"Fat Man" was possibly named after Winston Churchill, though Robert Serber said in his memoirs that as the "Fat Man" bomb was round and fat, he named it after Sydney Greenstreet's character of "Kasper Gutman" in The Maltese Falcon.
The device was 128 inches (3,300 mm) long, 5 feet (1.5 m) in diameter, and weighed 10,200 pounds (4,600 kg). In accordance with the name, it was more than twice as wide as Little Boy, which was dropped on Hiroshima three days earlier; however, the mass was only 15% more than that of Little Boy.
"Fat Man" was an implosion-type device using plutonium-239. A subcritical sphere of plutonium was placed in the center of a hollow sphere of high explosives. Thirty-two pairs of detonators located on the surface of the high explosive were fired simultaneously to produce a powerful inward pressure on the core, squeezing it and increasing its density, resulting in a supercritical condition and a nuclear initiation.
At first it was thought that two pieces of subcritical plutonium (Pu-239) could simply be shot into one another to create a nuclear explosion, and a plutonium gun-type design of this sort (known as the "Thin Man" bomb) was worked on for some time during the Manhattan Project. In April 1944, Emilio Segrè discovered that plutonium created for the bomb in the nuclear reactors at Hanford, Washington—even though it was supergrade plutonium containing only about 0.9% Pu-240—was not as pure as the initial samples of plutonium developed at the cyclotrons at Ernest O. Lawrence's Radiation Laboratory in Berkeley, California. Because of the presence of the Pu-240 isotope, reactor-bred plutonium had a much higher rate of spontaneous neutron emission than was previously thought, and if a gun-type device was used it would most likely pre-initiate and result in a messy and costly "fizzle". The spontaneous fission rate of Pu-240 is 40,000 times greater than that of Pu-239, so that in a gun-type plutonium device of the sort planned during the Manhattan Project, the last few centimeters would have to be traveled in less than 40 microseconds. After this problem was realized, the entire Los Alamos laboratory re-organized around the problem of the implosion bomb, the "Fat Man" starting in June 1944.
The difficulty in the design of an implosion device lay primarily in properly compressing the plutonium core into a near-perfect sphere; if the compression was not symmetrical it would cause the plutonium to be ejected from the weapon, making it an inefficient "dirty bomb". In order to accomplish the compression, the high explosive system had to be carefully designed as a series of explosive lenses which used alternating fast- and slow-burning explosives to shape the explosive shock wave into the desired spherical shape. An early idea of this sort had been raised by physicist Richard Tolman during early discussions of possible bomb designs, specifically in having many pieces of fissile material attached to explosives that would then assemble them in a spherical fashion. This idea was further developed by Seth Neddermeyer, who attempted to find a way to collapse a hollow sphere of plutonium onto a solid sphere of it inside itself. Neither of these ideas relied on compression of the plutonium, and neither would assemble the device fast enough to avoid preinitiation (see discussion below).
The idea of using shaped charges came from James L. Tuck and was developed by mathematician John von Neumann, and the idea that under such pressures the plutonium metal itself would be compressed may have come about from conversations with Edward Teller, whose knowledge of how dense metals behaved under heavy pressure was influenced by his theoretical studies of the Earth's core with George Gamow. Von Neumann and George Kistiakowsky eventually became the principal architects behind the lens system. Robert Christy is credited with doing the final calculations that showed that a solid subcritical sphere of plutonium could be compressed to a critical state greatly simplifying the task since earlier efforts had attempted the more difficult compression of 3D shapes like spherical shells. After Christy's report, the solid-plutonium core weapon was referred to as the "Christy Gadget".
Because of its complicated firing mechanism, and the need for previously untested synchronization of explosives and precision design, it was felt that a full test of the concept was needed before the scientists and military representatives could be confident it would perform correctly under combat conditions. On July 16, 1945, a device using a similar mechanism (called the "gadget" for security reasons) detonated in a test explosion at a remote site in New Mexico, known as the "Trinity" test. It gave about 20 kt (80 TJ), 2 to 4 times the expected yield. The gun-type method, though inadequate for plutonium, could still be used for highly enriched uranium, and was employed in the "Little Boy" device, used against Hiroshima. The implosion method is more efficient than the gun-type method, and also far safer, as a perfect synchronization of the explosion lenses is required for the core to properly detonate, greatly reducing the chances of an accidental nuclear initiation. After the success of the first implosion "gadget", almost all subsequent American fission designs utilized implosion, with a rare few that used the gun-type design out of special design requirements (like extreme narrowness of weapon, such as nuclear artillery). The Soviet Union's first nuclear weapon detonated at Operation First Lightning (known as "Joe 1" in the West) was closely based on the "Fat Man" device, on which they had obtained detailed information from the spies Klaus Fuchs, Theodore Hall, and David Greenglass.
The device was 128 inches (3,300 mm) long, 5 feet (1.5 m) in diameter, and weighed 10,200 pounds (4,600 kg). In accordance with the name, it was more than twice as wide as Little Boy, which was dropped on Hiroshima three days earlier; however, the mass was only 15% more than that of Little Boy.
"Fat Man" was an implosion-type device using plutonium-239. A subcritical sphere of plutonium was placed in the center of a hollow sphere of high explosives. Thirty-two pairs of detonators located on the surface of the high explosive were fired simultaneously to produce a powerful inward pressure on the core, squeezing it and increasing its density, resulting in a supercritical condition and a nuclear initiation.
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The idea of using shaped charges came from James L. Tuck and was developed by mathematician John von Neumann, and the idea that under such pressures the plutonium metal itself would be compressed may have come about from conversations with Edward Teller, whose knowledge of how dense metals behaved under heavy pressure was influenced by his theoretical studies of the Earth's core with George Gamow. Von Neumann and George Kistiakowsky eventually became the principal architects behind the lens system. Robert Christy is credited with doing the final calculations that showed that a solid subcritical sphere of plutonium could be compressed to a critical state greatly simplifying the task since earlier efforts had attempted the more difficult compression of 3D shapes like spherical shells. After Christy's report, the solid-plutonium core weapon was referred to as the "Christy Gadget".
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