W79 The W79 was an American nuclear artillery shell , fired from any standard 8 inch ( 203 mm ) howitzer e . g . the M115 . The W79 was produced in two models , the " W79 Mod 0 " and " W79 Mod 1 " . Both were a plutonium - based linear - implosion , nuclear weapon . The " Mod 0 " was a variable yield device with three yields , ranging from 100 tons up to 1 . 1 kiloton and an enhanced - radiation ( popularly known as Neutron bomb ) mode which could be turned on or off . The " Mod 1 " was fission only , without the enhanced - radiation option , and had a fixed 0 . 8 kiloton yield ( 800 tons of TNT ) . This probably corresponds with the maximum pure - fission yield of the " Mod 0 " . Both models were 8 inches ( 203 mm ) in diameter , 44 inches ( 112 cm ) long and weighed 200 pounds ( 90 kg ) . The W79 was produced starting at least as early as 1976 and continuing into 1986 . All units were retired from active service by the end of 1992 . Linear implosion . Linear - implosion uses a mass of nuclear material which is more than one critical mass at normal pressure and in a spherical configuration . The mass , known as pit , is configured in a lower density non - spherical configuration prior to firing the weapon and then , small to moderate amounts of explosive collapse and slightly reshape the nuclear - material into a supercritical - mass which then undergoes chain - reaction and explodes . Three methods are known to compress and reshape the nuclear - material : 1 ) collapsing hollow spaces inside the nuclear material ; 2 ) using plutonium - gallium alloy , which is stabilized in the low - density delta - phase at a density of 16 . 4 ( and which collapses to denser alpha - phase under moderate explosive - compression ) ; and 3 ) shaping an explosive and nuclear material so that the explosive pressure changes a stretched - out , elliptical or football shape to collapse towards a spherical or more spherical end - shape . A bare critical mass of plutonium at normal density and without additional neutron reflector material is roughly 10 kilograms . To achieve a large explosive - yield , a linear - implosion weapon needs somewhat more material , on the order of 13 kilograms . 13 kilograms of alpha - phase ( highest density ) plutonium at a density of 19 . 8 g / cm³ is 657 cubic centimeters , a sphere of radius 5 . 4 cm ( diameter 10 . 8 cm / 4 . 25 inches ) . Linear - implosion weapons could use tampers or reflectors , but the overall diameter of the fissile - material plus tamper / reflector increases compared to the volume required for an untamped , unreflected pit . To fit weapons into small artillery - shells ( 155 mm and 152 mm are known ; 105 mm has been alleged to be possible by nuclear - weapon designer Ted Taylor ) , bare pits may be required . Linear - implosion weapons have much lower efficiency due to low pressure , and require two to three times more nuclear - material than conventional implosion weapons . They are also considerably heavier , and much smaller than conventional implosion weapons . The W54 nuclear warhead used for special purposes and the Davy Crockett nuclear - artillery unit was about 11 inches diameter and weighs 51 pounds . The 155 mm W48 is 6 inches in diameter and weighs over twice as much , and probably requires twice as much plutonium . Independent researchers have determined that one model of US Army conventional implosion fission - weapon cost $ 1 . 25 million per - unit produced , of which $ 0 . 25 million was the total cost for all non - nuclear components and $ 1 million the cost of the plutonium . Linear - implosion weapons , requiring two to three times more plutonium , are considerably more expensive .