B41 (nuclear weapon)

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The highest-yield nuclear weapon (up to 25 Mt [note 1] ) ever deployed by the United States, the B41 nuclear weapon was carried by the B-52 bomber and had been planned for B-70 Valkyrie armament. As accuracy improved, it was replaced by the B53. It was the only "three-stage" (i.e., fission-fusion-fission) fusion device produced by the United States of America.

There were two versions, produced between 1960 and 1962. Approximately 500 were built. As B53 replacements became available, the first B41s were retired in 1963, and the last in 1976. When deployed, there were two versions:[1]

  • Y1 − 25 Mt yield, "dirty" version, probably with a 238uranium tamper and a third stage
  • Y2 − "less than 10 Mt" yield, "clean", perhaps with a lead casing

Its published weight was 10,670 lb (4,840 kg), but that included parachutes, fuzes, etc. According to weapons designer Ted Taylor, the maximum possible yield-to-weight ratio is 6 kt/kg. When the precursor test devices, such as BASSOON, are considered, their weight, in the 9,000 lb (4,082 kg) range, suggests that the B41 yield may have been 5.2 kt/kg, the highest ratio in a United States weapon.


  1. The energy yields of nuclear weapons are commonly expressed in units of TNT equivalent, meaning the energy yield from the explosion of a stated amount of trinitrotoluene (TNT). The commonly used units are a kilotonne or a megatonne of TNT equivalent. A kilotonne (kt) of TNT equivalent is equal to 1012 joules and a megatonne (Mt) of TNT equivalent is equal to 1015 joules. The kilotonne and megatonne are often taken to be synonymous with kiloton and megaton.


  1. The B-41 (Mk-41) Bomb: High yield strategic thermonuclear bomb, Nuclear Weapons Archive, 21 October 1997