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Nuclear Reactors

Nuclear Reactors

LARRY GILMAN

Nuclear reactors are complex devices in which fissionable elements such as uranium, thorium, or plutonium are made to undergo a sustainable nuclear chain reaction.

This chain reaction releases energy in the form of radiation that (a) sustains the chain reaction; (b) transmutes (i.e., alters the nuclear characteristics of) nearby atoms, including the nuclear fuel itself; and (c) may be harvested as heat. Transmutation in nuclear reactors of the common but weakly fissionable nuclide uranium-238 (238U) into plutonium-239 (239Pu) is an important source of explosive material for nuclear weapons, and heat from nuclear reactors is used to generate approximately 16 percent of the world's electricity and to propel submarines, aircraft carriers, and some other military vessels. Nuclear reactors have also been used on satellites and proposed as power sources for locomotives, aircraft, and rockets.

How a nuclear reactor works. A nuclear reactor exploits the innate instability of some atomsin general, those that have a large atomic number or that contain an imbalance of protons and neutronswhich break apart (fission) at random times, releasing photons, neutrons, electrons, and alpha particles. For some nuclides (atomic species having a specific number of protons and neutrons in the nucleus), the average wait until a given atom spontaneously fissions is shorter. When enough atoms of such an unstable isotope are packed close together, the neutrons released by fissioning atoms are more likely to strike the nuclei of nearby unstable atoms. These may fission at once, releasing still more neutrons, which may trigger still other fission events, and so forth. This is the chain reaction on which nuclear reactors and fission-type nuclear bombs depend. In a reactor, however, the fission rate is approximately constant, whereas in a bomb it grows exponentially, consuming most of the fissionable material in a small fraction of a second.

To produce a sustained chain reaction rather than a nuclear explosion, a reactor must not pack its fissionable atoms too closely together. They are therefore mixed with less-fissionable atoms that do not sustain the chain reaction. For example, in a reactor utilizing235U as its primary fuel, only 3 percent of the fuel is actually235U; the rest is mostly238U, a much less fissionable isotope of uranium. The higher the ratio of active fuel atoms to inert atoms in a given fuel mix, the more "enriched" the fuel is said to be; commercial nuclear power plant fuel is enriched only 3 to 5 percent235U, and so cannot explode. For a fission bomb, 90 percent enrichment would be typical (although bombs could be made with less-enriched uranium). Naval nuclear reactors, discussed further below, have used fuels enriched to between 20 and 93 percent.

Having diluted its active fuel component (e.g.,235U), a typical nuclear reactor must compensate by assuring that the neutrons produced by this diluted fuel can keep the chain reaction going. This is done, in most reactors, by embedding the fuel as small chunks or "fuel elements" in a matrix of a material termed a "moderator." The moderator's function is to slow (moderate) neutrons emitted by fissioning atoms in the fuel. Paradoxically, a slow neutron is more likely to trigger fission in a uranium, plutonium, or thorium nucleus than a fast neutron; a moderator, by slowing most neutrons before allowing them to strike nuclei, thus increases the probability that each neutron will contribute to sustaining the chain reaction. Graphite (a form of pure carbon), water, heavy water (deuterium dioxide or2H2), and zirconium hydride can all be used as moderators. Ordinary water is the most commonly used moderator.

If the chain reaction sustained by a nuclear reactor produces enough heat to damage the reactor itself, that heat must be carried off constantly by a gas or liquid as long as the reactor is operating. Once removed from the reactor, this energy may be ejected into the environment as waste heat or used, in part, to generate electricity. (Electricity, if generated, is an intermediate energy form; all the energy generated in a nuclear reactor or other power plant eventually winds up in the environment as heat.) In the case of a nuclear-powered rocket, such as the one the U.S. National Aeronautics and Space Administration (NASA) seeks to develop with its Project Phoenix, heat is removed from the system by ejected propellant. Liquid sodium, pressurized water, boiling water, and helium have all been used as cooling media for nuclear reactors, with pressurized or boiling water being used by commercial nuclear power plants. Typically, heat energy removed from the reactor is first turned into kinetic energy by using hot gas or water vapor to drive turbines (essentially enclosed, high-speed windmills), then into electrical energy by using the turbines to turn generators.

Nuclear power sources that do not produce enough heat to melt themselves, and which therefore require no circulating coolant, have been used on some space probes and satellites, both U.S. and Russian. Such a power source, termed a radioactive thermoelectric generator or RTG, consists of a mass of highly radioactive material, usually plutonium, that radiates enough heat to allow the generation of a modest but steady flow of electricity via the thermoelectric effect. The efficiency of an RTG is low but its reliability is very high.

