nuclear reactor

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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.

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 have raised concern over the safety of reactors. Another concern over fission reactors is the storage of hazardous radioactive waste . In the United States, where nuclear fission now is neither politically acceptable nor economically attractive, no new plants have been ordered since 1978, but nuclear fission is used extensively for power generation in France, Japan, and a few other nations.

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).

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

nuclear reaction A reaction involving one or more nuclei of atoms in which there is a change to the nuclei. Nuclear reactions can release enormous amounts of energy; examples are nucleosynthesis, fission, fusion, and radioactive decay. However, other nuclear reactions require an input of energy to build up elements heavier than iron, such as the r-process and s-process.

nuclear reactor a facility in which fissile material is used in a self-supporting chain reaction (nuclear fission) to produce heat and/or radiation for both practical application and research and development.

nu·cle·ar re·ac·tor • n. see reactor.

nuclear reactor see nuclear reactor .