breeder reactor

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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.


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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The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Breeder Reactor


a nuclear reactor in which an initial quantity of nuclear fuel is consumed and then reproduced in a greater quantity as a fissile nuclear fuel. As a rule, both the fuel consumed and the fuel produced in the reactor are made up of the same chemical element, either plutonium or uranium. Fuel is produced as a result of the interaction of the neutrons released in the fission of the initial fuel with the nuclei of a substance known as the fuel blanket (source material), which is placed in the reactor. In uranium-plutonium fast-breeder reactors 239Pu serves as the initial fuel, and 238U as the fuel blanket. The fissile material produced, 239Pu, is formed through the capture of free neutrons by the uranium nuclei. In uranium-thorium fast or slow breeder reactors, 233U serves as the initial fuel, and 232Th as the fuel blanket; 233U is the fuel produced. The doubling time, the time in which the mass of the produced fuel becomes twice as great as the beginning mass of the initial fuel, is an important quantity characterizing the operation of a breeder reactor.

The only natural nuclear fuel is 235U, and it constitutes in the natural mixture of uranium isotopes only 0.71 percent. Breeder reactors thus markedly increase the fuel base of the nuclear power industry by employing substances that cannot themselves sustain a fission reaction. Therefore, in the industrially developed countries, great attention is devoted to the problem of constructing reliable and economical breeder reactors. In the USSR, such work was started in 1949 under the direction of A. I. Leipunskii. After the construction of a series of experimental breeder reactors, the first large breeder reactor in the world, the BN-350, went into operation in 1973 at a 150-megawatt nuclear power plant in the city of Shevchenko in the Kazakh SSR. A BN-600 breeder reactor, designed for use in a 600-megawatt plant, is currently under construction.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

breeder reactor

[′brēd·ər rē′ak·tər]
A nuclear reactor that produces more fissionable material that it consumes.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

breeder reactor

a type of nuclear reactor that produces more fissionable material than it consumes
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
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Such breeder reactors will actually breed more fissionable fuel in the jackets than is consumed in the core, and this means that all the uranium and thorium supply of the world can serve as potential fission fuel, rather than the rare uranium-235 alone.