Reactor byproducts. The neutron flow inside a reactor bombards, and by bombarding changes, the nuclei of many atoms in the reactor. The longer a unit of nuclear fuel remains in a reactor, therefore, the more altered nuclei it contains. Most of the new atoms formed are radioactive nuclides such as cesium-144 or ruthenium-106; a significant number are, if238U is present, isotopes of plutonium, mostly 239Pu. (Absorption of one neutron by a 238U nucleus turns it into a239Pu nucleus; absorption of one, two, or three neutrons by a239Pu nucleus turns it into a240Pu,241Pu, or242Pu nucleus.) Plutonium is found in nature only in trace amounts, but is present in all spent nuclear fuel containing238U. If it is extracted for use as a reactor fuel or a bomb material, it is considered a useful by-product of the nuclear reactor; otherwise, it is a waste product. In either case, plutonium is highly toxic and radioactive, and remains so for tens of thousands of years unless it is further transmuted by particle bombardment, as in a particle accelerator, reactor, or nuclear explosion. Reactors specially designed to turn otherwise inert238U into239Pu by neutron bombardment are termed fast breeder reactors, and can produce more nuclear fuel than they consume; however, all nuclear reactors, whether designed to "breed" or not, produce plutonium.

This fact has a basic military consequence: Every nation that possesses a nuclear power plant produces plutonium, which can be used to build atomic bombs. Plutonium sufficiently pure to be used in a bomb is termed bomb-grade or weapons-grade plutonium, and the process of extracting plutonium from irradiated nuclear fuel is termed reprocessing. (The alloy used in sophisticated nuclear weapons is nearly pure plutonium, but the U.S. Department of Energy has estimated that an unwieldy bomb could be made with material that is only 15 to 25 percent plutonium, with less-unwieldy bombs being possible with more-enriched alloys.) Every nation that possesses a nuclear reactor and reprocessing capability thus possesses most of what it needs to build nuclear weapons. Several nations, including India and Pakistan, have in fact built nuclear weapons using plutonium reprocessed from "peaceful" nuclear-reactor programs. A large (100 MW electric) nuclear power plant produces enough plutonium for several dozen bombs a year.

Besides producing plutonium that can, and sometimes is, extracted to produce nuclear weapons, every nuclear reactor has the feature that if bombed, its radioactive contents could be released into the environment, greatly amplifying the destructive effects of a wartime or terrorist attack. Nuclear reactors thus have a two-edged aspect: as producers, potentially, of weapons for use against an enemy, and as weapons, if attacked, for an enemy.

Naval nuclear reactors. The primary military use of nuclear reactors, apart from the production of material for nuclear weapons, is the propulsion of naval vessels. Nuclear power sources enable naval vessels to remain at sea for long periods without refueling; modern replacement cores for aircraft carriers are designed to last at least 50 years without refueling, while those for submarines are designed to last 30 to 40 years. In the case of submarines, nuclear power also makes it possible to remain submerged for months at a time without having to surface for oxygen. Furthermore, reactors have the general design advantage of high power density, that is, they provide high power output while consuming relatively little shipboard space. A large nuclear-powered vessel may be propelled by more than one reactor; the U.S. aircraft carrier USS Enterprise, launched in 1960, is powered by eight reactors. Britain, France, China, and Russia (formerly the Soviet Union) have also built nuclear-powered submarines and other vessels.

Although the design details of the nuclear reactors used on submarines and aircraft carriers are secret, they are known to differ in several ways from the large land-based reactors typically used for generating electricity. The primary difference is that in order to achieve high power density, naval reactors use more-highly-enriched fuel. Older designs used uranium enriched to at least 93 percent 235U; later Western reactors have used uranium enriched to only 20 to 25 percent, while Russian reactors have used fuels enriched to up to 45 percent. Small quantities of ex-Soviet submarine fuel have appeared on the global black market; larger quantities could be used as a bomb material.

The first nuclear-powered vessel, was a U.S. submarine launched in 1955, the USS Nautilus. Only three civil vessels (one U.S.-made, one German, and one Japanese) have ever been propelled by nuclear power; all proved too expensive to operate. About 160 nuclear-powered ships, mostly military, are presently at sea; at the peak of the Cold War, there were approximately 250.

FURTHER READING:

BOOKS:

Glasstone, Samuel, and Alexander Sesonske. Nuclear Reactor Engineering. Vol. I: Reactor Design Basics. New York: Chapman & Hall, 1994.

Todreas, Neil E., and Mujid S. Kazimi. Nuclear Systems I: Thermal Hydraulic Fundamentals. New York: Hemisphere Publishing Corporation, 1990.

