W48
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The W48 was an American nuclear artillery shell, fired from a standard 155 mm (6.1 inch) howitzer. It was manufactured starting in 1963, and all units were retired in 1992.
The W48 was 6.1 inches (155 mm) in diameter and 33.3 inches long. It came in two models, Mod 0 and Mod 1, which are reported to have weighed 118 and 128 pounds respectively. It had an explosive yield equivalent to 72 tons of TNT (0.072 kiloton), which is very small for a nuclear weapon.
Linear implosion
The W48 was a small diameter linear implosion nuclear fission weapon. Larger, "normal" implosion nuclear weapons use less nuclear material than is required to form a critical mass at normal pressure and configurations, and use large precise explosive assemblies to collapse the nuclear material to many times normal density and attain critical mass.
Linear implosion uses a mass of nuclear material which is more than one critical mass at normal pressure and a spherical configuration. The mass, known as the "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 begins chain reaction, goes critical, and then expands in a small nuclear blast. Three methods are known to compress and reshape the nuclear material: collapsing hollow spaces inside the nuclear material, 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 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 (highest density) alpha-phase plutonium at a density of 19.8 g/cm³ is 657 cm³, 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 2-3 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 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 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 2-3 times more plutonium, are extremely expensive.