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Originally Posted by Locke
As far as I understand - enriched uranium is just one of the many steps required to produce a working atomic weapon. Through the use of various pieces of equipment, you seperate the U235 (middle weight isotope) from the mined uranium, usually by gaseous diffusion (uranium tetra-chloride), or centrifuges (iran is using the latter iirc).
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If I recall correctly, gaseous diffusion isn't being used anymore, unless the plants are already built. It's inefficient when compared to centrifuges or other somewhat still experimental methods.
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Originally Posted by Locke
Even though this is a major step - they are still far away from creating a working weapon - they still have to deal with obtaining the right weights for a critical mass, and perfecting the timers and explosives to the accuracy needed to acheive that critical mass. etc...
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Sadly enough, this is not entirely true. What Iran has is enriched uranium, not the usual plutonium which can be produced using U238, in very large quantities. This might look like a good thing, as less material means less bombs, but there are other differences between Pu239 and U235. I explain those in the next paragraph, skip it if you want, it's not absolutely necessary.
(During the production of Pu239 through neutron absorption in a nuclear reactor, a whole load of different nuclear reactions take place in the fuel. You have the fission of U235 itself, used to produce energy, but you have impurities in the fuel which might be tranformed into other elements, or even a minority of "unusual" reactions in the U238, U235 or Pu239. One element resulting of those reaction is a rather uncommon isotope of uranium or plutonium, I can't remember which. I *think* it's Pu240, doesn't really matter. Anyway, that isotope has a somewhat low half-life, meaning it's likely to desintegrate and release a number of products, including neutrons. Neutrons happen to be what is used to split U235 or Pu239 atoms in an atomic bomb. The process is essentially this, you'll have different masses coming together, forming a supercritical mass, meaning more neutrons are produced than lost, leading to a chain reaction. The longer the masses stay together, the more energy you'll have time to release. If the masses do not stay supercritical for long enough, the bomb will fail to work correctly. Now, the thing is, the neutrons liberated by the Pu240 are enough to make the reaction begin much sooner, when the masses are not completely together, or right after they've touched. Enough energy will be generated to separate the masses, but not much else. In the end, the bomb will fizzle out.)
Now, what this means is that to use Pu239 you need to have a much greater force holding the masses of fissile material at first. Those found in an implosion-type device are enough. Those in a gun-type device are not. However, U235 does not have this same problem, meaning a gun-type device will work with U235. And gun-type devices are a whole lot simpler than implosion-type devices. First of all, there's no need to worry about explosion timing, no need to worry about explosive lenses used to focus shockwaves at particular points and all the calculations that follow. Second, the masses of fissile materials don't have to be made as precisely. Figuring out how much you need isn't the toughest thing ever. There are calculations to be made, of course, but they're not complex when compared to other things you encounter in physics or engineering.
This is somewhat similar to the path South Africa followed. They want to get a bomb, any bomb. It doesn't matter if it's inefficient and that they can't make it into an H-bomb afterwards. And they want it soon. Of course, there are other circumstances; unlike North Korea, Iran doesn't have a reprocessing plant or any other source of plutonium, and they're not building an arsenal capable of destroying the US, as the USSR was, so they have no reason NOT to make a gun-type device if they want a bomb.