nuclear energy

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nuclear energy,

the energy stored in the nucleusnucleus,
in physics, the extremely dense central core of an atom. The Nature of the Nucleus
Composition

Atomic nuclei are composed of two types of particles, protons and neutrons, which are collectively known as nucleons.
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 of an atom and released through fission, fusion, or radioactivityradioactivity,
spontaneous disintegration or decay of the nucleus of an atom by emission of particles, usually accompanied by electromagnetic radiation. The energy produced by radioactivity has important military and industrial applications.
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. In these processes a small amount of mass is converted to energyenergy,
in physics, the ability or capacity to do work or to produce change. Forms of energy include heat, light, sound, electricity, and chemical energy. Energy and work are measured in the same units—foot-pounds, joules, ergs, or some other, depending on the system of
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 according to the relationship E = mc2, where E is energy, m is mass, and c is the speed of light (see relativityrelativity,
physical theory, introduced by Albert Einstein, that discards the concept of absolute motion and instead treats only relative motion between two systems or frames of reference.
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). The most pressing problems concerning nuclear energy are the possibility of an accident or systems failure at a nuclear reactornuclear 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
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 or fuel plant, such as those which occurred at Three Mile IslandThree Mile Island,
site of a nuclear power plant 10 mi (16 km) south of Harrisburg, Pa. On Mar. 28, 1979, failure of the cooling system of the No. 2 nuclear reactor led to overheating and partial melting of its uranium core and production of hydrogen gas, which raised fears of
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 (1979), ChernobylChernobyl
, Ukr. Chornobyl, abandoned city, N Ukraine, near the Belarus border, on the Pripyat River. Ten miles (16 km) to the north, in the town of Pripyat, is the Chernobyl nuclear power station, site of the worst nuclear reactor disaster in history. On Apr.
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 (1986), and FukushimaFukushima
, city (1990 pop. 277,528), capital of Fukushima prefecture, N Honshu, Japan, on the Kiso plain. A silk-textile center, it is a major commercial city of NE Japan, also producing cameras, automobiles, fruits, and bonsai trees.

Fukushima prefecture (1990 pop.
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 (2011), and the potential threat to the continued existence of the human race posed by nuclear weapons (see disarmament, nucleardisarmament, nuclear,
the reduction and limitation of the various nuclear weapons in the military forces of the world's nations. The atomic bombs dropped (1945) on Japan by the United States in World War II demonstrated the overwhelming destructive potential of nuclear weapons
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).

Nuclear Reactions

The release of nuclear energy is associated with changes from less stable to more stable nuclei and produces far more energy for a given mass of fuel than any other source of energy. In fission processes, a fissionable nucleus absorbs a neutron, becomes unstable, and splits into two nearly equal nuclei. In fusion processes, two nuclei combine to form a single, heavier nucleus. The most stable nuclei—those with the highest binding energies per nucleon holding their components together—are in the middle range of atomic weights, with the maximum stability at weights near 60. Thus, fission, which produces two lighter fragments, occurs for very heavy nuclei, while fusion occurs for the lightest nuclei.

Nuclear Fission

The process of nuclear fission was discovered in 1938 by Otto Hahn and Fritz Strassmann and was explained in early 1939 by Lise Meitner and Otto Frisch. The fissionable isotopeisotope
, in chemistry and physics, one of two or more atoms having the same atomic number but differing in atomic weight and mass number. The concept of isotope was introduced by F.
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 of uranium, U-235, can be split by bombarding it with a slow, or thermal, neutron. (Slow neutrons are called "thermal" because their average kinetic energies are about the same as those of the molecules of air at ordinary temperatures.) The atomic numbers of the nuclei resulting from the fission add up to 92, which is the atomic number of uranium. A number of pairs of product nuclei are possible, with the most frequently produced fragments being krypton and barium.

Since this reaction also releases an average of 2.5 neutrons, a chain reactionchain reaction,
self-sustaining reaction that, once started, continues without further outside influence. Proper conditions for a chain reaction depend not only on various external factors, such as temperature, but also on the quantity and shape of the substance undergoing the
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 is possible, provided at least one neutron per fission is captured by another nucleus and causes a second fission. In an atomic bombatomic bomb
or A-bomb,
weapon deriving its explosive force from the release of nuclear energy through the fission (splitting) of heavy atomic nuclei. The first atomic bomb was produced at the Los Alamos, N.Mex., laboratory and successfully tested on July 16, 1945.
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, the number is greater than 1 and the reaction increases rapidly to an explosion. In a nuclear reactornuclear 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
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, where the chain reaction is controlled, the number of neutrons producing additional fission must be exactly 1.0 in order to maintain a steady flow of energy.

