W79 Artillery-Fired Atomic Projectile
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The W79 was an American nuclear artillery shell, capable of being fired from any standard 8 inch (203 mm) howitzer e.g. the M115 & M110 howitzer.[1]
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.[2]
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 rugby-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.