SEE ALSO

Nuclear Detection Devices
Nuclear Emergency Support Team, United States
Nuclear Power Plants, Security
Russian Nuclear Materials, Security Issues

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nuclear reactor

nuclear reactor, device for producing controlled release of nuclear energy. Reactors can be used for research or for power production. A research reactor is designed to produce various beams of radiation for experimental application; the heat produced is a waste product and is dissipated as efficiently as possible. In a power reactor the heat produced is of primary importance for use in driving conventional heat engines; the beams of radiation are controlled by shielding.

Fission Reactors

A fission reactor consists basically of a mass of fissionable material usually encased in shielding and provided with devices to regulate the rate of fission and an exchange system to extract the heat energy produced. A reactor is so constructed that fission of atomic nuclei produces a self-sustaining nuclear chain reaction, in which the neutrons produced are able to split other nuclei. A chain reaction can be produced in a reactor by using uranium or plutonium in which the concentration of fissionable isotopes has been artificially increased. Even though the neutrons move at high velocities, the enriched fissionable isotope captures enough neutrons to make possible a self-sustaining chain reaction. In this type of reactor the neutrons carrying on the chain reaction are fast neutrons.

A chain reaction can also be accomplished in a reactor by employing a substance called a moderator to retard the neutrons so that they may be more easily captured by the fissionable atoms. The neutrons carrying on the chain reaction in this type of reactor are slow (or thermal) neutrons. Substances that can be used as moderators include graphite, beryllium, and heavy water (deuterium oxide). The moderator surrounds or is mixed with the fissionable fuel elements in the core of the reactor.

Types of Fission Reactors

A nuclear reactor is sometimes called an atomic pile because a reactor using graphite as a moderator consists of a pile of graphite blocks with rods of uranium fuel inserted into it. Reactors in which the uranium rods are immersed in a bath of heavy water are often referred to as "swimming-pool" reactors. Reactors of these types, in which discrete fuel elements are surrounded by a moderator, are called heterogeneous reactors. If the fissionable fuel elements are intimately mixed with a moderator, the system is called a homogeneous reactor (e.g., a reactor having a core of a liquid uranium compound dissolved in heavy water).

The breeder reactor is a special type used to produce more fissionable atoms than it consumes. It must first be primed with certain isotopes of uranium or plutonium that release more neutrons than are needed to continue the chain reaction at a constant rate. In an ordinary reactor, any surplus neutrons are absorbed in nonfissionable control rods made of a substance, such as boron or cadmium, that readily absorbs neutrons. In a breeder reactor, however, these surplus neutrons are used to transmute certain nonfissionable atoms into fissionable atoms. Thorium (Th-232) can be converted by neutron bombardment into fissionable U-233. Similarly, U-238, the most common isotope of uranium, can be converted by neutron bombardment into fissionable plutonium-239.

Production of Heat and Nuclear Materials

The transmutation of nonfissionable materials to fissionable materials in nuclear reactors has made possible the large-scale production of atomic energy. The excess nuclear fuel produced can be extracted and used in other reactors or in nuclear weapons. The heat energy released by fission in a reactor heats a liquid or gas coolant that circulates in and out of the reactor core, usually becoming radioactive. Outside the core, the coolant circulates through a heat exchanger where the heat is transferred to another medium. This second medium, nonradioactive since it has not circulated in the reactor core, carries the heat away from the reactor. This heat energy can be dissipated or it can be used to drive conventional heat engines that generate usable power. Submarines and surface ships propelled by nuclear reactors and nuclear-powered electric generating stations are in operation. However, nuclear accidents in 1979 at Three Mile Island and in 1986 at Chernobyl raised concern over the safety of reactors, and these concerns were revived somewhat in 2011 after an earthquake and tsunami resulted in a nuclear disaster in Fukushima, Japan. Another concern over fission reactors is the storage of hazardous radioactive waste. In the United States, the events at Three Mile Island made nuclear fission plants politically unacceptable and economically unattractive for many years; no new plants were approved for construction until 2012. In contrast, in France, Japan, and a few other nations nuclear fission has been used extensively for power generation. The Japanese and French adopted a more cautious approach in the aftermath of Fukushima; Germany, which has been less dependent on nuclear reactors, chose to accelerate its planned phase out of nuclear power generation.

Fusion Reactors

Fusion reactors are being studied as an alternative to fission reactors. The design of nuclear fusion reactors, which are still in the experimental stage, differs considerably from that of fission reactors. In a fusion reactor, the principal problem is the containment of the plasma fuel, which must be at a temperature of millions of degrees in order to initiate the reaction. Magnetic fields have been used in several ways to hold the plasmas in a "magnetic bottle." If development should reach a practical stage of application, it is expected that fusion reactors would have many advantages over fission reactors. Fusion reactors, for instance, would produce less hazardous radioactive waste. Because their fuel, deuterium (an isotope of hydrogen readily separated from water), is far less expensive to obtain than enriched uranium, fusion reactors also would be far more economical to operate.

