Nuclear fission is the splitting of a heavy nucleus to form two nuclei of roughly the same mass, in addition to several neutrons. Fission rarely occurs spontaneously and is induced when a single neutron collides with a nucleus being captured as a result. This releases neutrons, which in turn may cause further fission and so on in a chain reaction. This process releases a sizeable amount of energy such that the energy released per atom by fission is roughly 50 million times greater than that per atom from a chemical reaction such as burning.

Uncontrolled chain reactions are used in nuclear weapons while controlled ones take place in nuclear reactors and realease at a steady rate. To maintain a chain reaction, a minimum of one neutron from each fission must cause further fission. If the fission material is less than a certain critical size, too many neutrons escape without hitting nuclei.

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ANDREW SIDELL / May 2001 /