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The name Fat Man refers to the early design of the bomb because it had a wide, round shape. Fat Man was an implosion-type nuclear weapon with a solid plutonium core. The first of that type to be detonated was the Gadget in the Trinity nuclear test less than a month earlier on 16 July at the Alamogordo Bombing and Gunnery Range in New Mexico.
The Mark 5 design was the first production American nuclear weapon which, with a diameter of 39 in (990 mm), was significantly smaller than the 60 in (1,520 mm) diameter implosion system of the 1945 Fat Man nuclear bomb design.
The Fat Man device specifically used 6.2 kg (14 lb), about 350 ml or 12 US fl oz in volume, of Pu-239, which is only 41% of bare-sphere critical mass (see Fat Man article for a detailed drawing). Surrounded by a U-238 reflector/tamper, the Fat Man's pit was brought close to critical mass by the neutron-reflecting properties of the U-238. During ...
Fat Man was the second nuclear weapon to be deployed in combat after the US dropped a 5-ton atomic bomb, called "Little Boy," on the Japanese city of Hiroshima.
Mark 3 – "Fat Man" plutonium implosion weapon (used against Nagasaki), effectively the same as the "Gadget" device used in the Trinity nuclear test with minor design differences. (21 kilotons, 1945–1950) Mark 4 – Post-war "Fat Man" redesign. Bomb designed with weapon characteristics as the foremost criteria. (1949–1953)
The pits of the first nuclear weapons were solid, with an urchin neutron initiator in their center. The Gadget and Fat Man used pits made of 6.2 kg of solid hot pressed plutonium-gallium alloy (at 400 °C and 200 MPa in steel dies – 750 °F and 29,000 psi) half-spheres of 9.2 cm (3.6 in) diameter, with a 2.5 cm (1 in) internal cavity for the initiator.
The original gun-type plutonium weapon, code-named "Thin Man", had to be abandoned as a result—the increased number of spontaneous neutrons meant that nuclear pre-detonation was likely. [90] The entire plutonium weapon design effort at Los Alamos was soon changed to the more complicated implosion device, code-named "Fat Man".
Theodore Alvin Hall (October 20, 1925 – November 1, 1999) was an American physicist and an atomic spy for the Soviet Union, who, during his work on United States efforts to develop the first and second atomic bombs during World War II (the Manhattan Project), gave a detailed description of the "Fat Man" plutonium bomb, and of several processes for purifying plutonium, to Soviet intelligence.