Bibliography

See G. I. Bell, Nuclear Reactor Theory (1970); R. J. Watts, Elementary Primer of Diffusion Theory and the Chain Reaction (1982).

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nuclear reactor

nuclear reactor Device in which nuclear fission (and sometimes nuclear fusion) reactions are used for power generation or for the production of radioactive materials. In the reactor, the fuel is a radioactive heavy metal: uranium-235, uranium-233, or plutonium-239. In these metals, atoms break down spontaneously, undergoing a process called radioactive decay. Some neutrons released in this process strike the nuclei of fuel atoms, causing them to undergo fission and emit more neutrons. These, in turn, strike more nuclei, and in this way a chain reaction is set up. Usually a material, called a moderator, is used to slow down the neutrons to a speed at which the chain reaction is self-sustaining. This process occurs within the reactor core. The chain reaction is regulated by inserting control rods, which contain neutron-absorbing material such as cadmium or boron, into the core. The heat generated by the nuclear reaction is absorbed by a circulating coolant, and transferred to a boiler to raise steam. The steam drives a turbine that turns a generator, that in turn produces electricity. There are a variety of nuclear reactors, named after the type of coolant they use. For example, a boiling-water reactor (BWR) and a pressurized-water reactor (PWR), presently the most common type of reactor, both use water as the coolant and the moderator. In advanced gas-cooled reactors (AGR), the coolant is a gas – most commonly carbon dioxide. Fast reactors do not use a moderator, and fission is caused by fast neutrons. This type of reactor generates greater temperatures, and the coolant used is a liquid metal (usually liquid sodium). Sometimes called ‘fast-breeder’ reactors, fast reactors produce (‘breed’) more fissionable material than they consume. Excess neutrons from the fission of a fuel such as Ur235, instead of being absorbed in control rods, are used to bombard atoms of relatively inactive Ur238 which transmutes to the active isotope Pu239. When the original fuel is spent, the plutonium can be used as a nuclear fuel in other reactors or nuclear weapons. See also electricity sources; transmutation

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nuclear reactor

nu·cle·ar re·ac·tor • n. see reactor.

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reactor, nuclear

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Nuclear Reactor

Nuclear Reactor

Theory of fission reactors

Reactor core

Moderators

Control rods

Reactor types

Applications

Resources

A nuclear reactor is a device by which energy is produced as the result of a nuclear reaction, either fission or fusion. All commercially available nuclear reactors make use of fission reactions, in which the nuclei of large atoms such as uranium are broken apart into smaller nuclei with attendant release of energy. It is theoretically possible to construct reactors that operate on the principle of nuclear fusion, in which small nuclei are combined with each other with the release of energy. But after a half century of research on fusion reactors, no practicable device has yet been developed.

Theory of fission reactors

When neutrons strike the nucleus of a large atom, they can cause that nucleus to split apart into two roughly equal pieces known as fission products. In that process, additional neutrons and very large amounts of energy are also released. Only three isotopes are known to be fissionable, uranium235, uranium233, and plutonium239. Of these, only the first, uranium235, occurs naturally. Plutonium239 is produced synthetically when nuclei of uranium238 are struck by neutrons and transformed into plutonium. Since uranium238 always outweighs uranium235 in commercial nuclear reactor fuel, plutonium239 is made as a byproduct in all commercial reactors now in operation. Uranium233 can also be produced synthetically by the bombardment of thorium with neutrons. Thus far, however, this isotope has not been put to practical use in nuclear reactors.

The release of neutrons during fission makes it possible for a rapid and continuous repetition of the reaction. Suppose that a single neutron strikes a onegram block of uranium235. The fission of one uranium nucleus in that block releases, on an average, about two to three more neutrons. Each of those neutrons, then, is available for the fission of three more uranium nuclei. In the next stage, about nine neutrons (three from each of three fissioned uranium nuclei) are released. As long as more neutrons are being released, the fission of uranium nuclei can continue. (In practice, a single neutron cannot begin a chain reaction even in a pure 1g block of uranium235; too many neutrons will simply escape from the sample without causing a fission event. However, the chainreaction principle is used in practice.)

A reaction of this type that continues on its own once under way is known as a chain reaction. During a nuclear chain reaction, many billions of uranium nuclei may fission in less than a second. Enormous amounts of energy are released in a very short time, a fact that becomes visible with the explosion of a nuclear weapon.

Arranging for the uncontrolled, largescale release of energy produced during nuclear fission is a relatively simple task. Fission (atomic) bombs are essentially devices in which a chain reaction is initiated and

then allowed to continue on its own. The problems of designing a system by which fission energy is released at a constant and useable rate, however, are much more difficult.

