Science
CRITICAL MASSScience > Nuclear PhysicsThe critical mass of a fissionable substance is the minimum amount of fissionable material that will support a self-sustaining chain reaction. At this mass the neutrons released as a product of one fission reaction can cause neighboring atoms to fission. A mass of material that is made to exceed the critical mass and fissions at an increasing rate is "supercritical" while masses insufficient to support a chain reaction are termed "subcritical." If the reaction is uncontrolled, as is the case in an atomic bomb, the critical mass of fissionable material creates an explosion. If the reaction is controlled, as in reactors, the critical mass can provide atomic power. Mirroring the language for bombs, self-sustaining reactors are said to have achieved "criticality" or "gone critical." The "critical size" of a reactor is the minimum size of the core and the reflector that surrounds it required to maintain a chain reaction. 
The critical mass of a fissionable substance depends upon its density, purity, and design. When a mass of fissionable material increases in density, the mass required to achieve criticality decreases. Furthermore, the density of a substance will increas if its volume decreases. Thus rapidly decreasing the radius of a mass of fissionable material can rapidly achieve criticality. During the Manhattan Project, scientists designing the implosion type weapon, which was tested at Trinity and dropped on Nagasaki, exploited precisely this relationship between density and critical mass. Manhattan Project scientists designing the bomb also developed certain elements to help sustain a chain reaction and to make criticality easier to achieve. Material called tampers, used in both the gun and the implosion designs, for example, reflected escaping neutrons back into the mass, and thus decreased the critical mass required to sustain a chain reaction. Click the "Los Alamos Primer" at right to read Robert Serber's lecture on critical mass estimates (pages 5-7).
|
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. John F. Hogerton, ed., "Critical Mass" and "Critical Size," 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), 113. The lecture on critical mass estimates comes from Serber, Robert, The Los Alamos Primer, Los Alamos, NM: April 1943.