![]() South Africa also developed six nuclear bombs based on the gun-type principle, and was working on missile warheads using the same basic design – See South Africa and weapons of mass destruction. The third family was an 8-inch (203 mm) artillery shell, the W33. The second program was a family of 11-inch (280 mm) nuclear artillery shells, the W9 and its derivative W19, plus a repackaged W19 in a 16-inch (406 mm) shell for US Navy battleships, the W23. Both the Mark 8 and Mark 11 designs were intended for use as earth-penetrating bombs (see nuclear bunker buster), for which the gun-type method was preferred for a time by designers who were less than certain that early implosion-type weapons would successfully detonate following an impact. First, the " Little Boy" weapon which was detonated over Hiroshima and several additional units of the same design prepared after World War II, in 40 Mark 8 bombs, and their replacement, 40 Mark 11 bombs. The method was applied in four known US programs. The bomb would use the gun-type design "to bring the two halves together at high velocity and it is proposed to do this by firing them together with charges of ordinary explosive in a form of double gun". 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 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. Instead, gun type bombs assemble the supercritical mass by amassing such a large quantity of uranium that the overall distance through which daughter neutrons must travel has so many mean free paths it becomes very probable most neutrons will find uranium nuclei to collide with, before escaping the supercritical mass. The main reason for this is the uranium metal does not undergo compression (and resulting density increase) as does the implosion design. The required amount of uranium is relatively large, and thus the overall efficiency is relatively low. Production of impurity-free plutonium is very difficult and is impractical. Since it is a relatively slow method of assembly, plutonium cannot be used unless it is purely the 239 isotope. Its name is a reference to the fact that it is shooting the material through an artillery barrel as if it were a projectile. 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 spike, which fills the hole in its center. ![]() ![]() 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. Fission-based nuclear weapon The "gun" assembly method ![]()
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