Reactor core

The heart of any nuclear reactor is the core, which contains the fuel, a moderator, and control rods. The fuel used in some reactors consists of uranium oxide, enriched with about 34% of uranium235. In other reactors, the fuel consists of an alloy made of uranium and plutonium239. In either case, the amount of fissionable material is actually only a small part of the entire fuel assembly.

The fuel elements in a reactor core consist of cylindrical pellets about 0.6 in (1.5 cm) thick and 0.4 in (1.0 cm) in diameter. These pellets are stacked one on top of another in a hollow cylindrical tube known as the fuel rod and then inserted into the reactor core. Fuel rods tend to be about 12 ft (3.7 m) long and about 0.5 in (1.3 cm) in diameter. They are arranged in a grid pattern containing more than 200 rods each at the center of the reactor. The materials that fuel these pellets are made of must be replaced on a regular basis as the proportion of fissionable nuclei within them decreases.

Moderators

A nuclear reactor containing only fuel elements would be unusable because a chain reaction could probably not be sustained within it. The reason is that nuclear fission occurs best with neutrons that move at relatively modest speeds, called thermal neutrons. But the neutrons released from fission reactions tend to be moving very rapidly, at about 1/15 the speed of light. In order to maintain a chain reaction, therefore, it is necessary to introduce some material that will slow down the neutrons released during fission. Such a material is known as a moderator.

The most common moderators are substances of low atomic weight such as heavy water (deuterium oxide) or graphite. Hydrides (binary compounds containing hydrogen), hydrocarbons, and beryllium and beryllium oxide have also been used as moderators in certain specialized kinds of reactors.

Control rods

A chain reaction could easily be sustained in a reactor containing fuel elements and a moderator. In fact, the reaction might occur so quickly that the reactor would explode. In order to prevent such a disaster, the reactor core also contains control rods. Control rods are solid cylinders of metal constructed of some material that has an ability to absorb neutrons. One of the metals most commonly used in the manufacture of control rods is cadmium.

The purpose of control rods is to maintain the ratio of neutrons used up in fission compared to neutrons produced during fission at about 1:1. In such a case, for every one new neutron that is used up in causing a fission reaction, one new neutron becomes available to bring about the next fission reaction.

The problem is that the actual ratio of neutrons produced to neutrons used up in a fission reaction is closer to 2:1 or 3:1. That is, neutrons are produced so rapidly that the chain reaction goes very quickly and is soon out of control. By correctly positioning control rods in the reactor core, however, many of the excess neutrons produced by fission can be removed from the core and the reaction can be kept under control.

The control rods are, in a sense, the dial by which the rate of fission is maintained within the core. When the rods are inserted completely into the core, most neutrons released during fission are absorbed, and no chain reaction occurs. As the rods are slowly removed from the core, the rate at which fission occurs increases. At some point, the position of the control rods is such that the 1:1 ratio of produced to use up neutrons is achieved. At that point, the chain reaction goes forward, releasing energy, but under precise control of human operators.

Reactor types

In most cases, the purpose of a nuclear reactor is to capture the energy released from fission reactions and put it to some useful service. For example, the heat generated by a nuclear reactor in a nuclear power plant is used to boil water and make steam, which can then be used to generate electricity. The way that heat is removed from a reactor core is the basis for defining a number of different reactor types.

For example, one of the earliest types of nuclear reactors is the boiling water reactor (BWR) in which the reactor core is surrounded by ordinary water. As the reactor operates, the water is heated, begins to boil, and changes to steam. The steam produced is piped out of the reactor vessel and delivered (usually) to a turbine and generator, where electrical power is produced.

Another type of reactor is the pressurized water reactor (PWR). In a PWR, coolant water surrounding the reactor core is kept under high pressure, preventing it from boiling. This water is piped out of the reactor vessel into a second building where it is used to heat a secondary set of pipes also containing ordinary water. The water in the secondary system is allowed to boil, and the steam formed is then transferred to a turbine and generator, as in the BWR.

Some efforts have been made to design nuclear reactors in which liquid metals are used as heat transfer agents. Liquid sodium is the metal most often suggested. Liquid sodium has many attractive properties as a heat transfer agent, but it has one serious drawback. It reacts violently with water and great care must be taken, therefore, to make sure that the two materials do not come into contact with each other.

At one time, there was also some enthusiasm for the use of gases as heat transfer agents. A group of reactors built in Great Britain, for example, were designed to use carbon dioxide to move heat from the reactor to the power generating station. Gas reactors have, however, proved technically troublesome and have not experienced much popularity in other nations.

