NUCLEAR FISSION

Sequence of events in the fission of a uranium nucleus by a neutron. Sequence of events in the fission of a uranium nucleus by a neutron. subdivision of a heavy atomic nucleus, such as that of uranium or plutonium, into two fragments of roughly equal mass. This process is accompanied by the release of a large amount of energy. Nuclear fission was discovered in 1938 by Otto Hahn and Fritz Strassmann while attempting to produce elements heavier than uranium by bombarding uranium with neutrons. The two German chemists did not realize that they had in fact induced a fission reaction. Subsequent theoretical studies by others provided a description of the process and the principles involved. It was established that nuclear fission occurs when a particle such as a neutron strikes the nucleus of a uranium atom and causes it to split into two fission fragments, each of which is composed of a nucleus with roughly half the neutrons and protons of the original nucleus. This fission process releases a large quantity of thermal energy as well as gamma rays and two or more free neutrons (i.e., neutrons not bound to the fission fragments). These free neutrons fission other uranium nuclei, which then give off neutrons that split still more nuclei. A series of fissions of this kind constitutes a chain reaction, which yields a continuous supply of nuclear energy. Research by Niels Bohr of Denmark and John A. Wheeler of the United States suggested that the fission observed in uranium involved uranium-235, an isotope comprising only 1 part in 140 of natural uranium, rather than the much more abundant uranium-238 species. The latter does not fission as readily as uranium-235 because it absorbs most of the neutrons that strike it. The prediction by Bohr and Wheeler was later confirmed through the use of minute quantities of a target material somewhat richer in uranium-235 than normal. In 1942 the Italian-born American physicist Enrico Fermi and his coworkers at the University of Chicago produced the first controlled self-sustaining fission reaction. They accomplished this feat with a simple nuclear reactor, the first such device ever constructed. Their so-called atomic pile consisted of large lumps of natural uranium embedded in hundreds of tons of specially purified graphite. The graphite was intended to slow down free neutrons to ensure their capture by the small number of uranium-235 nuclei in the natural uranium fuel. Fermi realized that unless this was done the bombarding neutrons would simply be absorbed by the more plentiful uranium-238 nuclei in the natural uranium, making it impossible to sustain a chain reaction. The advanced nuclear reactors of today still use the basic technique devised by Fermi (i.e., employ a moderator to reduce the speed of free neutrons) to promote neutron capture by uranium-235 nuclei. Nuclear fission is one of three types of nuclear reactions that release substantial amounts of energy, the other two being radioactive decay and nuclear fusion. The energy derived from fission has come to be utilized on a relatively large scale for generating electric power and for propelling ships, notably submarines. Such energy is also the source of the vast destructive power of atomic bombs and neutron warheads. subdivision of a heavy atomic nucleus, such as that of uranium or plutonium, into two fragments of roughly equal mass. The process is accompanied by the release of a large amount of energy. In nuclear fission the nucleus of an atom breaks up into two lighter nuclei. The process may take place spontaneously in some cases or may be induced by the excitation of the nucleus with a variety of particles (e.g., neutrons, protons, deuterons, or alpha particles) or with electromagnetic radiation in the form of gamma rays. In the fission process, a large quantity of energy is released, radioactive products are formed, and several neutrons are emitted. These neutrons can induce fission in a nearby nucleus of fissionable material and release more neutrons that can repeat the sequence, causing a chain reaction in which a large number of nuclei undergo fission and an enormous amount of energy is released. If controlled in a nuclear reactor, such a chain reaction can provide power for society's benefit. If uncontrolled, as in the case of the so-called atomic bomb, it can lead to an explosion of awesome destructive force. The discovery of nuclear fission has opened a new erathe Atomic Age. The potential of nuclear fission for good or evil and the risk/benefit ratio of its applications have not only provided the basis of many sociological, political, economic, and scientific advances but grave concerns as well. Even from a purely scientific perspective, the process of nuclear fission has given rise to many puzzles and complexities, and a complete theoretical explanation is still not at hand. Additional reading Louis A. Turner, Nuclear Fission, Reviews of Modern Physics, 12(1):129 (January 1940), an excellent review of the early studies on nuclear fission; Henry DeWolf Smyth, Atomic Energy for Military Purposes: The Official Report on the Development of the Atomic Bomb Under the Auspices of the United States Government, 19401945, new and enlarged ed. (1948, reprinted 1978); and Samuel Glasstone, Sourcebook on Atomic Energy, 3rd ed. (1967, reprinted 1979), a comprehensive text on the atom and nuclear energy. For a detailed, authoritative treatment of all aspects of nuclear fission, see Earl K. Hyde, Isadore Perlman, and Glenn T. Seaborg, The Nuclear Properties of the Heavy Elements, vol. 3, Fission Phenomena (1964, reissued 1971); and Robert Vandenbosch and John R. Huizenga, Nuclear Fission (1973). Also useful are Wolf-Udo Schrder (ed.), Nuclear Fission and Heavy-Ion-Induced Reactions (1987), papers from a conference; and a multivolume proceedings series published by the International Atomic Energy Agency, Physics and Chemistry of Fission. For more popular accounts of nuclear energy and its uses, see Grace Marmor Spruch and Larry Spruch (eds.), The Ubiquitous Atom (1974); and Martin Mann, Peacetime Uses of Atomic Energy, 3rd rev. ed. (1975), a brief description of nuclear reactors and the uses of radioisotopes in industry, medicine, and scientific research. The story of the atomic bomb is told in William L. Laurence, Men and Atoms: The Discovery, the Uses, and the Future of Atomic Energy (1959, reissued 1962); James W. Kunetka, City of Fire: Los Alamos and the Atomic Age, 19431945, rev. ed. (1978); and Richard Rhodes, The Making of the Atomic Bomb (1986). Ellis P. Steinberg