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A nuclear reactor , formerly known as an atomic pile so-called because the graphite moderator of the first reactor was placed into a tall pile [1] , is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. Heat from nuclear fission is passed to a working fluid water or gas , which in turn runs through steam turbines.
These either drive a ship's propellers or turn electrical generators ' shafts. Nuclear generated steam in principle can be used for industrial process heat or for district heating. Some reactors are used to produce isotopes for medical and industrial use, or for production of weapons-grade plutonium. As of early , the IAEA reports there are nuclear power reactors and nuclear research reactors in operation around the world. Just as conventional thermal power stations generate electricity by harnessing the thermal energy released from burning fossil fuels , nuclear reactors convert the energy released by controlled nuclear fission into thermal energy for further conversion to mechanical or electrical forms.
When a large fissile atomic nucleus such as uranium or plutonium absorbs a neutron, it may undergo nuclear fission. The heavy nucleus splits into two or more lighter nuclei, the fission products , releasing kinetic energy , gamma radiation , and free neutrons.
A portion of these neutrons may be absorbed by other fissile atoms and trigger further fission events, which release more neutrons, and so on. This is known as a nuclear chain reaction.
To control such a nuclear chain reaction, control rods containing neutron poisons and neutron moderators can change the portion of neutrons that will go on to cause more fission. A kilogram of uranium U converted via nuclear processes releases approximately three million times more energy than a kilogram of coal burned conventionally 7.
The fission of one kilogram of uranium releases about 19 billion kilocalories , so the energy released by 1 kg of uranium corresponds to that released by burning 2. A nuclear reactor coolant — usually water but sometimes a gas or a liquid metal like liquid sodium or lead or molten salt — is circulated past the reactor core to absorb the heat that it generates.
The heat is carried away from the reactor and is then used to generate steam. Most reactor systems employ a cooling system that is physically separated from the water that will be boiled to produce pressurized steam for the turbines , like the pressurized water reactor.
However, in some reactors the water for the steam turbines is boiled directly by the reactor core ; for example the boiling water reactor. The rate of fission reactions within a reactor core can be adjusted by controlling the quantity of neutrons that are able to induce further fission events. Nuclear reactors typically employ several methods of neutron control to adjust the reactor's power output. Some of these methods arise naturally from the physics of radioactive decay and are simply accounted for during the reactor's operation, while others are mechanisms engineered into the reactor design for a distinct purpose.
The fastest method for adjusting levels of fission-inducing neutrons in a reactor is via movement of the control rods. Control rods are made of neutron poisons and therefore absorb neutrons. When a control rod is inserted deeper into the reactor, it absorbs more neutrons than the material it displaces — often the moderator.
This action results in fewer neutrons available to cause fission and reduces the reactor's power output. Conversely, extracting the control rod will result in an increase in the rate of fission events and an increase in power. The physics of radioactive decay also affects neutron populations in a reactor. One such process is delayed neutron emission by a number of neutron-rich fission isotopes.
These delayed neutrons account for about 0. The fission products which produce delayed neutrons have half-lives for their decay by neutron emission that range from milliseconds to as long as several minutes, and so considerable time is required to determine exactly when a reactor reaches the critical point. Keeping the reactor in the zone of chain reactivity where delayed neutrons are necessary to achieve a critical mass state allows mechanical devices or human operators to control a chain reaction in "real time"; otherwise the time between achievement of criticality and nuclear meltdown as a result of an exponential power surge from the normal nuclear chain reaction, would be too short to allow for intervention.
This last stage, where delayed neutrons are no longer required to maintain criticality, is known as the prompt critical point. There is a scale for describing criticality in numerical form, in which bare criticality is known as zero dollars and the prompt critical point is one dollar , and other points in the process interpolated in cents.
In some reactors, the coolant also acts as a neutron moderator. A moderator increases the power of the reactor by causing the fast neutrons that are released from fission to lose energy and become thermal neutrons. Thermal neutrons are more likely than fast neutrons to cause fission. A higher temperature coolant would be less dense, and therefore a less effective moderator. In other reactors the coolant acts as a poison by absorbing neutrons in the same way that the control rods do.
In these reactors power output can be increased by heating the coolant, which makes it a less dense poison. Nuclear reactors generally have automatic and manual systems to scram the reactor in an emergency shut down. These systems insert large amounts of poison often boron in the form of boric acid into the reactor to shut the fission reaction down if unsafe conditions are detected or anticipated. Most types of reactors are sensitive to a process variously known as xenon poisoning, or the iodine pit.