Applications

At the end of World War II, great hopes were expressed for the use of nuclear reactors as a way of providing power for many human energy needs. For example, some optimists envisioned the use of small nuclear reactors as power sources in airplanes, ships, and automobiles. These hopes have been realized to only a limited extent. Nuclear powered submarines and aircraft carriers, for example, have become a practical reality. But other forms of transportation do not make use of this source of energy (although NASA continues research into nuclearpowered rockets).

Instead, the vast majority of nuclear reactors in use today are employed in nuclear power plants, where they supply the energy needed to manufacture electrical energy. In a power reactor, energy released within the reactor core is transferred by a coolant to an external building in which are housed a turbine and generator. Steam obtained from water boiled by reactor heat energy is used to drive the turbine and generator, thereby producing electrical energy.

Reactors with other functions are also in use. Reactors are also used to make plutonium and tritium for nuclear weapons. Enriched uraniumuranium with a high percentage of uranium235can also be used for making nuclear weapons. A breeder reactor is a type of reactor in which new reactor fuel (plutonium) is manufactured from uranium238. By far the most common material in any kind of nuclear reactor is uranium238. This isotope of uranium does not undergo fission and does not, therefore, make any direct contribution to the production of energy. But the vast numbers of neutrons produced in the reactor core do react with uranium238 in a different way, producing plutonium239 as a product. This plutonium239 can then be removed from the reactor core and used as a fuel in other reactors. Reactors whose primary function it is to generate plutonium239 are known as breeder reactors.

Research reactors may have one or both of two functions. First, such reactors are often built simply to test new design concepts for the nuclear reactor. When the test of the design element has been completed, the primary purpose of the reactor has been accomplished.

Second, research reactors can also be used to take advantage of the various forms of radiation released during fission reactions. These forms of radiation can be used to bombard a variety of materials to study the effects of the radiation on the materials.

KEY TERMS

Chain reaction An ongoing process in which a neutron produced from fission enters a nearby nucleus, causes it to fission, and releases additional neutrons to continue the process.

Electron volt A unit for measuring energy. An electron volt is the energy gained or lost by an electron as it passes through a potential difference of one volt.

Generator A device for converting kinetic energy (the energy of movement) into electrical energy.

Isotopes Two molecules in which the number of atoms and the types of atoms are identical, but their arrangement in space is different, resulting in different chemical and physical properties.

Neutron A subatomic particle that carries no electrical charge and that has a mass of zero.

Nuclear fission A reaction in which a larger atomic nucleus breaks apart into two roughly equal smaller nuclei.

Nuclear fusion A reaction in which two small nuclei combine with each other to from one larger nucleus.

Turbine A device consisting of a series of baffles mounted on a wheel around a central shaft used to convert the energy of a moving fluid into the energy of mechanical rotation.

Nuclear reactors have experienced several notable failures to contain their radioactive contents, especially the Chernobyl disaster in Ukraine in 1986. Proponents of nuclear power argue that reactor dangers are exaggerated by public fears; opponents of nuclear power argue that reactors remain intrinsically dangerous, and that the nuclear fuel cycle produces other dangers as well that make nuclear power an unwise option.

Resources

BOOKS

Trefil, James. Encyclopedia of Science and Technology. The reference Works, Inc., 2001.

PERIODICALS

Effect Of Heating And Power Plants On The Environment. Atomic Energy 92, no. 66 (2002): 523528.

David E. Newton

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Nuclear Reactor

Nuclear reactor

A nuclear reactor is a device by which energy is produced as the result of a nuclear reaction, either fission or fusion. At the present time, all commercially available nuclear reactors make use of fission reactions, in which the nuclei of large atoms such as uranium (the fuel) are broken apart into smaller nuclei, with the release of energy. It is theoretically possible to construct reactors that operate on the principle of nuclear fusion , in which small nuclei are combined with each other with the release of energy. But after a half century of research on fusion reactors, no practicable device has yet been developed.

Theory of fission reactors

When neutrons strike the nucleus of a large atom, they cause that nucleus to split apart into two roughly equal pieces known as fission products. In that process, additional neutrons and very large amounts of energy are also released. Only three isotopes are known to be fissionable, uranium-235, uranium-233, and plutonium-239. Of these, only the first, uranium-235, occurs naturally. Plutonium-239 is produced synthetically when nuclei of uranium-238 are struck by neutrons and transformed into plutonium. Since uranium-238 always occurs along with uranium-235 in a nuclear reactor, plutonium-239 is produced as a byproduct in all commercial reactors now in operation. As a result, it has become as important in the production of nuclear power as uranium-235. Uranium-233 can also be produced synthetically by the bombardment of thorium with neutrons. Thus far, however, this isotope has not been put to practical use in nuclear reactors.

