of nuclear fission or nuclear fusion, it consists of the release of free neutrons from atoms, and these free neutrons react with nuclei of other atoms to form new isotopes, which, in turn, may produce...

    .

Classification by moderator material


Used by thermal reactors:
  • Graphite moderated reactors
    Graphite moderated reactors
    There are several types of graphite moderated nuclear reactors that have been used in commercial electricity generation:*Gas-cooled reactors**Magnox**Advanced gas-cooled reactor *Water-cooled reactors**RBMK*High temperature gas-cooled reactors...

  • Water moderated reactors
    • Heavy water reactor
      Heavy water reactor
      A pressurised heavy water reactor is a nuclear power reactor, commonly using unenriched natural uranium as its fuel, that uses heavy water as its coolant and moderator. The heavy water coolant is kept under pressure in order to raise its boiling point, allowing it to be heated to higher...

      s
    • Light water moderated reactors
      Light water reactor
      The light water reactor is a type of thermal reactor that uses normal water as its coolant and neutron moderator. Thermal reactors are the most common type of nuclear reactor, and light water reactors are the most common type of thermal reactor...

       (LWRs). Light water reactors use ordinary water to moderate and cool the reactors. When at operating temperature
      Operating temperature
      An operating temperature is the temperature at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the...

      , 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
      Negative feedback
      Negative feedback occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.- Overview :...

       stabilizes the reaction rate. Graphite and heavy water reactors tend to be more thoroughly thermalised than light water reactors. Due to the extra thermalization, these types can use natural uranium
      Natural uranium
      Natural uranium refers to refined uranium with the same isotopic ratio as found in nature. It contains 0.7 % uranium-235, 99.3 % uranium-238, and a trace of uranium-234 by weight. In terms of the amount of radioactivity, approximately 2.2 % comes from uranium-235, 48.6 % uranium-238, and 49.2 %...

      /unenriched fuel.
  • Light element moderated reactors. These reactors are moderated by lithium or beryllium.
    • Molten salt reactor
      Molten salt reactor
      A molten salt reactor is a type of nuclear fission reactor in which the primary coolant, or even the fuel itself is a molten salt mixture...

      s (MSRs) are moderated by a light elements such as lithium or beryllium, which are constituents of the coolant/fuel matrix salts LiF and BeF2.
    • Liquid metal cooled reactor
      Liquid metal cooled reactor
      A liquid metal cooled nuclear reactor, liquid metal fast reactor or LMFR is an advanced type of nuclear reactor where the primary coolant is a liquid metal. Liquid metal cooled reactors were first adapted for nuclear submarine use but have also been extensively studied for power generation...

      s, such as one whose coolant is a mixture of Lead and Bismuth, may use BeO as a moderator.
  • Organically moderated reactors (OMR) use biphenyl
    Biphenyl
    Biphenyl is an organic compound that forms colorless crystals. It has a distinctively pleasant smell. Biphenyl is an aromatic hydrocarbon with a molecular formula 2...

     and terphenyl
    Terphenyl
    Terphenyls are a group of closely related aromatic hydrocarbons. Also known as diphenylbenzenes or triphenyls, they consist of a central benzene ring substituted with two phenyl groups. The three isomers are ortho-terphenyl, meta-terphenyl, and para-terphenyl. Commercial grade terphenyl is...

     as moderator and coolant.

Classification by coolant


  • Water cooled reactor. There are 104 operating reactors in the United States. Of these, 69 are pressurized water reactors (PWR), and 35 are boiling water reactors (BWR).
    • Pressurized water reactor
      Pressurized water reactor
      Pressurized water reactors constitute a large majority of all western nuclear power plants and are one of three types of light water reactor , the other types being boiling water reactors and supercritical water reactors...

       (PWR)
      • A primary characteristic of PWRs is a pressurizer, a specialized pressure vessel
        Pressure vessel
        A pressure vessel is a closed container designed to hold gases or liquids at a pressure substantially different from the ambient pressure.The pressure differential is dangerous and many fatal accidents have occurred in the history of their development and operation. Consequently, their design,...

        . 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.
      • Pressurised heavy water reactors are a subset of pressurized water reactors, sharing the use of a pressurized, isolated heat transport loop, but using heavy water
        Heavy water
        Heavy water is water highly enriched in the hydrogen isotope deuterium; e.g., heavy water used in CANDU reactors is 99.75% enriched by hydrogen atom-fraction...

         as coolant and moderator for the greater neutron economies it offers.
    • Boiling water reactor
      Boiling water reactor
      The boiling water reactor is a type of light water nuclear reactor used for the generation of electrical power. It is the second most common type of electricity-generating nuclear reactor after the pressurized water reactor , also a type of light water nuclear 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 that are submerged in water and housed in a steel vessel. 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.
    • Pool-type reactor
  • Liquid metal cooled reactor
    Liquid metal cooled reactor
    A liquid metal cooled nuclear reactor, liquid metal fast reactor or LMFR is an advanced type of nuclear reactor where the primary coolant is a liquid metal. Liquid metal cooled reactors were first adapted for nuclear submarine use but have also been extensively studied for power generation...

    . Since water is a moderator, it cannot be used as a coolant in a fast reactor. Liquid metal coolants have included sodium
    Sodium
    Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...

    , NaK
    NaK
    NaK, or sodium-potassium alloy, an alloy, of potassium , and sodium , is usually liquid at room temperature. Various commercial grades are available. NaK is highly reactive with water and may catch fire when exposed to air, so must be handled with special precautions...

    , lead
    Lead
    Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...

    , lead-bismuth eutectic
    Lead-bismuth eutectic
    Lead-Bismuth Eutectic or LBE is a eutectic alloy of lead and bismuth used as a coolant in some nuclear reactors, and is a proposed coolant for the lead-cooled fast reactor, part of the Generation IV reactor initiative....

    , and in early reactors, mercury
    Mercury (element)
    Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...

    .
    • Sodium-cooled fast reactor
      Sodium-cooled fast reactor
      The sodium-cooled fast reactor or SFR is a Generation IV reactor project to design an advanced fast neutron reactor.It builds on two closely related existing projects, the LMFBR and the Integral Fast Reactor, with the objective of producing a fast-spectrum, sodium-cooled reactor.The reactors are...

    • Lead-cooled fast reactor
  • Gas cooled reactor
    Gas Cooled Reactor
    A gas-cooled reactor is a nuclear reactor that uses graphite as a neutron moderator and carbon dioxide as coolant...

    s are cooled by a circulating inert gas, often helium
    Helium
    Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

     in high-temperature designs, while carbon dioxide
    Carbon dioxide
    Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

     has been used in past British and French nuclear power plants. Nitrogen
    Nitrogen
    Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

     has also been used. Utilization of the heat varies, depending on the reactor. Some reactors run hot enough that the gas can directly power a gas turbine. Older designs usually run the gas through a heat exchanger
    Heat exchanger
    A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...

     to make steam for a steam turbine.
  • Molten Salt Reactor
    Molten salt reactor
    A molten salt reactor is a type of nuclear fission reactor in which the primary coolant, or even the fuel itself is a molten salt mixture...

    s (MSRs) are cooled by circulating a molten salt, typically a eutectic mixture of fluoride salts, such as FLiBe
    FLiBe
    FLiBe is a mixture of lithium fluoride and beryllium fluoride . As a molten salt it is proposed as a nuclear reactor coolant, and two different mixtures were used in the Molten-Salt Reactor Experiment....