The common fission product Xenon produced in the fission process acts as a neutron poison that absorbs neutrons and therefore tends to shut the reactor down. Xenon accumulation can be controlled by keeping power levels high enough to destroy it by neutron absorption as fast as it is produced.
Fission also produces iodine , which in turn decays with a half-life of 6. When the reactor is shut down, iodine continues to decay to xenon, making restarting the reactor more difficult for a day or two, as the xenon decays into cesium, which is not nearly as poisonous as xenon, with a half-life of 9.
This temporary state is the "iodine pit. As the extra xenon is transmuted to xenon, which is much less a neutron poison, within a few hours the reactor experiences a "xenon burnoff power transient". Control rods must be further inserted to replace the neutron absorption of the lost xenon Failure to properly follow such a procedure was a key step in the Chernobyl disaster.
Reactors used in nuclear marine propulsion especially nuclear submarines often cannot be run at continuous power around the clock in the same way that land-based power reactors are normally run, and in addition often need to have a very long core life without refueling.
For this reason many designs use highly enriched uranium but incorporate burnable neutron poison in the fuel rods. The energy released in the fission process generates heat, some of which can be converted into usable energy. A common method of harnessing this thermal energy is to use it to boil water to produce pressurized steam which will then drive a steam turbine that turns an alternator and generates electricity. The neutron was discovered in by British physicist James Chadwick.
He filed a patent for his idea of a simple reactor the following year while working at the Admiralty in London. Inspiration for a new type of reactor using uranium came from the discovery by Lise Meitner , Fritz Strassmann and Otto Hahn in that bombardment of uranium with neutrons provided by an alpha-on-beryllium fusion reaction, a " neutron howitzer " produced a barium residue, which they reasoned was created by the fissioning of the uranium nuclei.
The U. The following year the U. Government received the Frisch—Peierls memorandum from the UK, which stated that the amount of uranium needed for a chain reaction was far lower than had previously been thought.
Eventually, the first artificial nuclear reactor, Chicago Pile-1 , was constructed at the University of Chicago , by a team led by Italian physicist Enrico Fermi , in late By this time, the program had been pressured for a year by U.
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. The primary purpose for the largest reactors located at the Hanford Site in Washington , was the mass production of plutonium for nuclear weapons. Fermi and Szilard applied for a patent on reactors on 19 December Its issuance was delayed for 10 years because of wartime secrecy.
Atomic Energy Commission produced 0. Besides the military uses of nuclear reactors, there were political reasons to pursue civilian use of atomic energy. This diplomacy led to the dissemination of reactor technology to U. It produced around 5 MW electrical. It was built after the F-1 nuclear reactor which was the first reactor to go critical in Europe, and was also built by the Soviet Union.
All commercial power reactors are based on nuclear fission. They generally use uranium and its product plutonium as nuclear fuel , though a thorium fuel cycle is also possible. Fission reactors can be divided roughly into two classes, depending on the energy of the neutrons that sustain the fission chain reaction :. In principle, fusion power could be produced by nuclear fusion of elements such as the deuterium isotope of hydrogen.
While an ongoing rich research topic since at least the s, no self-sustaining fusion reactor for any purpose has ever been built. More than a dozen advanced reactor designs are in various stages of development. Rolls-Royce aims to sell nuclear reactors for the production of synfuel for aircraft. 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 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.
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. Generation V reactors are designs which are theoretically possible, but which are not being actively considered or researched at present.
Though some generation V reactors could potentially be built with current or near term technology, they trigger little interest for reasons of economics, practicality, or safety. Controlled nuclear fusion could in principle be used in fusion power plants to produce power without the complexities of handling actinides , but significant scientific and technical obstacles remain.
Several fusion reactors have been built, but reactors have never been able to release more energy than the amount of energy used in the process.
Despite research having started in the s, no commercial fusion reactor is expected before The ITER project is currently leading the effort to harness fusion power.
Thermal reactors generally depend on refined and enriched uranium. Some nuclear reactors can operate with a mixture of plutonium and uranium see MOX. The process by which uranium ore is mined, processed, enriched, used, possibly reprocessed and disposed of is known as the nuclear fuel cycle. Enrichment involves increasing the percentage of U and is usually done by means of gaseous diffusion or gas centrifuge. The enriched result is then converted into uranium dioxide powder, which is pressed and fired into pellet form.
These pellets are stacked into tubes which are then sealed and called fuel rods. Many of these fuel rods are used in each nuclear reactor. Theft risk of this fuel potentially used in the production of a nuclear weapon has led to campaigns advocating conversion of this type of reactor to low-enrichment uranium which poses less threat of proliferation. Fissile U and non-fissile but fissionable and fertile U are both used in the fission process.
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