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How much U-235 will make a big bomb? The type of U-235-based atom bomb that was dropped on Hiroshima is known as the gun bomb design. In essence, super-critical mass is achieve within the device by uniting one sub-critical chunk of U-235 with another, by shooting it down the barrel of an artillery tube, where the propellant used is similar to conventional artillery-shell propellant. The Hiroshima weapon's yield was about 12 kilotons. Some sources put the total amount of U-235 refined into bomb grade (say, 90% or better purity) that's needed in this design as something like forty pounds. Less than that, and the gun bomb won't accomplish super-criticality: no big blast and no mushroom cloud. So, it would seem that forty pounds is a barrier, a limiting factor, in this approach to mass destruction. Less than forty pounds, the bad guys don't got an A-bomb.
However, less widely known is that uranium can be used in the different type of bomb design that was tested in New Mexico and dropped on Nagasaki. This other approach, the implosion design, must be used if the fuel is plutonium. (Because of its differing, more temperamental nuclear properties, plutonium in a gun bomb would predetonate, or fizzle.) An implosion bomb is a much more subtle and delicate thing, harder to construct from scratch than a gun bomb. However, an implosion bomb can indeed be fueled by weapons grade uranium instead of plutonium. Sources estimate that an implosion design U-235 bomb would need only twenty pounds of fuel, instead of forty. This is because the implosion approach assembles a super-critical mass more efficiently. The barrier to WMD implementation has just been cut in half. Not good news.
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It gets worse. There are variants of the implosion design. The simplest one uses specially shaped high-explosive lenses to collapse a hollow sphere of plutonium - or uranium. The hollow sphere represents latent atomic blast power, waiting to be unleashed. The weight of fuel is adequate to achieve super-criticality. Its spatial arrangement, however, is all wrong - not enough fuel packed down to small enough volume. But when that hollow sphere is squashed at extremely rapid speed into a smaller but solid sphere, the new configuration has the density needed to achieve a "fast prompt critical disassembly" - the technobabble euphemism for a nuclear detonation.
More sophisticated implosion designs do more than simply collapse a hollow sphere into a solid one. They achieve such a strong and perfectly shaped implosive force that the solid sphere is actually compressed. Compression into a volume as small as one-fourth that of unstressed solid metal has been reported. In other words, the density of the bomb fuel is as much as quadrupled. Because of the physics involved, this higher density requires less total weight of fuel for a large atomic explosion to occur. The degree of reduction in the amount of fuel required by such a compression-implosion device is classified. I make a semi-educated guess that if the bomb quadruples the density, the weight of fuel needed might be cut in half. If so, ten pounds of weapons grade U-235 would achieve a yield of several kilotons, maybe even a dozen - Hiroshima-scale devastation with only one quarter the fuel. Or, viewed differently, four times as numerous an arsenal for the same amount of fuel. Frightening thoughts.
Conclusion: A centrifuge cascade alone does not an atom bomb make. You need the raw uranium, and you need the means to assemble a super-critical mass of bomb grade fuel. But yellow cake in quantities of interest to rogue states and terrorists is everywhere, so restriction of the supply is much more difficult that it looks. And advanced implosion bomb designs, plus the technology to make them, might become more widely available soon, if they aren't already. When these designs greatly shrink the weight of bomb grade U-235 needed to produce a Hiroshima-sized nuclear blast, Armageddon may be closer than we realize.
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© 2005 Joe Buff. All opinions expressed
in this article are the author's and do not necessarily reflect
those of Military.com.
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