    . In a typical MSR, the coolant is also used as a matrix in which the fissile material is dissolved.

Classification by generation

  • Generation I reactor
  • Generation II reactor
    Generation II reactor
    A generation II reactor is a design classification for a nuclear reactor, and refers to the class of commercial reactors built up to the end of the 1990s...

     (most current nuclear power plant
    Nuclear power plant
    A nuclear power plant is a thermal power station in which the heat source is one or more nuclear reactors. As in a conventional thermal power station the heat is used to generate steam which drives a steam turbine connected to a generator which produces electricity.Nuclear power plants are usually...

    s)
  • Generation III reactor
    Generation III reactor
    A generation III reactor is a development of any of the generation II nuclear reactor designs incorporating evolutionary improvements in design developed during the lifetime of the generation II reactor designs...

     (evolutionary improvements of existing designs)
  • Generation IV reactor
    Generation IV reactor
    Generation IV reactors are a set of theoretical nuclear reactor designs currently being researched. Most of these designs are generally not expected to be available for commercial construction before 2030...

     (technologies still under development)


The "Gen IV"-term was dubbed by the United States Department of Energy
United States Department of Energy
The United States Department of Energy is a Cabinet-level department of the United States government concerned with the United States' policies regarding energy and safety in handling nuclear material...

 (DOE) for developing new plant types in 2000. In 2003, the French Commissariat à l'Énergie Atomique
Commissariat à l'Énergie Atomique
The Commissariat à l'énergie atomique et aux énergies alternatives or CEA, is a French “public establishment related to industrial and commercial activities” whose mission is to develop all applications of nuclear power, both civilian and military...

 (CEA) was the first to refer to Gen II types in Nucleonics Week; . First mentioning of Gen III was also in 2000 in conjunction with the launch of the Generation IV International Forum
Generation IV International Forum
Nine countries, as well as the member states of the European Union are working together as the Generation IV International Forum to lay the groundwork for the Generation IV reactor....

 (GIF) plans.

Classification by phase of fuel

  • Solid fueled
  • Fluid fueled
    • Aqueous homogeneous reactor
      Aqueous homogeneous reactor
      Aqueous homogeneous reactors are a type of nuclear reactor in which soluble nuclear salts have been dissolved in water. The fuel is mixed with the coolant and the moderator, thus the name "homogeneous" The water can be either heavy water or light water, both which need to be very pure...

    • Molten salt reactor
      Molten salt reactor
      A molten salt reactor is a type of nuclear fission reactor in which the primary coolant, or even the fuel itself is a molten salt mixture...

  • Gas fueled
    Gaseous fission reactor
    A gas nuclear reactor is a nuclear reactor in which the nuclear fuel is in a gaseous state rather than liquid or solid. In this type of reactor, the only temperature-limiting materials are the reactor walls. Conventional reactors have stricter limitations because the core would melt if the fuel...

     (theoretical)

Classification by use

  • Electricity
    • Nuclear power plant
      Nuclear power plant
      A nuclear power plant is a thermal power station in which the heat source is one or more nuclear reactors. As in a conventional thermal power station the heat is used to generate steam which drives a steam turbine connected to a generator which produces electricity.Nuclear power plants are usually...

      s
  • Propulsion, see nuclear propulsion
    Nuclear propulsion
    Nuclear propulsion includes a wide variety of propulsion methods that fulfil the promise of the Atomic Age by using some form of nuclear reaction as their primary power source.- Surface ships and submarines :...

    • Nuclear marine propulsion
      Nuclear marine propulsion
      Nuclear marine propulsion is propulsion of a ship by a nuclear reactor. Naval nuclear propulsion is propulsion that specifically refers to naval warships...

    • Various proposed forms of rocket propulsion
  • Other uses of heat
    • Desalination
      Desalination
      Desalination, desalinization, or desalinisation refers to any of several processes that remove some amount of salt and other minerals from saline water...

    • Heat for domestic and industrial heating
    • Hydrogen production for use in a hydrogen economy
      Hydrogen economy
      The hydrogen economy is a proposed system of delivering energy using hydrogen. The term hydrogen economy was coined by John Bockris during a talk he gave in 1970 at General Motors Technical Center....

  • Production reactors for transmutation
    Nuclear transmutation
    Nuclear transmutation is the conversion of one chemical element or isotope into another. In other words, atoms of one element can be changed into atoms of other element by 'transmutation'...

     of elements
    • Breeder reactor
      Breeder reactor
      A breeder reactor is a nuclear reactor capable of generating more fissile material than it consumes because its neutron economy is high enough to breed fissile from fertile material like uranium-238 or thorium-232. Breeders were at first considered superior because of their superior fuel economy...

      s are capable of producing more fissile material than they consume during the fission chain reaction (by converting fertile
      Fertile material
      Fertile material is a term used to describe nuclides which generally themselves do not undergo induced fission but from which fissile material is generated by neutron absorption and subsequent nuclei conversions...

       U-238 to Pu-239, or Th-232 to U-233). Thus, a uranium breeder reactor, once running, can be re-fueled with natural
      Natural uranium
      Natural uranium refers to refined uranium with the same isotopic ratio as found in nature. It contains 0.7 % uranium-235, 99.3 % uranium-238, and a trace of uranium-234 by weight. In terms of the amount of radioactivity, approximately 2.2 % comes from uranium-235, 48.6 % uranium-238, and 49.2 %...

       or even depleted uranium
      Depleted uranium
      Depleted uranium is uranium with a lower content of the fissile isotope U-235 than natural uranium . Uses of DU take advantage of its very high density of 19.1 g/cm3...

      , and a thorium breeder reactor can be re-fueled with thorium
      Thorium
      Thorium is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder....

      ; however, an initial stock of fissile material is required.
    • Creating various radioactive
      Radiation
      In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...

       isotope
      Isotope
      Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...

      s, such as americium
      Americium
      Americium is a synthetic element that has the symbol Am and atomic number 95. This transuranic element of the actinide series is located in the periodic table below the lanthanide element europium, and thus by analogy was named after another continent, America.Americium was first produced in 1944...

       for use in smoke detector
      Smoke detector
      A smoke detector is a device that detects smoke, typically as an indicator of fire. Commercial, industrial, and mass residential devices issue a signal to a fire alarm system, while household detectors, known as smoke alarms, generally issue a local audible and/or visual alarm from the detector...

      s, and cobalt-60, molybdenum-99 and others, used for imaging and medical treatment.
    • Production of materials for nuclear weapon
      Nuclear weapon
      A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion. Both reactions release vast quantities of energy from relatively small amounts of matter. The first fission bomb test released the same amount...

      s such as weapons-grade
      Weapons-grade
      A weapons-grade substance is one that is pure enough to be used to make a weapon or has properties that make it suitable for weapons use. Weapons-grade plutonium and uranium are the most common examples, but it may also be used to refer to chemical and biological weapons...

       plutonium
      Plutonium
      Plutonium is a transuranic radioactive chemical element with the chemical symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation...

