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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

nuclear reactor: see nuclear reactor.

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