Nuclear fission is a promising source of energy for two reasons. First, the amount of energy released during fission is very large compared to that obtained from conventional energy sources. For example, the fissioning of a single uranium-235 nucleus results in the release of about 200 million electron volts of energy. In comparison, the oxidation of a single carbon atom (as it occurs in the burning of coal or oil) releases about four electron volts of energy. When the different masses of carbon and uranium atoms are taken into consideration, the fission reaction still produces about 2.5 million times more energy than does the oxidation reaction.

Second, the release of neutrons during fission makes it possible for a rapid and continuous repetition of the reaction. Suppose that a single neutron strikes a one gram block of uranium-235. The fission of one uranium nucleus in that block releases, on an average, about two to three more neutrons. Each of those neutrons, then, is available for the fission of three more uranium nuclei. In the next stage, about nine neutrons (three from each of three fissioned uranium nuclei) are released. As long as more neutrons are being released, the fission of uranium nuclei can continue.

A reaction of this type that continues on its own once under way is known as a chain reaction. During a nuclear chain reaction, many billions of uranium nuclei may fission in less than a second. Enormous amounts of energy are released in a very short time, a fact that becomes visible with the explosion of a nuclear weapon.

Arranging for the uncontrolled, large-scale release of energy produced during nuclear fission is a relatively simple task. Fission (atomic) bombs are essentially devices in which a chain reaction is initiated and then allowed to continue on its own. The problems of designing a system by which fission energy is released at a constant and useable rate, however, are much more difficult.


Reactor core

The heart of any nuclear reactor is the core, which contains the fuel, a moderator, and control rods. The fuel used in some reactors consists of uranium oxide, enriched with about 3-4 % of uranium-235. In other reactors, the fuel consists of an alloy made of uranium and plutonium-239. In either case, the amount of fissionable material is actually only a small part of the entire fuel assembly.

The fuel elements in a reactor core consist of cylindrical pellets about 0.6 in (1.5 cm) thick and 0.4 in (1.0 cm) in diameter. These pellets are stacked one on top of another in a hollow cylindrical tube known as the fuel rod and then inserted into the reactor core. Fuel rods tend to be about 12 ft (3.7 m) long and about 0.5 in (1.3 cm) in diameter. They are arranged in a grid pattern containing more than 200 rods each at the center of the reactor. The materials that fuel these pellets are made of must be replaced on a regular basis as the proportion of fissionable nuclei within them decreases.


Moderators

A nuclear reactor containing only fuel elements would be unusable because a chain reaction could probably not be sustained within it. The reason is that nuclear fission occurs best with neutrons that move at relatively modest speeds, called thermal neutrons. But the neutrons released from fission reactions tend to be moving very rapidly, at about 1/15 the speed of light . In order to maintain a chain reaction, therefore, it is necessary to introduce some material that will slow down the neutrons released during fission. Such a material is known as a moderator.

The most common moderators are substances of low atomic weight such as heavy water (deuterium oxide) or graphite. Hydrides (binary compounds containing hydrogen ), hydrocarbons, and beryllium and beryllium oxide have also been used as moderators in certain specialized kinds of reactors.


Control rods

A chain reaction could easily be sustained in a reactor containing fuel elements and a moderator. In fact, the reaction might occur so quickly that the reactor would explode. In order to prevent such a disaster, the reactor core also contains control rods. Control rods are solid cylinders of metal constructed of some material that has an ability to absorb neutrons. One of the metals most commonly used in the manufacture of control rods is cadmium.

The purpose of control rods is to maintain the ratio of neutrons used up in fission compared to neutrons produced during fission at about 1:1. In such a case, for every one new neutron that is used up in causing a fission reaction, one new neutron becomes available to bring about the next fission reaction.

The problem is that the actual ratio of neutrons produced to neutrons used up in a fission reaction is closer to 2:1 or 3:1. That is, neutrons are produced so rapidly that the chain reaction goes very quickly and is soon out of control. By correctly positioning control rods in the reactor core, however, many of the excess neutrons produced by fission can be removed from the core and the reaction can be kept under control.

The control rods are, in a sense, the dial by which the rate of fission is maintained within the core. When the rods are inserted completely into the core, most neutrons released during fission are absorbed, and no chain reaction occurs. As the rods are slowly removed from the core, the rate at which fission occurs increases. At some point, the position of the control rods is such that the 1:1 ratio of produced to used up neutrons is achieved. At that point, the chain reaction goes forward, releasing energy, but under precise control of human operators.


Reactor types

In most cases, the purpose of a nuclear reactor is to capture the energy released from fission reactions and put it to some useful service. For example, the heat generated by a nuclear reactor in a nuclear power plant is used to boil water and make steam, which can then be used to generate electricity . The way that heat is removed from a reactor core is the basis for defining a number of different reactor types.

