NUCLEAR REACTOR

any of a class of devices that can initiate and control a self-sustaining series of nuclear fissions. In the fission process, a heavy nucleus such as uranium absorbs a neutron, after which it splits into two fragments of nearly equal mass. A substantial amount of energy is released simultaneously, as are several neutrons. These may then strike other heavy nuclei and cause them to fission. The continuous recurrence of this process results in a chain reaction in which many billions of nuclei may fission within a small fraction of a second. In a nuclear reactor such a self-sustaining series of fissions is carefully controlled, making it possible to utilize the enormous amount of energy released. This energy occurs in the form of radiation and the kinetic energy of the fission products expelled at high speeds. Much of it becomes thermal energy as the fission products come to rest. Some of the thermal energy is used to heat water and convert it to high-pressure steam. This steam drives a turbine, and the turbine's mechanical energy is then converted into electricity by means of a generator. Besides providing a valuable source of electric power for commercial use, nuclear reactors also serve to propel certain types of military surface vessels, submarines, and some unmanned spacecraft. Another major application of reactors is the production of radioactive isotopes that are used extensively in scientific research, medical therapy, and industry. These isotopes are created by bombarding nonradioactive substances with the neutrons released during fission. The only material occurring in nature that is readily fissionable and able to sustain a chain reaction is the uranium isotope U-235. It is a rare isotope; in natural uranium it is outnumbered roughly 140 to 1 by another isotope, U-238. When a single slow-moving neutron collides with the nucleus of a U-235 atom, the nucleus becomes suddenly so unstable that it splits into two major fragments and releases, on the average, two or three neutrons. Of these neutrons, at least one must succeed in producing another fission if the chain reaction is to persist. This is difficult to accomplish with natural uranium. Because its concentration of U-235 nuclei is so small, the neutrons may escape the nuclear fuel without colliding with the fissionable nuclei, or they may strike the more plentiful U-238 nuclei and be absorbed. To reduce the possibility of this happening, enriched uranium, which contains a higher percentage of U-235 than does natural uranium, is often used instead of the latter as reactor fuel. Enrichment can be achieved by various processes, as, for example, gaseous diffusion. Since the energy from nuclear fuels would be limited if only U-235 were utilized, reactors known as breeder reactors have been designed to convert nonfissionable uranium or other elements into fissionable isotopes. See also breeder reactor. Most commercial power reactors of the conventional variety require a moderator to slow neutrons to a tiny fraction of their initial speed so as to increase their likelihood of causing fissions of U-235. Such substances as ordinary water (termed light water), deuterium oxide (heavy water), and graphite have been found to be effective moderators because they are able to slow down neutrons during the fission process without appreciably reducing their number by absorption. Control of the rate at which neutrons are emitted, and thus of the reaction, is achieved by introducing materials, usually in the form of rods, that readily absorb slow neutrons. Typically, rods made of cadmium or boron are gradually withdrawn from the reactor core until a chain reaction is initiated. They are reinserted into the core if the series of fissions begins to proceed at too great a rate, which would result in the release of an excessive amount of thermal energy and radiation and possibly cause meltdown of the core. The heat released by fission is removed from the reactor core by a coolant, either liquid or gaseous, that is circulated through the core. Desirable coolants have good heat-transfer properties as well as low neutron-absorbing properties. Both light water and heavy water have been employed, as have liquid metals (e.g., sodium), helium, and several other substances. As a chain reaction continues, fission products accumulate in the reactor core. Most of these fragments are intensely radioactive and emit harmful gamma rays and neutrons. Consequently, the reactor must be surrounded by thick, heavy concrete shielding to protect operators and other individuals in the vicinity against radiation from these fragments and from that arising directly from the fission process itself. The disposal of radioactive fission products and spent-fuel assemblies poses a more difficult problem than does the containment of radiation in the reactor core. Some of these nuclear wastes remain dangerously radioactive for thousands of years and thus must be eliminated or stored permanently. As yet, however, no practical method of safe and permanent disposal has been implemented. The world's first nuclear reactor was built at the University of Chicago under the direction of the Italian-born physicist Enrico Fermi; it produced a chain reaction on Dec. 2, 1942. Immediately following World War II, scientists and engineers in various other countries undertook efforts to develop reactors for large-scale power production. In 1956 Great Britain opened at Calder Hall, Eng., the first full-scale commercial nuclear-power plant. The first such American power station went into operation in 1957. The number of large nuclear-power plants proliferated rapidly in many industrialized countries until about the late 1970s. Since that time, there has been a significant slowdown in the deployment of nuclear energy for commercial power production. This has resulted largely because of a sharp decrease in the projected rate of increase in electric-power demand, rising costs of constructing new nuclear plants, and public fears of nuclear power due to major accidents at the Three Mile Island power station, near Harrisburg, Pa., in the United States, and the Chernobyl installation in the Soviet Union. France, Japan, South Korea, and Taiwan, which have few alternative-energy resources, however, have continued to increase their use of nuclear power. any of a class of devices that can initiate and control a self-sustaining series of nuclear fissions. Such devices are used as research tools, as systems for producing radioisotopes, and most prominently as energy sources. The latter are commonly called power reactors. Fission is the process in which a heavy nucleus splits into two smaller fragments. A large amount of energy is released in this process, and this energy is the basis of fission power systems. The nuclear fragments are in very excited states and emit neutrons and other forms of radiation. The neutrons can then cause new fissions, which in turn yield more neutrons, and so forth. Such a continuous self-sustaining series of fissions constitutes a fission chain reaction. For a detailed discussion of nuclear fission, see nuclear fission. In an atomic bomb the chain reaction is designed to increase in intensity until much of the material has fissioned. This increase is very rapid and produces the extremely sharp, tremendously energetic explosions characteristic of such bombs. In a nuclear reactor the chain reaction is maintained at a controlled, nearly constant level. Nuclear reactors are so designed that they cannot explode like atomic bombs. Most of the energy of fissionabout 85 percent of itis released within a very short time after the process occurs. The rest of the energy comes from the radioactive decay of fission products, which is what the fragments are called after they have emitted neutrons. Radioactive decay continues when the fission chain has been stopped, and its energy must be dealt with in any proper reactor design. Additional reading Richard Rhodes, The Making of the Atomic Bomb (1986), chronicles developments leading to the first reactor and first atomic bomb. An elementary text covering reactor concepts, radiation, nuclear fuel cycles, reactor systems, safety and safeguards, and fusion concepts is Ronald Allen Knief, Nuclear Energy Technology: Theory and Practice of Commercial Nuclear Power (1981); the same concepts are treated at a more advanced mathematical level in John R. Lamarsh, Introduction to Nuclear Engineering, 2nd ed. (1983). James J. Duderstadt and Louis J. Hamilton, Nuclear Reactor Analysis (1976), discusses the theory of neutron behaviour in matter, criticality, neutron spectrum, and reactor core design and control, with emphasis on methods of calculation. Manson Benedict, Thomas H. Pigford, and Hans Wolfgang Levi, Nuclear Chemical Engineering, 2nd ed. (1981), includes coverage of fuel cycles, the chemistry of uranium and heavy elements, the theory of multistage systems, enrichment processes and theory, the reprocessing of nuclear fuel, and nuclear waste management. Current developments in domestic and international nuclear power, safety, research, and opinion are published in Nuclear News (monthly), the newsletter of the American Nuclear Society. Bernard I. Spinrad