  • Providing a source of neutron radiation
    Neutron radiation
    Neutron radiation is a kind of ionizing radiation which consists of free neutrons. A result of nuclear fission or nuclear fusion, it consists of the release of free neutrons from atoms, and these free neutrons react with nuclei of other atoms to form new isotopes, which, in turn, may produce...

     (for example with the pulsed Godiva device
    Godiva device
    The Lady Godiva device was an unshielded, pulsed nuclear reactor originally situated at the Los Alamos National Laboratory , New Mexico, U.S. It was one of a number of criticality devices within Technical Area 18 . Specifically, it was used to produce bursts of neutrons and gamma rays for...

    ) and positron radiation (e.g. neutron activation analysis
    Neutron activation analysis
    In chemistry, neutron activation analysis is a nuclear process used for determining the concentrations of elements in a vast amount of materials. NAA allows discrete sampling of elements as it disregards the chemical form of a sample, and focuses solely on its nucleus. The method is based on...

     and potassium-argon dating
    Potassium-argon dating
    Potassium–argon dating or K–Ar dating is a radiometric dating method used in geochronology and archeology. It is based on measurement of the product of the radioactive decay of an isotope of potassium into argon . Potassium is a common element found in many materials, such as micas, clay minerals,...

    )
  • Research reactor
    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 for electricity production, heat generation, or maritime propulsion.-Purpose:...

    : 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


There are two types of nuclear power in current use:
  • The Radioisotope thermoelectric generator
    Radioisotope thermoelectric generator
    A radioisotope thermoelectric generator is an electrical generator that obtains its power from radioactive decay. In such a device, the heat released by the decay of a suitable radioactive material is converted into electricity by the Seebeck effect using an array of thermocouples.RTGs can be...

     produces heat through passive radioactive decay
    Radioactive decay
    Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...

    . Some radioisotope thermoelectric generators have been created to power space probes (for example, the Cassini
    Cassini-Huygens
    Cassini–Huygens is a joint NASA/ESA/ASI spacecraft mission studying the planet Saturn and its many natural satellites since 2004. Launched in 1997 after nearly two decades of gestation, it includes a Saturn orbiter and an atmospheric probe/lander for the moon Titan, although it has also returned...

     probe), some lighthouse
    Lighthouse
    A lighthouse is a tower, building, or other type of structure designed to emit light from a system of lamps and lenses or, in older times, from a fire, and used as an aid to navigation for maritime pilots at sea or on inland waterways....

    s in the former Soviet Union
    Soviet Union
    The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....

    , and some pacemakers. The heat output of these generators diminishes with time; the heat is converted to electricity utilising the thermoelectric effect
    Thermoelectric effect
    The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice-versa. A thermoelectric device creates a voltage when there is a different temperature on each side. Conversely, when a voltage is applied to it, it creates a temperature difference...

    .
  • Nuclear fission reactors produce heat through a controlled nuclear chain reaction
    Nuclear chain reaction
    A nuclear chain reaction occurs when one nuclear reaction causes an average of one or more nuclear reactions, thus leading to a self-propagating number of these reactions. The specific nuclear reaction may be the fission of heavy isotopes or the fusion of light isotopes...

     in a critical mass of fissile
    Fissile
    In nuclear engineering, a fissile material is one that is capable of sustaining a chain reaction of nuclear fission. By definition, fissile materials can sustain a chain reaction with neutrons of any energy. The predominant neutron energy may be typified by either slow neutrons or fast neutrons...

     material. All current nuclear power plant
    Nuclear power plant
    A nuclear power plant is a thermal power station in which the heat source is one or more nuclear reactors. As in a conventional thermal power station the heat is used to generate steam which drives a steam turbine connected to a generator which produces electricity.Nuclear power plants are usually...

    s are critical fission reactors, which are the focus of this article. The output of fission reactors is controllable. There are several subtypes of critical fission reactors, which can be classified as Generation I, Generation II
    Generation II reactor
    A generation II reactor is a design classification for a nuclear reactor, and refers to the class of commercial reactors built up to the end of the 1990s...

     and Generation III
    Generation III reactor
    A generation III reactor is a development of any of the generation II nuclear reactor designs incorporating evolutionary improvements in design developed during the lifetime of the generation II reactor designs...

    . All reactors will be compared to the Pressurized Water Reactor
    Pressurized water reactor
    Pressurized water reactors constitute a large majority of all western nuclear power plants and are one of three types of light water reactor , the other types being boiling water reactors and supercritical water reactors...

     (PWR), as that is the standard modern reactor design.
Pressurized Water Reactor
Pressurized water reactor
Pressurized water reactors constitute a large majority of all western nuclear power plants and are one of three types of light water reactor , the other types being boiling water reactors and supercritical water reactors...

s (PWR)
: These reactors use a pressure vessel to contain the nuclear fuel, control rods, moderator, and coolant. They are cooled and moderated by high pressure liquid water. The hot radioactive water that leaves the pressure vessel is looped through a steam generator, which in turn heats a secondary (non-radioactive) loop of water to steam that can run turbines. They are the majority of current reactors, and are generally considered the safest and most reliable technology currently in large scale deployment. This is a thermal neutron reactor design, the newest of which are the VVER-1200, Advanced Pressurized Water Reactor
Advanced Pressurized Water Reactor
Westinghouse Electric Company's AP1000 reactor design is the first Generation III+ reactor to receive final design approval from the U.S. Nuclear Regulatory Commission ....

 and the European Pressurized Reactor
European Pressurized Reactor
The EPR is a third generation pressurized water reactor design. It has been designed and developed mainly by Framatome , Electricité de France in France, and Siemens AG in Germany...

. United States Naval reactor
United States Naval reactor
United States Naval reactor refers to nuclear reactors used by the United States Navy aboard certain ships to produce power for propulsion, electric power, catapulting airplanes in aircraft carriers, and a few more minor uses. Such Naval nuclear reactors have a complete power plant associated with...

s are of this type

Boiling Water Reactor
Boiling water reactor
The boiling water reactor is a type of light water nuclear reactor used for the generation of electrical power. It is the second most common type of electricity-generating nuclear reactor after the pressurized water reactor , also a type of light water nuclear reactor...

s (BWR)
: A BWR is like a PWR without the steam generator. A boiling water reactor is cooled and moderated by water like a PWR, but at a lower pressure, which allows the water 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
Advanced Boiling Water Reactor
The Advanced Boiling Water Reactor is a Generation III boiling water reactor. The ABWR is currently offered by GE Hitachi Nuclear Energy and Toshiba...

 and the Economic Simplified Boiling Water Reactor
Economic Simplified Boiling Water Reactor
The type of nuclear reactor formally known as the Economic Simplified Boiling Water Reactor is a passively safe generation III+ reactor derived from the predecessor Simplified Boiling Water Reactor and the Advanced Boiling Water Reactor...


