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There are two modes to building what we learned as ABC-weapons; now known as WMD. The first is in "person mode". They don't have to work all that well, it is alright if the nuke just fizzles with a lot of dirty radiation; or most of the mustard gas has oxidised. You make the point anyway. A rouge state that really just wants to blackmail someone can get away with this.
The other mode is "military mode". Then the nuke must be able to stop the tanks at the border even after two decades of storage, and the mustard gas must be able to knock out the enemy's infantry and oxidise before the counterattack.
The last mode is what give you a ticket to military superiority; and this is what Iran (and Saddam) want, and what Israel is rumoured to have, This mode requires a substantial military-industrial capacity. One "back of the envelope" test is ; If you cannot launch a successful car production company you haven't got what it takes.
It will require a huge pooling of resources for the smaller countries that try. You must get hold of enough raw materials, and you must constantly enrich them.
Plutonium decays very rapidly. Expected lifetime for US produced plutonium cores are 7 years. They are visibly decayed when they are inspected after 4. It is the same for enriched uranium; even if the timespans are a little longer.
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A handful of years. For the very highest grade of Pu, the one solely composed of Pu239, the half life is...
Building one construction pipeline is hard. Building many are reserved for the superpowers plus Germany and Japan and possibly Brazil. It may not be all co-located, but still one pipeline. So, therefore, yes; the Iranian facilities are probably in one place.
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Nitpik. It's not just a rogue state. A culture that has no value on human life will also not care about clean weapons. When that culture is run by religious...