TheMark 18 nuclear bomb, also known as theSOB orSuperOralloy Bomb, was an Americannuclear bomb design which was thehighest yieldfission bomb produced by the US. The Mark 18 had a design yield of 500kilotons. Nuclear weapon designerTed Taylor was the lead designer for the Mark 18.
The Mark 18 was tested once, in theIvy King nuclear test at theEnewetak atoll in thePacific Ocean on November 16 1952. The test was a complete success at full yield.
The Mark 18 bomb design used an advanced 92-point implosion system, derived from theMark 13 nuclear bomb and its ancestors theMark 6 nuclear bomb,Mark 4 nuclear bomb, andFat Man Mark 3 nuclear bomb ofWorld War II. Its normal mixeduranium/plutoniumfissile core ("pit") was replaced with over 60 kg of purehighly enriched uranium or HEU. With a natural uraniumtamper layer, the bomb had over fourcritical masses of fissile material in the core, and was unsafe: the accidental detonation of even one of the detonator triggers would likely cause a significant (many kilotons of energy yield) explosion. Analuminum/boron chain designed to absorbneutrons was placed in the fissile pit to reduce the risk of accidental high yield detonation, and removed during the last steps of the arming sequence.[1][2]
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 bythermonuclear weapons in the mid-1950s. The Mark 18 weapons were all modified into lower yieldMark 6 nuclear bomb variants in 1956.