Pressurized Heavy Water Reactor (PHWR)
: A Canadian
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

 design (known as CANDU), these reactors are heavy-water
Heavy water
Heavy water is water highly enriched in the hydrogen isotope deuterium; e.g., heavy water used in CANDU reactors is 99.75% enriched by hydrogen atom-fraction...

-cooled and -moderated Pressurized-Water reactors. 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
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...

 and are thermal neutron reactor designs. PHWRs can be refueled while at full power, 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
Argentina
Argentina , officially the Argentine Republic , is the second largest country in South America by land area, after Brazil. It is constituted as a federation of 23 provinces and an autonomous city, Buenos Aires...

, China
People's Republic of China
China , officially the People's Republic of China , is the most populous country in the world, with over 1.3 billion citizens. Located in East Asia, the country covers approximately 9.6 million square kilometres...

, India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...

, Pakistan
Pakistan
Pakistan , officially the Islamic Republic of Pakistan is a sovereign state in South Asia. It has a coastline along the Arabian Sea and the Gulf of Oman in the south and is bordered by Afghanistan and Iran in the west, India in the east and China in the far northeast. In the north, Tajikistan...

, Romania
Romania
Romania is a country located at the crossroads of Central and Southeastern Europe, on the Lower Danube, within and outside the Carpathian arch, bordering on the Black Sea...

, and South Korea
South Korea
The Republic of Korea , , is a sovereign state in East Asia, located on the southern portion of the Korean Peninsula. It is neighbored by the People's Republic of China to the west, Japan to the east, North Korea to the north, and the East China Sea and Republic of China to the south...

. 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
Smiling Buddha
The Smiling Buddha, formally designated as Pokhran-I, was the codename given to Republic of India's first nuclear test explosion that took place at the long-constructed Indian Army base, Pokhran Test Range at Pokhran municipality, Rajasthan state on 18 May 1974 at 8:05 a.m....

 nuclear weapon test. :mage:Elektrownia Ignalina.jpg|thumb|The Ignalina Nuclear Power Plant
Ignalina Nuclear Power Plant
The Ignalina Nuclear Power Plant is a closed two-unit RBMK-1500 nuclear power station in Visaginas, Lithuania. It was named after the nearby city of Ignalina...

 — a RBMK type (closed 2009)]
Reaktor Bolshoy Moschnosti Kanalniy (High Power Channel Reactor) (RBMK
RBMK
RBMK is an initialism for the Russian reaktor bolshoy moshchnosti kanalniy which means "High Power Channel-type Reactor", and describes a class of graphite-moderated nuclear power reactor which was built in the Soviet Union. The RBMK reactor was the type involved in the Chernobyl disaster...

)
: A Soviet design, built to produce plutonium as well as power. RBMKs are water cooled with a graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...

 moderator. 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
Containment building
A containment building, in its most common usage, is a steel or reinforced concrete structure enclosing a nuclear reactor. It is designed, in any emergency, to contain the escape of radiation to a maximum pressure in the range of 60 to 200 psi...

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
Chernobyl disaster
The Chernobyl disaster was a nuclear accident that occurred on 26 April 1986 at the Chernobyl Nuclear Power Plant in Ukraine , which was under the direct jurisdiction of the central authorities in Moscow...

. Their main attraction is their use of light water and un-enriched uranium. As of 2010, 11 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
Soviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....



Gas Cooled Reactor (GCR) and Advanced Gas Cooled Reactor (AGR)
: These are generally graphite moderated and CO2
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 cooled. They can 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
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

, where the concept was developed. Older designs (i.e. Magnox
Magnox
Magnox is a now obsolete type of nuclear power reactor which was designed and is still in use in the United Kingdom, and was exported to other countries, both as a power plant, and, when operated accordingly, as a producer of plutonium for nuclear weapons...

 stations) are either shut down or will be in the near future. However, the AGCRs 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
Liquid Metal
Breeder reactor
A breeder reactor is a nuclear reactor capable of generating more fissile material than it consumes because its neutron economy is high enough to breed fissile from fertile material like uranium-238 or thorium-232. Breeders were at first considered superior because of their superior fuel economy...

 Fast Breeder Reactor (LMFBR)
: This is a reactor design that is cooled by liquid metal, totally unmoderated, and 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
Neutron capture is a kind of nuclear reaction in which an atomic nucleus collides with one or more neutrons and they merge to form a heavier nucleus. Since neutrons have no electric charge they can enter a nucleus more easily than positively charged protons, which are repelled...

. 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. BN-350 and BN-600 in USSR and Superphénix
Superphénix
Superphénix or SPX was a nuclear power station on the Rhône River at Creys-Malville in France, close to the border with Switzerland. A fast breeder reactor, it halted electricity production in 1996 and was closed as a commercial plant in 1997....

 in France were a reactor of this type, as was Fermi-I
Enrico Fermi Nuclear Generating Station
The Enrico Fermi Nuclear Generating Station is a nuclear power plant on the shore of Lake Erie near Monroe, in Frenchtown Charter Township, Michigan. It is approximately halfway between Detroit, Michigan, and Toledo, Ohio. It is also visible from parts of Amherstburg, Ontario. Two units have been...

 in the United States. The Monju reactor in Japan suffered a sodium leak in 1995 and was restarted in May 2010. All of them use/used liquid sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...

. These reactors are fast neutron, not thermal neutron designs. These reactors come in two types

Lead cooled
Lead cooled fast reactor
The lead-cooled fast reactor is a nuclear power Generation IV reactor that features a fast neutron spectrum, molten lead or lead-bismuth eutectic coolant. Options include a range of plant ratings, including a number of 50 to 150 MWe units featuring long-life, pre-manufactured cores...

Using lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...

 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
Lead-bismuth eutectic
Lead-Bismuth Eutectic or LBE is a eutectic alloy of lead and bismuth used as a coolant in some nuclear reactors, and is a proposed coolant for the lead-cooled fast reactor, part of the Generation IV reactor initiative....

 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
Alfa class submarine
The Soviet Union/Russian Navy Project 705 was a class of hunter/killer nuclear powered submarines. The class is also known by the NATO reporting name of Alfa...

 uses a lead-bismuth-cooled fast reactor as its main power plant.
Sodium cooled
Sodium-cooled fast reactor
The sodium-cooled fast reactor or SFR is a Generation IV reactor project to design an advanced fast neutron reactor.It builds on two closely related existing projects, the LMFBR and the Integral Fast Reactor, with the objective of producing a fast-spectrum, sodium-cooled reactor.The reactors are...

