Science
IMPLOSION DESIGN
Science > Bomb Design and Components
While the gun-type bomb design seemed to be the most simple, a second bomb design used the principle of implosion. Conventional explosives located on the outside of the bomb core would detonate and create shock waves that could compress a shell or a sphere of sub-critical fissionable material into a denser super-critical assembly, and thereby trigger a fission chain reaction and a nuclear explosion. This technique was straightforward in principle, but required great precision in the explosives. The explosives around the bomb needed to be set off simultaneously, and needed to maintain perfect spherical symmetry in order to compress the fissile material without deforming it. A device called an initiator was added to improve the timing of the device by releasing neutrons to trigger fission at the optimal moment of compression. 
The first nuclear bomb, detonated at the Trinity test site, was an implosion design with a plutonium core, as was the "Fat Man" bomb dropped on Nagasaki on August 9, 1945. The basic idea had been discussed by Bob Serber and Richard Tolman in the early stages of bomb design, though it languished due to its difficulty until Seth Neddermeyer strongly pushed for implosion research in April 1943. This research program consisted of experiments with high explosives at the S-Site near Los Alamos. 
Implosion research became even more urgent with the discovery of spontaneous fission in plutonium in 1944, and the subsequent realization that a plutonium bomb could only be built with an implosion design. Further research into explosives helped make spherical compression possible, and Robert Christy suggested in September 1944 that a solid core would maintain its shape better than a hollow one. What the Christy design gained in reliability it lost in efficiency, though the addition of a sensitive initiator made it a useful design.
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The text for this page is original to the Department of Energy's Office of History and Heritage Resources. For further information, see Lillian Hoddeson, et al., Critical Assembly: A Technical History of Los Alamos during the Oppenheimer Years, 1943-1945 (Cambridge: Cambridge University Press, 1993) and Richard G. Hewlett and Oscar E. Anderson, Jr., The New World, 1939-1946: Volume I, A History of the United States Atomic Energy Commission (Washington: U.S. Atomic Energy Commission, 1972). The sketch of the implosion design is reproduced from Robert Serber's April 1943 "Los Alamos Primer," 21-22. The photograph of the unidentified man next to the gadget is courtesy of the Los Alamos National Laboratory. The diagram illustrating implosion is reproduced from the Department of Energy report Linking Legacies: Connecting the Cold War Nuclear Weapons Production Processes to their Environmental Consequences (Washington: Center for Environmental Management Information, Department of Energy, January 1997), 13.