"Fat Man" is the codename for the atomic bomb that was detonated over Nagasaki, Japan, by the United States on August 9, 1945. It was the second of the only two nuclear weapons to be used in warfare and was the third man-made nuclear explosion at that time. The name also refers more generically to the early nuclear weapon designs of U.S. weapons based on the "Fat Man" model. It was an implosion-type weapon with a plutonium core.
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 design of "Fat Man" nuclear assembly was substantially the same as "the gadget" detonated at the Trinity test in July 1945.
"Fat Man" was detonated at an altitude of about 1,800 feet (550 m) over the city, and was dropped from a B-29 bomber Bockscar, piloted by Major Charles Sweeney of the 393d Bombardment Squadron, Heavy. The bomb had a yield of about 21 kilotons of TNT, or 8.78×1013 joules = 88 TJ (terajoules). Because of Nagasaki's hilly terrain, the damage was somewhat less extensive than that in relatively flat Hiroshima. An estimated 39,000 people were killed outright by the bombing at Nagasaki, and about 25,000 were injured. Thousands more would die later from related blast and burn injuries, and hundreds more from radiation illnesses from exposure to the bomb's initial radiations. The aerial bombing raid on Nagasaki had the third highest fatality rate in World War II after the nuclear strike on Hiroshima and the March 9/10 1945 fire bombing raid on Tokyo.
The weapon was 10 feet 8 inches (3.25 m) long, five feet (1.52 m) in diameter, and weighed 10,200 pounds (4,630 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 10% more than that of Little Boy.
"Fat Man" was an implosion-type weapon using plutonium 239. A subcritical sphere of plutonium was placed in the center of a hollow sphere of high explosive. 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 explosion.
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. However in April 1944 it was discovered by Emilio Segrè that plutonium created for the bomb in the nuclear reactors at Hanford, Washington—even though it was supergrade weapon 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-detonate 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 weapon 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 shockwave 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 predetonation (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 Gamov. 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 utilizing 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 somewhere around 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" weapon, which was 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 detonation. 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, for 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 names for all three projects ("Fat Man", "Thin Man", and "Little Boy") were created by Robert Serber, a former student of Los Alamos director Robert Oppenheimer's who worked on the project, according to Serber. According to his later memoirs, he chose them based on their design shapes; the "Thin Man" would be a very long device, and the name came from the Dashiell Hammett detective novel and series of movies by the same name; the "Fat Man" bomb would be round and fat and was named after Sidney Greenstreet's character in The Maltese Falcon. "Little Boy" would come last and be named only to contrast to the "Thin Man" bomb. Other sources say the two initial names came from an Air Force cover story that a pair B-29 bombers were being modified to serve as transport planes for Franklin Roosevelt ("Thin Man"), and Winston Churchill ("Fat Man").
Below is a diagram of the main parts of the "Fat Man" bomb itself, followed by a more detailed look at the different materials used in the physics package of the bomb (the part responsible for the nuclear detonation).
To allow insertion of the plutonium pit as late as possible in the bomb's assembly, the spherical U-238 tamper had a 4" diameter cylindrical hole running through it, like the hole in a cored apple. The missing cylinder, containing the plutonium pit, could be slipped in through a hole in the surrounding aluminium pusher. In August 1945, it was assembled on Tinian Island. When the physics package was fully assembled and wired, it was placed inside its ellipsoidal aerodynamic bombshell and wheeled to the bomb bay of a nameless B-29 only called #77. This was later named "Bockscar" after its flight to Nagasaki on August 9 to honor command pilot, Fred Bock, according to the Atomic Museum. http://www.atomicmuseum.com/Tour/dd2.cfm
In 2003, these concentric spheres and cylinder were recreated as the centerpiece of an art installation called "Critical Assembly" by sculptor Jim Sanborn. Using non-nuclear materials, he replicated the internal components of the "Trinity" bomb, which had the same design as Fat Man. Critical Assembly was first displayed at the Corcoran Gallery of Art, in Washington, DC.
The plutonium must be compressed to twice its normal density before free neutrons are added to start the fission chain reaction:
The result is that about two and a half of the thirteen pounds of plutonium in the pit, (about 20% of the 6.2 kg ) underwent fission, and converted probably less than 1 gram of mass into energy, and released the energy equivalent of twenty-one thousand tons (21 kT) of TNT.