Most LMFBRs are 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 vastly more violent than (for example) a leak of superheated fluid from a SCWR
Supercritical water reactor
The supercritical water reactor is a Generation IV reactor concept that uses supercritical water as the working fluid...

 or PWR. EBR-I, the first reactor to have a core meltdown, was of this type.
Pebble Bed Reactor
Pebble bed reactor
The pebble bed reactor is a graphite-moderated, gas-cooled, nuclear reactor. It is a type of very high temperature reactor , one of the six classes of nuclear reactors in the Generation IV initiative...

s (PBR)
: 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
AVR reactor
The AVR reactor was a prototype pebble bed reactor at Jülich Research Centre in West Germany. Construction began in 1960, first grid connection was in 1967 and operation ceased in 1988....

Molten Salt Reactor
Molten salt reactor
A molten salt reactor is a type of nuclear fission reactor in which the primary coolant, or even the fuel itself is a molten salt mixture...

s
:These dissolve the fuels in fluoride
Fluoride
Fluoride is the anion F−, the reduced form of fluorine when as an ion and when bonded to another element. Both organofluorine compounds and inorganic fluorine containing compounds are called fluorides. Fluoride, like other halides, is a monovalent ion . Its compounds often have properties that are...

 salts, or use fluoride salts for coolant. These have many safety features, high efficiency and a high power density suitable for vehicles. Notably, they have no high pressures or flammable components in the core. The prototype was the MSRE
Molten-Salt Reactor Experiment
The Molten-Salt Reactor Experiment was an experimental molten-salt reactor at the Oak Ridge National Laboratory researching this technology through the 1960s; constructed by 1964, it went critical in 1965 and was operated until 1969....

, which also used Thorium's fuel cycle to produce 0.1% of the radioactive waste of standard reactors
Aqueous Homogeneous Reactor
Aqueous homogeneous reactor
Aqueous homogeneous reactors are a type of nuclear reactor in which soluble nuclear salts have been dissolved in water. The fuel is mixed with the coolant and the moderator, thus the name "homogeneous" The water can be either heavy water or light water, both which need to be very pure...

 (AHR)
: These reactors use soluble nuclear salts dissolved in water and mixed with a coolant and a neutron moderator
Neutron moderator
In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....


Advanced reactors


More than a dozen advanced reactor designs are in various stages of development. Some are evolutionary from the PWR
Pressurized water reactor
Pressurized water reactors constitute a large majority of all western nuclear power plants and are one of three types of light water reactor , the other types being boiling water reactors and supercritical water reactors...

, BWR
Boiling water reactor
The boiling water reactor is a type of light water nuclear reactor used for the generation of electrical power. It is the second most common type of electricity-generating nuclear reactor after the pressurized water reactor , also a type of light water nuclear reactor...

 and PHWR designs above, some are more radical departures. The former include the Advanced Boiling Water Reactor
Advanced Boiling Water Reactor
The Advanced Boiling Water Reactor is a Generation III boiling water reactor. The ABWR is currently offered by GE Hitachi Nuclear Energy and Toshiba...

 (ABWR), two of which are now operating with others under construction, and the planned passively safe ESBWR and AP1000 units (see Nuclear Power 2010 Program
Nuclear Power 2010 Program
The "Nuclear Power 2010 Program" was unveiled by the U.S. Secretary of Energy Spencer Abraham on February 14, 2002 as one means towards addressing the expected need for new power plants...

).
  • The Integral Fast Reactor
    Integral Fast Reactor
    The Integral Fast Reactor is a design for a nuclear reactor using fast neutrons and no neutron moderator . IFR is distinguished by a nuclear fuel cycle that uses reprocessing via electrorefining at the reactor site.The U.S...

     (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
    Pebble bed reactor
    The pebble bed reactor is a graphite-moderated, gas-cooled, nuclear reactor. It is a type of very high temperature reactor , one of the six classes of nuclear reactors in the Generation IV initiative...

    , a High Temperature Gas Cooled Reactor (HTGCR), is designed so high temperatures reduce power output by doppler broadening
    Doppler broadening
    In atomic physics, Doppler broadening is the broadening of spectral lines due to the Doppler effect caused by a distribution of velocities of atoms or molecules. Different velocities of the emitting particles result in different shifts, the cumulative effect of which is the line broadening.The...

     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
    SSTAR is an acronym for the "small, sealed, transportable, autonomous reactor" - being primarily researched and developed in the USA by Lawrence Livermore National Laboratory. It is designed as a fast breeder nuclear reactor that is passively safe...

     (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
    Clean And Environmentally Safe Advanced Reactor
    The Clean And Environmentally Safe Advanced Reactor is a nuclear reactor concept that uses steam as a moderator. Steam's density can be controlled very finely so, according to its developer Dr...

     (CAESAR) is a nuclear reactor concept that uses steam as a moderator — this design is still in development.
  • The Hydrogen Moderated Self-regulating Nuclear Power Module
    Hydrogen Moderated Self-regulating Nuclear Power Module
    The Hydrogen Moderated Self-Regulating Nuclear Power Module , also referred to as the Compact Self-regulating Transportable Reactor , is a new type of nuclear power reactor using hydride as a neutron moderator. The design is inherently safe, as the fuel and the neutron moderator are uranium hydride...

     (HPM) is a reactor design emanating from the Los Alamos National Laboratory
    Los Alamos National Laboratory
    Los Alamos National Laboratory is a United States Department of Energy national laboratory, managed and operated by Los Alamos National Security , located in Los Alamos, New Mexico...

     that uses uranium hydride
    Uranium hydride
    Uranium hydride, also called uranium trihydride is an inorganic compound, a hydride of uranium.-Properties:Uranium hydride is a highly toxic, brownish gray to brownish black pyrophoric powder or brittle solid. Its specific gravity at 20 °C is 10.95, much lower than that of uranium...

     as fuel.
  • Subcritical reactor
    Subcritical reactor
    A subcritical reactor is a nuclear fission reactor that produces fission without achieving criticality. Instead of a sustaining chain reaction, a subcritical reactor uses additional neutrons from an outside source...

    s are designed to be safer and more stable, but pose a number of engineering and economic difficulties. One example is the Energy amplifier
    Energy amplifier
    In nuclear physics, an energy amplifier is a novel type of nuclear power reactor, a subcritical reactor, in which an energetic particle beam is used to stimulate a reaction, which in turn releases enough energy to power the particle accelerator and leave an energy profit for power generation...

    .
  • 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 more plentiful 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
      Advanced Heavy Water Reactor
      The Advanced Heavy Water Reactor is the latest Indian design for a next generation nuclear reactor that will burn thorium in its fuel core. It is slated to form the third stage in India's 3 stage fuel cycle plan. Thorium is an element that is 3 times more abundant globally than uranium...

