Thin Man (nuclear bomb) Thin Man (nuclear bomb) "Thin Man" was the code name for a proposed plutonium gun-type nuclear bomb using plutonium-239 which the United States was developing during the Manhattan Project. They aborted its development when they discovered that the spontaneous fission rate of their nuclear reactor-bred plutonium was too high for use in a gun-type design, due to the high concentration of the isotope plutonium-240. In 1942, prior to the Army taking over wartime atomic research, Robert Oppenheimer held conferences in Chicago in June and Berkeley, California, in July at which various engineers and physicists discussed nuclear bomb design issues. A
Mark 18 nuclear bomb placed in the fissile pit to reduce the risk of accidental high yield detonation, and removed during the last steps of the arming sequence. Beginning in March 1953, the United States deployed a number of Mark 18 bombs. A total of 90 were manufactured and placed in service. The weapon had a short lifetime, and was replaced by thermonuclear weapons in the mid-1950s. The Mark 18 weapons were all modified into lower yield Mark 6 nuclear bomb variants in 1956. Mark 18 nuclear bomb The Mark 18 nuclear bomb, also known as the SOB or Super Oralloy Bomb, was an