For example, one of the earliest types of nuclear reactors is the boiling water reactor (BWR) in which the reactor core is surrounded by ordinary water. As the reactor operates, the water is heated, begins to boil, and changes to steam. The steam produced is piped out of the reactor vessel and delivered (usually) to a turbine and generator , where electrical power is produced.

Another type of reactor is the pressurized water reactor (PWR). In a PWR, coolant water surrounding the reactor core is kept under high pressure , preventing it from boiling. This water is piped out of the reactor vessel into a second building where it is used to heat a secondary set of pipes also containing ordinary water. The water in the secondary system is allowed to boil, and the steam formed is then transferred to a turbine and generator, as in the BWR.

Some efforts have been made to design nuclear reactors in which liquid metals are used as heat transfer agents. Liquid sodium is the metal most often suggested. Liquid sodium has many attractive properties as a heat transfer agent, but it has one serious drawback. It reacts violently with water and great care must be taken, therefore, to make sure that the two materials do not come into contact with each other.

At one time, there was also some enthusiasm for the use of gases as heat transfer agents. A group of reactors built in Great Britain, for example, were designed to use carbon dioxide to move heat from the reactor to the power generating station. Gas reactors have, however, not experienced much popularity in other nations.




Applications

At the end of World War II, great hopes were expressed for the use of nuclear reactors as a way of providing power for many human energy needs. For example, some optimists envisioned the use of small nuclear reactors as power sources in airplanes, ships, and automobiles. These hopes have been realized to only a limited extent. Nuclear powered submarines, for example, have become a practical reality. But other forms of transportation seldom make use of this source of energy.

Instead, the vast majority of nuclear reactors in use today are employed in nuclear power plants where they supply the energy needed to manufacture electrical energy. In a power reactor, energy released within the reactor core is transferred by a coolant to an external building in which are housed a turbine and generator. Steam obtained from water boiled by reactor heat energy is used to drive the turbine and generator, thereby producing electrical energy.

Reactors with other functions are also in use. For example, a breeder reactor is one in which new reactor fuel is manufactured. By far the most common material in any kind of nuclear reactor is uranium-238. This isotope of uranium does not undergo fission and does not, therefore, make any direct contribution to the production of energy. But the vast numbers of neutrons produced in the reactor core do react with uranium-238 in a different way, producing plutonium-239 as a product. This plutonium-239 can then be removed from the reactor core and used as a fuel in other reactors. Reactors whose primary function it is to generate plutonium-239 are known as breeder reactors.

Research reactors may have one or both of two functions. First, such reactors are often built simply to test new design concepts for the nuclear reactor. When the test of the design element has been completed, the primary purpose of the reactor has been accomplished.

Second, research reactors can also be used to take advantage of the various forms of radiation released during fission reactions. These forms of radiation can be used to bombard a variety of materials to study the effects of the radiation on the materials.

Resources

books

Carlisle, Rodney P. Encyclopedia of the Atomic Age. New York: Facts on File, 2001.

Croall, C., and S. Sempler. Nuclear Power for Beginners. New York: State Mutual Books, 1990.

Ebinger, Charles K. Nuclear Power: The Promise of New Technologies. Washington, DC: CSI Studies, 1991.

Jones, P. M., ed. Nuclear Power: Policy and Prospects. New York: John Wiley & Sons, 1987.

Macaulay, David. The New Way Things Work. Boston: Houghton Mifflin Company, 1998.

Nuclear Power, Nuclear Fuel Cycle and Waste Management,Part C: Status and Trends, 1993. Lanham, MD: UNIPUB, 1993.

Trefil, James. Encyclopedia of Science and Technology. The Reference Works, Inc., 2001.


periodicals

"Effect Of Heating And Power Plants On The Environment." Atomic Energy 92, no. 6-6 (2002): 523-528.

David E. Newton

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chain reaction

—An on-going process in which a neutron produced from fission enters a nearby nucleus, causes it to fission, and releases additional neutrons to continue the process.

Electron volt

—A unit for measuring energy. An electron volt is the energy gained or lost by an electron as it passes through a potential difference of one volt.

Generator

—A device for converting kinetic energy (the energy of movement) into electrical energy.

Isotopes

—Two molecules in which the number of atoms and the types of atoms are identical, but their arrangement in space is different, resulting in different chemical and physical properties.

Neutron

—A subatomic particle that carries no electrical charge and that has a mass of zero.

Nuclear fission

—A reaction in which a larger atomic nucleus breaks apart into two roughly equal smaller nuclei.

Nuclear fusion

—A reaction in which two small nuclei combine with each other to from one larger nucleus.

Turbine

—A device consisting of a series of baffles mounted on a wheel around a central shaft used to convert the energy of a moving fluid into the energy of mechanical rotation.

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