       (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
      Bhabha Atomic Research Centre
      The Bhabha Atomic Research Centre is India's primary nuclear research facility based in Mumbai. It has a number of nuclear reactors, all of which are used for India's nuclear power and research programme.- History :...

       (BARC), India.
    • KAMINI
      KAMINI
      KAMINI is a research reactor at Indira Gandhi Center for Atomic Research in Kalpakkam, India. Its first criticality was on October 29, 1996...

       — A unique reactor using Uranium-233 isotope for fuel. Built in India by BARC
      Bhabha Atomic Research Centre
      The Bhabha Atomic Research Centre is India's primary nuclear research facility based in Mumbai. It has a number of nuclear reactors, all of which are used for India's nuclear power and research programme.- History :...

       and Indira Gandhi Center for Atomic Research (IGCAR
      IGCAR
      The Reactor Research Centre set up at Kalpakkam, India, 80 km south of Chennai in 1971 under the Department of Atomic Energy was renamed Indira Gandhi Center for Atomic Research in 1985. The center is primarily a research facility besides being a nuclear power plant aiding in meeting a...

      ).
    • 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.

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
    Lead cooled fast reactor
    The lead-cooled fast reactor is a nuclear power Generation IV reactor that features a fast neutron spectrum, molten lead or lead-bismuth eutectic coolant. Options include a range of plant ratings, including a number of 50 to 150 MWe units featuring long-life, pre-manufactured cores...

  • Molten salt reactor
    Molten salt reactor
    A molten salt reactor is a type of nuclear fission reactor in which the primary coolant, or even the fuel itself is a molten salt mixture...

  • Sodium-cooled fast reactor
    Sodium-cooled fast reactor
    The sodium-cooled fast reactor or SFR is a Generation IV reactor project to design an advanced fast neutron reactor.It builds on two closely related existing projects, the LMFBR and the Integral Fast Reactor, with the objective of producing a fast-spectrum, sodium-cooled reactor.The reactors are...

  • Supercritical water reactor
    Supercritical water reactor
    The supercritical water reactor is a Generation IV reactor concept that uses supercritical water as the working fluid...

  • Very high temperature reactor
    Very high temperature reactor
    The Very High Temperature Reactor , or High Temperature Gas-cooled Reactor , is a Generation IV reactor concept that uses a graphite-moderated nuclear reactor with a once-through uranium fuel cycle. The VHTR is a type of High Temperature Reactor that can conceptually have an outlet temperature of...


Generation V+ reactors


Generation V reactors are designs which are theoretically possible, but which are not being actively considered or researched at present. Though such reactors could be built with current or near term technology, they trigger little interest for reasons of economics, practicality, or safety.
  • Liquid Core reactor. A closed loop liquid core nuclear reactor, where the fissile material is molten uranium cooled by a working gas pumped in through holes in the base of the containment vessel.
  • Gas core reactor. A closed loop version of the nuclear lightbulb rocket
    Nuclear lightbulb
    A nuclear lightbulb is a hypothetical type of spacecraft engine using a Fission reactor to achieve Nuclear propulsion. Specifically it would be a type of Gas core reactor rocket that separates the nuclear fuel from the coolant/propellant with a quartz wall. It would be operated at such high...

    , 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. 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
    Neutron flux
    The neutron flux is a quantity used in reactor physics corresponding to the total length travelled by all neutrons per unit time and volume . The neutron fluence is defined as the neutron flux integrated over a certain time period....

    .
  • Gas core EM reactor. As in the Gas Core reactor, but with photovoltaic arrays converting the UV light directly to electricity.
  • Fission fragment reactor
    Fission fragment reactor
    Similar to how the fission-fragment rocket produces thrust, 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...


Fusion reactors


Controlled nuclear fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...

 could in principle be used in fusion power
Fusion power
Fusion power is the power generated by nuclear fusion processes. In fusion reactions two light atomic nuclei fuse together to form a heavier nucleus . In doing so they release a comparatively large amount of energy arising from the binding energy due to the strong nuclear force which is manifested...

 plants to produce power without the complexities of handling actinides, but significant scientific and technical obstacles remain. Several fusion reactors have been built, but as yet none has 'produced' more thermal energy than electrical energy consumed. Despite research having started in the 1950s, no commercial fusion reactor is expected before 2050. The ITER
ITER
ITER is an international nuclear fusion research and engineering project, which is currently building the world's largest and most advanced experimental tokamak nuclear fusion reactor at Cadarache in the south of France...

 project is currently leading the effort to commercialize fusion power.

Nuclear fuel cycle



Thermal reactors generally depend on refined and enriched uranium
Enriched uranium
Enriched uranium is a kind of uranium in which the percent composition of uranium-235 has been increased through the process of isotope separation. Natural uranium is 99.284% 238U isotope, with 235U only constituting about 0.711% of its weight...

. Some nuclear reactors can operate with a mixture of plutonium and uranium (see MOX
Mox
MOX might be a name or acronym for:*Malaysian Oxygen Berhad - A Malaysian company that is specializes in providing total gas solutions.*Mixed Oxide Fuel, from nuclear reprocessing*An alien race in the TimeSplitters 2 video game, the Mox...

). The process by which uranium ore is mined, processed, enriched, used, possibly reprocessed
Nuclear reprocessing
Nuclear reprocessing technology was developed to chemically separate and recover fissionable plutonium from irradiated nuclear fuel. Reprocessing serves multiple purposes, whose relative importance has changed over time. Originally reprocessing was used solely to extract plutonium for producing...

 and disposed of is known as the nuclear fuel cycle
Nuclear fuel cycle
The nuclear fuel cycle, also called nuclear fuel chain, is the progression of nuclear fuel through a series of differing stages. It consists of steps in the front end, which are the preparation of the fuel, steps in the service period in which the fuel is used during reactor operation, and steps in...

.

Under 1% of the uranium found in nature is the easily fissionable U-235 isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...

 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
Gaseous diffusion
Gaseous diffusion is a technology used to produce enriched uranium by forcing gaseous uranium hexafluoride through semi-permeable membranes. This produces a slight separation between the molecules containing uranium-235 and uranium-238 . By use of a large cascade of many stages, high separations...

 or gas centrifuge
Gas centrifuge
A gas centrifuge is a device that performs isotope separation of gases. A centrifuge relies on the principles of centripetal force accelerating molecules so that particles of different masses are physically separated in a gradient along the radius of a rotating container.A prominent use of gas...

