The W80 is a low to intermediate yield two-stage thermonuclear warhead deployed by the U.S. enduring stockpile with a variable yield ("dial-a-yield") of . It was designed for deployment on cruise missiles and is the warhead used in all nuclear-armed ALCM and ACM missiles deployed by the US Air Force, and in the US Navy's BGM-109 Tomahawk. It is essentially a modification of the widely deployed B61 weapon, which forms the basis of most of the current US stockpile of nuclear gravity bombs. The very similar W84 warhead was deployed on the retired BGM-109G Ground Launched Cruise Missile. Dimensions The W80 is physically quite small: the physics package itself is about the size of a conventional Mk.81 bomb, in diameter and long, and only slightly heavier at about . History Early development The Los Alamos National Laboratory began development on the W80 in June 1976, with the brief of producing a custom weapon for the cruise missiles then under construction. With "the basic design" being derived from the B61. The main design differences are presumably a smaller secondary producing only yield (the B61 producing a maximum of in the tactical variants and in the strategic variants) and simplification of the design giving the weapon only two yield settings; . Production of the W80 mod 1 (W80-1) to arm the ALCM started in January 1979, and a number of warheads had been completed by January 1981 when the first low-temperature test was carried out. To everyone's surprise the test delivered a much lower yield than was expected, apparently due to problems in the TATB based insensitive high explosives used to fire the primary. This problem turned out to affect several models of the B61-based line, and production of all weapons was suspended while a solution was worked on. Production restarted in February 1982. In March 1982, designers began working on a W80 variant intended for the Navy's Tomahawk program. The W80 mod 0 (W80-0) used "supergrade" fission fuel, which has less radioactivity, in the primary in place of the conventional plutonium used in the Air Force's version. "Supergrade" is industry parlance for plutonium alloy bearing an exceptionally high fraction of Pu-239 (>95%), leaving a very low amount of Pu-240 which is a gamma emitter in addition to being a high spontaneous fission isotope. Such plutonium is produced from fuel rods that have been irradiated a very short time as measured in