Science
FISSION
Science > Nuclear Physics
A fission reaction occurs when a nucleus splits apart, releasing energy. Some unstable atoms randomly undergo "spontaneous fission" and are said to be radioactive. However, in order to release large amounts of energy (such as in a bomb), naturally occurring radioactive processes are insufficient. To create a nuclear explosion or generate useable power in a reactor, a self-sustaining chain reaction (at right) is required. In a nuclear fission chain reaction, a free neutron collides with the nucleus of an atom and causes that nucleus to split apart. The ruptured nucleus in turn releases additional neutrons, which can cause additional nuclei to split, and so on. (In the case of uranium-235, for example, each fission reaction produces two or three additional free neutrons, each of which may in turn strike additional uranium nuclei and cause them to fission.) A controlled chain reaction of this sort can be used to generate nuclear power; an uncontrolled chain reaction can result in a nuclear explosion. Chain reactions are initiated when a quantity of fissionable material, such as uranium-235 or plutonium, reaches critical mass. 
Fission of an atom's nucleus results in the formation of lighter elements (fission products), other miscellaneous material, and the release of energy. In the case of uranium-235 over 80% of this energy takes the form of the kinetic energy of fast-moving fission products, with the rest manifesting itself in the form of accelerated neutrons and various forms of radioactivity. In a nuclear reactor, about 90% of the total energy released manifests itself as heat, again mostly in the form of kinetic energy of fission products. Click the "Los Alamos Primer" at right to read Robert Serber's lecture on the Energy of Fission Process (page 1). The discovery of fission arose out of interpretations of work done by Enrico Fermi's group in Rome on the effects of neutron bombardment on known elements. In 1934 Fermi's group found that the bombardment of uranium (the heaviest known element) by slow neutrons yielded products with mysterious and unexpected radioactive properties. They subsequently concluded that the material produced was mysterious because it was a heretofore unknown element heavier than uranium; bombarding uranium with neutrons, they thought, had yielded the first "transuranic" element. This announcement was lauded in Italy, and Fermi would be awarded the 1938 Nobel Prize in physics for his work. Following up on Fermi's result, two important research centers in Paris and Berlin arose, focused on interpreting the products of Fermi's neutron irradiated uranium. By late 1938, Berlin chemists Otto Hahn and Friedrich Strassman working to make sense of a recent result out of Paris from Irene Joliot-Curie and Yugoslavian physicist Pavel Savitch, struggled to make sense of their growing evidence that one product of neutron bombarded uranium was, very unexpectedly, a much lighter element (read an English translation). Hahn and Strassman corresponded with physicists Lise Meitner (then living in Stockholm after fleeing Hitler's Reich in the summer) and her nephew Otto Frisch, searching for a possible mechanism that would allow a heavy element to transmute into a much lighter one. The physicists indeed found a theoretical foundation for the Hahn-Strassman result—the nucleus, they thought, could violently split to produce nuclei of much smaller elements. You can read their published result in a brief letter to the journal Nature.
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The text for this page is original to the Department of Energy's Office of History and Heritage Resources. Major sources consulted include the following. An excellent resource for learning about the history of the discovery of fission, from first hand recollections is the American Institute of Physics online exhibit http://www.aip.org/history/mod/fission/fission1/01.html. Book length studies include Helge Kragh, Quantum Generations: a History of Physics in the Twentieth Century (Princeton: Princeton University Press, 1999); and F. G. Gosling, The Manhattan Project: Making the Atomic Bomb (DOE/MA-0001; Washington: History Division, Department of Energy, January 1999). See also John F. Hogerton, ed., "Chain Reaction," "Fission (Nuclear)," and "Multiplication Factor," The Atomic Energy Deskbook (New York: Reinhold Publishing Corporation, 1963; prepared under the auspices of the Division of Technical Information, U.S. Atomic Energy Commission), 96-97, 168-170, and 317. The fission chain reaction graphic is adapted from a graphic originally produced by the Washington State Department of Health; the modifications are original to the Department of Energy's Office of History and Heritage Resources. The lecture on energy of fission process comes from Serber, Robert, The Los Alamos Primer, Los Alamos, NM: April 1943.