. The enriched result is then converted into uranium dioxide
Uranium dioxide
Uranium dioxide or uranium oxide , also known as urania or uranous oxide, is an oxide of uranium, and is a black, radioactive, crystalline powder that naturally occurs in the mineral uraninite. It is used in nuclear fuel rods in nuclear reactors. A mixture of uranium and plutonium dioxides is used...

 powder, which is pressed and fired into pellet form. These pellets are stacked into tubes which are then sealed and called fuel rod
Nuclear fuel
Nuclear fuel is a material that can be 'consumed' by fission or fusion to derive nuclear energy. Nuclear fuels are the most dense sources of energy available...

s. 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
Neutron economy
Neutron economy is defined as the ratio of an adjoint weighted average of the excess neutron production divided by an adjoint weighted average of the fission production....

 do not require the fuel to be enriched at all (that is, they can use natural uranium). According to the International Atomic Energy Agency
International Atomic Energy Agency
The International Atomic Energy Agency is an international organization that seeks to promote the peaceful use of nuclear energy, and to inhibit its use for any military purpose, including nuclear weapons. The IAEA was established as an autonomous organization on 29 July 1957...

 there are at least 100 research reactor
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 for electricity production, heat generation, or maritime propulsion.-Purpose:...

s 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
Fissile
In nuclear engineering, a fissile material is one that is capable of sustaining a chain reaction of nuclear fission. By definition, fissile materials can sustain a chain reaction with neutrons of any energy. The predominant neutron energy may be typified by either slow neutrons or fast neutrons...

 U-235 and non-fissile but fissionable and fertile
Fertile material
Fertile material is a term used to describe nuclides which generally themselves do not undergo induced fission but from which fissile material is generated by neutron absorption and subsequent nuclei conversions...

 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
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

, 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.

Fueling of nuclear reactors


The amount of energy in the reservoir of nuclear fuel
Nuclear fuel
Nuclear fuel is a material that can be 'consumed' by fission or fusion to derive nuclear energy. Nuclear fuels are the most dense sources of energy available...

 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
Fissile
In nuclear engineering, a fissile material is one that is capable of sustaining a chain reaction of nuclear fission. By definition, fissile materials can sustain a chain reaction with neutrons of any energy. The predominant neutron energy may be typified by either slow neutrons or fast neutrons...

 uranium-235
Uranium-235
- References :* .* DOE Fundamentals handbook: Nuclear Physics and Reactor theory , .* A piece of U-235 the size of a grain of rice can produce energy equal to that contained in three tons of coal or fourteen barrels of oil. -External links:* * * one of the earliest articles on U-235 for the...

 (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" and is discharged and replaced with new (fresh) fuel assemblies, although in practice it is the buildup of reaction poisons in nuclear fuel that determines the lifetime of nuclear fuel in a reactor. Long before all possible fission has taken place, the buildup of long-lived neutron absorbing fission byproducts impedes the chain reaction. The fraction of the reactor's fuel core replaced during refueling is typically one-fourth for a boiling-water reactor and one-third for a pressurized-water reactor. 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.

Not all reactors need to be shut down for refueling; for example, pebble bed reactor
Pebble bed reactor
The pebble bed reactor is a graphite-moderated, gas-cooled, nuclear reactor. It is a type of very high temperature reactor , one of the six classes of nuclear reactors in the Generation IV initiative...

s, RBMK reactors
RBMK
RBMK is an initialism for the Russian reaktor bolshoy moshchnosti kanalniy which means "High Power Channel-type Reactor", and describes a class of graphite-moderated nuclear power reactor which was built in the Soviet Union. The RBMK reactor was the type involved in the Chernobyl disaster...

, molten salt reactor
Molten salt reactor
A molten salt reactor is a type of nuclear fission reactor in which the primary coolant, or even the fuel itself is a molten salt mixture...

s, Magnox
Magnox
Magnox is a now obsolete type of nuclear power reactor which was designed and is still in use in the United Kingdom, and was exported to other countries, both as a power plant, and, when operated accordingly, as a producer of plutonium for nuclear weapons...

, AGR
Advanced gas-cooled reactor
An advanced gas-cooled reactor is a type of nuclear reactor. These are the second generation of British gas-cooled reactors, using graphite as the neutron moderator and carbon dioxide as coolant...

 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
Burnup
In nuclear power technology, burnup is a measure of how much energy is extracted from a primary nuclear fuel source...

, 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.

Safety



Nuclear safety covers the actions taken to prevent nuclear and radiation accidents
Nuclear and radiation accidents
A nuclear and radiation accident is defined by the International Atomic Energy Agency as "an event that has led to significant consequences to people, the environment or the facility...

 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 Fukushima
Timeline of the Fukushima nuclear accidents
For the timelines of the nuclear accidents at Fukushima Daiichi and Fukushima Daini after the 2011 Tohoku earthquake and tsunami, see:* Timeline of the Fukushima I nuclear accidents* Timeline of the Fukushima II nuclear accidents...

 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. 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 have estimated that given the expected growth of nuclear power from 2005 – 2055, at least four serious nuclear accidents would be expected in that period.

Accidents


Some serious nuclear and radiation accidents
Nuclear and radiation accidents by death toll
There have been more than 20 nuclear and radiation accidents involving fatalities. These involved nuclear power plant accidents, nuclear submarine accidents, radiotherapy accidents, and other mishaps.-Chernobyl disaster:...

 have occurred. Nuclear power plant
Nuclear power plant
A nuclear power plant is a thermal power station in which the heat source is one or more nuclear reactors. As in a conventional thermal power station the heat is used to generate steam which drives a steam turbine connected to a generator which produces electricity.Nuclear power plants are usually...

 accidents include the Chernobyl disaster
Chernobyl disaster
The Chernobyl disaster was a nuclear accident that occurred on 26 April 1986 at the Chernobyl Nuclear Power Plant in Ukraine , which was under the direct jurisdiction of the central authorities in Moscow...

 (1986), Fukushima Daiichi nuclear disaster
Fukushima Daiichi nuclear disaster
The is a series of equipment failures, nuclear meltdowns, and releases of radioactive materials at the Fukushima I Nuclear Power Plant, following the Tōhoku earthquake and tsunami on 11 March 2011. The plant comprises six separate boiling water reactors originally designed by General Electric ,...

 (2011), the Three Mile Island accident
Three Mile Island accident
The Three Mile Island accident was a core meltdown in Unit 2 of the Three Mile Island Nuclear Generating Station in Dauphin County, Pennsylvania near Harrisburg, United States in 1979....

 (1979), and SL-1
SL-1
The SL-1, or Stationary Low-Power Reactor Number One, was a United States Army experimental nuclear power reactor which underwent a steam explosion and meltdown on January 3, 1961, killing its three operators. The direct cause was the improper withdrawal of the central control rod, responsible for...

 accident (1961). Nuclear-powered submarine mishaps include the K-19
Soviet submarine K-19
K-19, KS-19, BS_19 was one of the first two Soviet submarines of the 658, 658м, 658с class , the first generation nuclear submarine equipped with nuclear ballistic missiles, specifically the R-13 . Its keel was laid down on 17 October 1958, christened on 8 April 1959 and launched on 11 October 1959...

 reactor accident (1961), the K-27
Soviet submarine K-27
The K-27 was the only submarine of Projekt 645 in the Soviet Navy. Project 645 did not have or need its own NATO reporting name. That project produced just one test model nuclear submarine, one which incorporated a pair of experimental VT-1 nuclear reactors that used a liquid-metal coolant ,...

 reactor accident (1968), and the K-431
Soviet submarine K-431
The Soviet submarine K-431 was a Soviet nuclear-powered submarine that had a reactor accident on August 10, 1985. An explosion occurred during refueling of the submarine at Chazhma Bay, Vladivostok...

 reactor accident (1985).

