W48 nuclear artillery
The US developed several nuclear artillery shells in the 155 mm caliber. The only one to be deployed was the W48 nuclear warhead developed by UCRL, packaged in the M45 AFAP (artillery fired atomic projectile) shell. The W48 nuclear warhead measured 86 cm (34") long and weighed 53.558 kg (118128 lbs). Its yield was on the order of 70 to 100 tons (it was tested in the Hardtack II Tamalpais shot with a yield of 72 tons, predicted yield was 100300 tons).
The smallest diameter US test device publicly known was the Livermore Swift device fired in the Redwing Yuma shot on 28 May 1956. It had a 5" (12.7 cm) diameter, a length of 62.2 cm (24.5 inches) and weighed 43.5 kg (96 lb). The test had a yield of 190 tons, but was intended to be fusion boosted (and thus would probably have had a yield in the kiloton range) but its yield was insufficient to ignite the fusion reaction and it failed to boost in this test. This test may have been a predecessor to the W48 design.
The W48 was 846 mm long and weighing 58 kg, it could be fitted in a 155 mm M45 AFAP (artillery fired atomic projectile) and used in the standard 155 mm howitzer. The fission warhead was a linear implosion type, consisting of a long cylinder of subcritical mass which is compressed and shaped by explosive into a supercritical mass. The W48 yielded just 72 tons TNT equivalent. The W48 entered production in 1963, and 135 of the Mod 0 variant were built. The Mod 0 variant was retired in 1968. It was retired. It was replaced by the Mod 1 which was manufactured from 1965 through 1969, with 925 of this type being built.
W48 was an atomic artillery shell for 155 mm howitzer M114 or M198. It was produced since 1963, and was withdrawn from service in 1992. The length of W48 was 85 cm. There were two modifications  Mod 0 and Mod 1, with a mass of 53.5 and 58 kg, respectively. The projectile capacity was 0.072 kt.
In W48, a linear implosion scheme with a small diameter of a nuclear device was used. In "conventional" implosive nuclear explosive devices, small amounts of fissile material are used, which are less than the critical mass under normal conditions. With the use of large and accurate explosive assemblies, the fissile material strongly compresses, resulting in a critical density.
With linear implosion, the mass of nuclear material used must exceed the critical mass. Initially, nuclear material, known as "pit", has a form different from spherical, in which the density is less than critical. When the device is triggered with a small amount of explosive, the shape of the fissile material changes so that it reaches a supercritical mass and a nuclear reaction begins, resulting in a nuclear explosion of low power.
Three methods of compression and change in the shape of nuclear material are known: the collapse of voids inside the pit, the use of a plutoniumgallium alloy that stabilizes in the delta phase of low density, and passes into a denser alpha phase under the influence of a blast wave, which is compressed into the sphere under the action of the explosion.
The net critical mass of plutonium at normal density and without using a neutron reflector is approximately 10 kilograms. To achieve the greatest power, linear implosion requires a greater amount of plutonium  about 13 kilograms of alphaphase plutonium with a density of 19.8 g / cm³. This is 657 cm³, that is, a sphere with a radius of 5.4 cm (diameter 10.8 cm).
Linear implosive weapons can use tamper or reflectors, but the total diameter of the fissile material plus the tamper / reflector increases in comparison with the volume required for a feed without a reflector. To accommodate the nuclear warhead, artillery shells of medium caliber (155 and 152 mm, according to Ted Taylor, a nuclear weapon designer, a 105mmhigh nuclear shell can be created), a "bare" pit may be required.
Nuclear weapons made by the linear implosion scheme are much less effective because of the lower pressure during the initiation process and requires two to three times more nuclear material than in the case of spherical implosion. It is also significantly heavier and much smaller in size than conventional implosion weapons. The nuclear warhead W54, used for the recoilless projectile Davy Crockett, had a diameter of about 280 mm and 23 kg. The diameter of W48 is 152 mm, while it weighs twice as much and requires about twice the amount of plutonium.
For the Livermoredesigned weapon systems being retired, the Lab had a continuing active responsibility to ensure safe and timely dismantlement and disposition of excess materials. In 1996, Livermore completed dismantlement of the W48 artillery projectiles, the W55 SUBROCs, and the W70 Lance warheads.
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