07-16-07, 10:08 AM
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#14
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Stowaway
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The easiest weapon to build uses a large amount (tens of kilograms) of enriched uranium. Because uranium releases neutrons at a very low rate, the weapon can use a relatively long "assembly time" to reach supercriticality. One design uses a sphere with a cylindrical hole in it, and a "gun" to fire a cylinder of uranium into that hole. Until the cylinder is inserted, both assemblies are well below critical mass, but when the cylinder is inserted, the mass rapidly rises to supercriticality. A neutron randomly released by the material during this process triggers the chain reaction. This weapon is so simple that the US used one against Nagasaki without ever testing the design. These weapons tend to be fairly large and inefficient, although the design was used in a US nuclear artillery shell.
A plutonium based weapon cannot use the "gun" approach, because plutonium releases too many neutrons, which would cause the chain reaction to start long before the mass was supercritical enough to cause a large explosion. Hence plutonium weapons require assembly by compressing a sphere or shell of plutonium very rapidly, using high velocity explosives. This neccessitates very high quality explosives, a very precise machining of all parts, and an electrical detonating system which can deliver very high energy pulses to a number of detonators with great timing precision. Hence plutonium based weapons are significantly harder to build.
http://www.tinyvital.com/Misc/nukes.htm
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