Natural nuclear reactors



Although nuclear fission reactors are often thought of as being solely a product of modern technology, the first nuclear fission reactors were in fact naturally occurring. A natural nuclear fission reactor
Natural nuclear fission reactor
A natural nuclear fission reactor is a uranium deposit where analysis of isotope ratios has shown that self-sustaining nuclear chain reactions have occurred. The existence of this phenomenon was discovered in 1972 at Oklo in Gabon, Africa, by French physicist Francis Perrin. The conditions under...

 can occur under certain circumstances that mimic the conditions in a constructed reactor. Fifteen natural fission reactors have so far been found in three separate ore deposits at the Oklo
Oklo
Oklo is a region near the town of Franceville, in the Haut-Ogooué province of the Central African state of Gabon. Several natural nuclear fission reactors were discovered in the uranium mines in the region in 1972.-History:...

 mine in Gabon
Gabon
Gabon , officially the Gabonese Republic is a state in west central Africa sharing borders with Equatorial Guinea to the northwest, Cameroon to the north, and with the Republic of the Congo curving around the east and south. The Gulf of Guinea, an arm of the Atlantic Ocean is to the west...

, West Africa
West Africa
West Africa or Western Africa is the westernmost region of the African continent. Geopolitically, the UN definition of Western Africa includes the following 16 countries and an area of approximately 5 million square km:-Flags of West Africa:...

. First discovered in 1972 by French physicist Francis Perrin
Francis Perrin
Francis Perrin was a French physicist,the son of Nobel prize-winning physicist Jean Perrin.- Physicist :Francis Perrin was born in Paris and attended École Normale Supérieure in Paris.In 1928 he obtained a doctorate in mathematical sciences from the faculté des sciences of Paris, based upon a...

, they are collectively known as the Oklo Fossil Reactors
Natural nuclear fission reactor
A natural nuclear fission reactor is a uranium deposit where analysis of isotope ratios has shown that self-sustaining nuclear chain reactions have occurred. The existence of this phenomenon was discovered in 1972 at Oklo in Gabon, Africa, by French physicist Francis Perrin. The conditions under...

. Self-sustaining nuclear fission
Nuclear fission
In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts , often producing free neutrons and photons , and releasing a tremendous amount of energy...

 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
University of Arkansas
The University of Arkansas is a public, co-educational, land-grant, space-grant, research university. It is classified by the Carnegie Foundation as a research university with very high research activity. It is the flagship campus of the University of Arkansas System and is located in...

.

Such reactors can no longer form on Earth: radioactive decay over this immense time span has reduced the proportion of U-235 in naturally occurring uranium to below the amount required to sustain a chain reaction.

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.

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.

See also



  • Advanced Test Reactor
    Advanced Test Reactor
    The Advanced Test Reactor is a research reactor at the Idaho National Laboratory, located east of Arco, Idaho. This reactor is primarily designed and used to test materials to be used in other, larger-scale and prototype reactors. It can operate at a maximum power of 250 MW and has a "Four...

     at Idaho National Laboratory
    Idaho National Laboratory
    Idaho National Laboratory is an complex located in the high desert of eastern Idaho, between the town of Arco to the west and the cities of Idaho Falls and Blackfoot to the east. It lies within Butte, Bingham, Bonneville and Jefferson counties...

  • Auxiliary feedwater
    Auxiliary feedwater
    Auxiliary feedwater is a backup water supply system found in pressurized water reactor nuclear power plants. This system, sometimes known as Emergency feedwater, can be used during shutdowns, including accident conditions, and sometimes during startup. It works by pumping water to the steam...

  • Containment building
    Containment building
    A containment building, in its most common usage, is a steel or reinforced concrete structure enclosing a nuclear reactor. It is designed, in any emergency, to contain the escape of radiation to a maximum pressure in the range of 60 to 200 psi...

  • Energy development
    Energy development
    Energy development is the effort to provide sufficient primary energy sources and secondary energy forms for supply, cost, impact on air pollution and water pollution, mitigation of climate change with renewable energy....

  • List of nuclear reactors
  • List of United States Naval reactors
  • Lists of nuclear disasters and radioactive incidents
  • List of small nuclear reactor designs
  • Nuclear marine propulsion
    Nuclear marine propulsion
    Nuclear marine propulsion is propulsion of a ship by a nuclear reactor. Naval nuclear propulsion is propulsion that specifically refers to naval warships...

  • Nuclear physics
    Nuclear physics
    Nuclear physics is the field of physics that studies the building blocks and interactions of atomic nuclei. The most commonly known applications of nuclear physics are nuclear power generation and nuclear weapons technology, but the research has provided application in many fields, including those...

  • Nuclear power by country
    Nuclear power by country
    Thirty countries operate nuclear power stations, and there are a considerable number of new reactors being built in China, South Korea, India, Pakistan, and Russia...

  • Nuclear Reactor Operator Badge
    Nuclear Reactor Operator Badge
    The Nuclear Reactor Operator Badge is a decoration of the United States Army which was issued between the years of 1965 and 1990. In 1991, the decoration was declared obsolete by Army Regulation 672-5-1, but uniform regulations permit the continued wearing of badges awarded before then...

  • Nuclear reactor physics
    Nuclear reactor physics
    Nuclear reactor physics is the branch of science that deals with the study and application of chain reaction to induce controlled rate of fission for energy in reactors....

  • SCRAM
    Scram
    A scram or SCRAM is an emergency shutdown of a nuclear reactor – though the term has been extended to cover shutdowns of other complex operations, such as server farms and even large model railroads...

  • Safety engineering
    Safety engineering
    Safety engineering is an applied science strongly related to systems engineering / industrial engineering and the subset System Safety Engineering...

  • Technology assessment
    Technology assessment
    Technology assessment Technology assessment Technology assessment (TA, German Tenteractive, and communicative process that aims to contribute to the formation of public and political opinion on societal aspects of science and technology.- General description :...

  • Enrico Fermi
    Enrico Fermi
    Enrico Fermi was an Italian-born, naturalized American physicist particularly known for his work on the development of the first nuclear reactor, Chicago Pile-1, and for his contributions to the development of quantum theory, nuclear and particle physics, and statistical mechanics...

  • World Nuclear Industry Status Report
    World Nuclear Industry Status Report
    The World Nuclear Industry Status Report is a yearly report that explores the global challenges facing the nuclear power industry. The reports show that the share of nuclear-generated electricity in the overall global energy production has decreased in the 2000s...


External links