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