Uranium-235, which occurs naturally as one part in 140 in a natural mixture of uranium isotopes, is not the only material fissionable by thermal neutrons. Uranium-233 and plutonium-239 can also be used but must be produced artificially. Uranium-233 is produced from thorium-232, which absorbs a neutron and then undergoes beta decay (the loss of an electron). Plutonium-239 is produced in a similar manner from uranium-238, which is the most common isotope of natural uranium. The average energy released by the fission of uranium-235 is 200 million electron volts, and that released by uranium-233 and plutonium-239 is comparable. Fission can also occur spontaneously, but the time required for a heavy nucleus to decay spontaneously by fission (10 million billion years in the case of uranium-238) is so long that induced fission by thermal neutrons is the only practical application of nuclear fission. However, spontaneous fission of uranium can be used in the datingdating,
the determination of the age of an object, of a natural phenomenon, or of a series of events. There are two basic types of dating methods, relative and absolute.
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 of very old rock samples.

The development of nuclear energy from fission reactions began with the program to produce atomic weapons in the United States. Early work was carried out at several universities, and the first sustained nuclear chain reaction was achieved at the Univ. of Chicago in 1942 by a group under Enrico Fermi. Later the weapons themselves were developed at Los Alamos, N.Mex., under the direction of J. Robert Oppenheimer (see Manhattan ProjectManhattan Project,
the wartime effort to design and build the first nuclear weapons (atomic bombs). With the discovery of fission in 1939, it became clear to scientists that certain radioactive materials could be used to make a bomb of unprecented power. U.S.
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).

Nuclear Fusion

Nuclear fusion, although it was known theoretically in the 1930s as the process by which the sun and most other stars radiate their great output of energy, was not achieved by scientists until the 1950s. Fusion reactions are also known as thermonuclear reactions because the temperatures required to initiate them are more than 1,000,000°C;. In the hydrogen bombhydrogen bomb
or H-bomb,
weapon deriving a large portion of its energy from the nuclear fusion of hydrogen isotopes. In an atomic bomb, uranium or plutonium is split into lighter elements that together weigh less than the original atoms, the remainder of the mass
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, such temperatures are provided by the detonation of a fission bomb. The energy released during fusion is even greater than that released during fission. Moreover, the fuel for fusion reactions, isotopes of hydrogen, is readily available in large amounts, and there is no release of radioactive byproducts.

In stars ordinary hydrogen, whose nucleus consists of a single proton, is the fuel for the reaction and is fused to form helium through a complex cycle of reactions (see nucleosynthesisnucleosynthesis
or nucleogenesis,
in astronomy, production of all the chemical elements from the simplest element, hydrogen, by thermonuclear reactions within stars, supernovas, and in the big bang at the beginning of the universe (see nucleus; nuclear energy).
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). This reaction takes place too slowly, however, to be of practical use on the earth. The heavier isotopes of hydrogen—deuteriumdeuterium
, isotope of hydrogen with mass no. 2. The deuterium nucleus, called a deuteron, contains one proton and one neutron. Deuterium is also called heavy hydrogen, and water in which the hydrogen atoms are deuterium is called heavy water (deuterium oxide, D2O).
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 and tritiumtritium
, radioactive isotope of hydrogen with mass number 3. The tritium nucleus, called a triton, contains one proton and two neutrons. It has a half-life of 12.5 years and decays by beta-particle emission. The symbol is T or 3H.
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—have much faster fusion reactions.

For sustained, controlled fusion reactions, a fission bomb obviously cannot be used to trigger the reaction. The difficulties of controlled fusion center on the containment of the nuclear fuel at the extremely high temperatures necessary for fusion for a time long enough to allow the reaction to take place. For deuterium-tritium fusion, this time is about 0.1 sec. At such temperatures the fuel is no longer in one of the ordinary states of matterstates of matter,
forms of matter differing in several properties because of differences in the motions and forces of the molecules (or atoms, ions, or elementary particles) of which they are composed.
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 but is instead a plasmaplasma,
in physics, fully ionized gas of low density, containing approximately equal numbers of positive and negative ions (see electron and ion). It is electrically conductive and is affected by magnetic fields.
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, consisting of a mixture of electrons and charged atoms. Obviously, no solid container could hold such a hot mixture; therefore, containment attempts have been based on the electrical and magnetic properties of a plasma, using magnetic fields to form a "magnetic bottle." In 1994 U.S. researchers achieved a fusion reaction that lasted about a second and generated 10.7 million watts, using deuterium and tritium in a magnetically confined plasma. The use of tritium lowers the temperature required and increases the rate of the reaction, but it also increases the release of radioactive neutronsneutron,
uncharged elementary particle of slightly greater mass than the proton. It was discovered by James Chadwick in 1932. The stable isotopes of all elements except hydrogen and helium contain a number of neutrons equal to or greater than the number of protons.
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. Another method uses laserlaser
[acronym for light amplification by stimulated emission of radiation], device for the creation, amplification, and transmission of a narrow, intense beam of coherent light. The laser is sometimes referred to as an optical maser.
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 beams aimed at tiny pellets of fusion fuel to create the necessary heat and pressure to initiate fusion.

