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Fusion Reactions

Scientists now believe that energy produced in the sun and other stars is released as the result of a series of thermonuclear fusion reactions. The term fusion refers to the fact that two small nuclei, such as two hydrogen nuclei, fuse—or join together—to form a larger nucleus. The term thermonuclear means that such reactions normally occur only at very high temperatures, typically a few millions of degrees Celsius. Interest in fusion reactions arises not only because of their role in the manufacture of stellar energy, but also because of their potential value as sources of energy here on Earth.

Deuterium plays a critical role in most thermonuclear fusion reactions. In the solar process, for example, the fusion sequence appears to begin when two protium nuclei fuse to form a single deuteron. The deuteron is used up in later stages of the cycle by which four protium nuclei are converted to a single helium nucleus.

In the late 1940s and early 1950s scientists found a way of duplicating the process by which the sun's energy is produced in the form of thermonuclear fusion weapons, the so-called hydrogen bomb. The detonating device in this type of weapon was lithium deuteride, a compound of lithium metal and deuterium. The detonator was placed on the casing of an ordinary fission ("atomic") bomb. When the fission bomb detonated, it set off further nuclear reactions in the lithium deuteride which, in turn, set of fusion reactions in the larger hydrogen bomb.

For more than four decades, scientists have been trying to develop a method for bringing under control the awesome fusion power of a hydrogen bomb for use in commercial power plants. One of the most promising approaches appears to be a process in which two deuterons are fused to make a proton and a triton (the nucleus of a hydrogen-3 isotope). The triton and another deuteron then fuse to produce a helium nucleus, with the release of very large amounts of energy. So far, the technical details for making this process a commercially viable source of energy have not been completely worked out.



Asimov, Isaac. Asimov's Biographical Encyclopedia of Science and Technology. 2nd revised edition. Garden City, NY: Doubleday & Company, Inc., 1982.

Greenwood, N.N., and A. Earnshaw. Chemistry of the Elements. 2nd ed. Oxford: Butterworth-Heinneman Press, 1997. Joesten, Melvin D., David O. Johnston, John T. Netterville, and

James L. Wood. World of Chemistry. Philadelphia: Saunders, 1991.

Thomson, John F. Biological Effects of Deuterium. New York: Macmillan, 1963.

David E. Newton


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—A proton and a neutron; the nucleus of a deuterium atom.


—The name given to the isotope of hydrogen with atomic mass of one.

Additional topics

Science EncyclopediaScience & Philosophy: Dependency - The Intellectual Roots Of Dependency Thinking to Dirac equationDeuterium - Discovery, Urey's Search For Deuterium, Properties And Preparation, Uses, Fusion Reactions