assemblage par insertion

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Gun-type fission weapons are fission-based nuclear weapons whose design assembles their fissile material into a supercritical mass by the use of the "gun" method: shooting one piece of sub-critical material into another. Although this is sometimes pictured as two sub-critical hemispheres driven together to make a supercritical sphere, typically, a hollow projectile is shot onto a cylindrical spike, which fills the hole in its center. Its name is a reference to the fact that it is shooting the material through an artillery barrel as if it were a projectile. Developed and deployed by the Manhattan Project, gun-type designs were quickly replaced by the more efficient implosion-type weapons. All known gun-type fission weapons have used highly enriched uranium (HEU). The high spontaneous fission rates of plutonium isotopes make it very impractical for use in gun-type designs, as in the abandoned Thin Man design. Additionally, the efficiency is low, increasing the amount of HEU required and weapon weight. The main reason for this is the fissile material does not undergo compression (and resulting density increase) as does the implosion design. Instead, gun-type bombs assemble the supercritical mass by amassing such a quantity of uranium that daughter neutrons must travel enough mean free paths of distance that they collide with more than one 235U nucleus on average, before escaping the supercritical mass. HEU could be more efficiently used by the composite cores of early implosion-type weapons. The first time gun-type fission weapons were discussed was as part of the British Tube Alloys nuclear bomb development program, the world's first nuclear bomb development program. The British MAUD Report of 1941 laid out how "an effective uranium bomb which, containing some 25 lb of active material, would be equivalent as regards destructive effect to 1,800 tons of T.N.T". The bomb would use the gun-type design...