If practical controlled fusion is achieved, it could have great advantages over fission as a source of energy. Deuterium is relatively easy to obtain, since it constitutes a small percentage of the hydrogen in water and can be separated by electrolysis, in contrast to the complex and expensive methods required to extract uranium-235 from its sources. In 2007 China, the European Union, India, Japan, Russia, South Korea, and the United States formally established the International Thermonuclear Experimental Reactor (ITER) Organization to build an experimental fusion reactor at Cadarache in S France that would use the "magnetic bottle" approach. The Lawrence Livermore National Laboratory's National Ignition Facility, based in Livermore, Calif., and dedicated in 2009, is exploring the use of high-energy lasers focused on hydrogen fuel to achieve nuclear fusion.

Bibliography

See H. Foreman, ed., Nuclear Power and the Public (1970); R. C. Lewis, Nuclear Power Rebellion: Citizen vs. the Atomic Industrial Establishment (1972); C. K. Ebinger, International Politics of Nuclear Energy (1978); S. Glasstone, Sourcebook on Atomic Energy (1979); G. S. Bauer and A. McDonald, ed., Nuclear Technologies in a Sustainable Energy System (1983); G. H. Clarfield and W. W. Wiecek, Nuclear America (1984).

Nuclear Energy

 

(also atomic energy), the internal energy of the atomic nucleus released in nuclear reactions.

The energy that must be expended to split the nucleus into its component parts is called the binding energy of the nucleus ℰb. Consequently, the binding energy is the maximum nuclear energy. The binding energy per nucleon is called the average binding energy ℰb/A, where A is the mass number.

Figure 1. Dependence of the average binding energy of nuclei on the number of nucleons

The binding energy of the nucleus comprises the energy of attraction of nucleons toward one another under the action of nuclear forces and the energy of mutual repulsion of the protons under the action of electrostatic forces. Each nucleon interacts strongly only with a small number of neighboring nucleons. Therefore, beginning as early as 4He, the average binding energy increases slowly with increasing A. The maximum is reached in the vicinity of iron (A = 56), after which there is a decline (see Figure 1). Such behavior can be attributed to the fact that some nucleons are on the periphery of the nucleus, and consequently their attraction to other nucleons is weaker. In light nuclei, the number of such nucleons is relatively high. As a result of the reduction in the role of the peripheral nucleons with increasing A, the value of ℰb/A increases. In heavy nuclei, ℰb/A decreases with increasing A, since the energy of attraction increases linearly with increasing A, but the energy of electrostatic repulsion of protons increases proportionally with the square of the proton number Z2. Thus, the reactions of nuclear fusion (the formation of light nuclei from lighter ones), the reactions of the splitting of heavy nuclei (the fissioning of nuclei into smaller fragments), and spontaneous alpha decay are exothermic. At magic values of Z and N (the number of neutrons in the nucleus), the dependence of ℰb/A on A shows slight maxima owing to the presence of closed shells in the nucleus (seeMAGIC NUMBER NUCLEI).

Because of the electrostatic repulsion of protons, reactions of nuclear fusion may develop if the kinetic energy of the nuclei is high, that is, at high temperatures of the medium. Nuclear fusion reactions are the source of the energy in stars. Reactions of the hydrogen cycle in stars proceed with the formation of 4He and the release of an energy of ~7 megaelectron volts per nucleon, or MeV/nucleon (1.8 × 108 killowatt-hours/kg, or kW-hr/kg). Two thermonuclear reactions have been produced under terrestrial conditions: the fusion of two deuterons, accompanied by the release of an energy of 1 MeV/nucleon, and the fusion of a deuteron and a triton, with the liberation of an energy of 3.5 MeV/-nucleon.

In the fission reaction of 235U under the action of neutrons, about 214 MeV is released in each fission event (4–5 percent higher for Pu isotopes). Of this amount, about 12 MeV is carried off into space by neutrinos. Thus, the nuclear energy actually released amounts to 0.85 MeV/nucleon, or 2.2 × 107 kW-hr/kg. This is 2 × 106 times the energy released upon the burning of 1 kg of petroleum. Thus far, only fission reactions have been used as an industrial source of nuclear energy.

A. M. PETROSIANTS

nuclear energy

[′nü·klē·ər ′en·ər·jē]
(nucleonics)
Energy released by nuclear fission or nuclear fusion. Also known as atomic energy.

nuclear energy

energy released during a nuclear reaction as a result of fission or fusion
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