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Notes -
An undercurrent of anxiety about Iran being nuclear-latent has come to my attention lately - I think I noticed it via Warographics on Youtube? In brief, they have fissile material, but thus far haven't demonstrated a complete device which requires a casing, fission initiation, etc, and the amount of max-effort work to cross that line seems small. Gun-type fission weapons are of course super-simple and reliable, the biggest problem to solve has always been the industrial capacity to refine the necessary amounts of weapons-grade uranium and plutonium. I don't know how hard the R&D and engineering problems of implosion-type weapons are, or multi-stage fission-boosted weapons, and I don't know how hard the miniaturization and operationalization problems are to create, say, a 10-KT device that is small enough to be man-portable, or rugged enough to be delivered via artillery or a ballistic reentry vehicle.
Except - isn't this old news?
Iran's been running gas centrifuge farms for a long time now, that was what Stuxnet was about, turning up the cost of operating them by abusing the bearings, ergo turning down the rate of weapons-grade uranium purification. Probably they've had multiple devices' worth of weapons-grade material for a long time. While that stockpile accumulates, it's not like the math for old designs of fusion weapons is hard to back out of modern physics, that gives you the prerequisites for your fuse engineering effort to run in parallel, up to testing with inert cores.
In summary, Iran has been "on the brink of getting access to nuclear weapons" in the same way they're claimed to be today for years. They've actively chosen not to cross that line in a way observable to Western intelligence. This is an open secret to everyone professionally in the game, and commentators of low-medium sophistication on up. So why are mutterings only surfacing now about the risk? I can only speculate it's a quiet angle of pressure against further increasing the scale of Israeli combat operations, but I don't understand to what end, or why it isn't simply being said in diplomatic meetings.
Why turn up public fear about the Iranian nuclear program? Why do it now? Why do it so subtly?
The only viable isotopes to use in a gun-type are 233U bred from thorium and 235U purified from natural uranium to >90%. The problem is that spontaneous fission can pre-ignite the reaction before the two masses are in the optimal configuration; even-neutron isotopes have a higher rate of spontaneous fission than odd-neutron isotopes and plutonium has a higher rate than uranium, so a little bit of contamination of a uranium core with 232U, 234U or 238U is tolerable, but even pure 239Pu is very hard to use and even a little bit of contamination with 240Pu (which you will always get if you're breeding it in a reactor) renders it impossible.
I couldn't build an implosion weapon myself with what I know (whereas I could build a gun-type if I had the materials), but with a few years and a well-stocked lab to perfect lens geometry and understanding of the material properties of Pu-Ga alloy it's not that hard AIUI.
EDIT: To clarify, 237Np is also suitable for a gun-type device (and there are no issues with contamination, as 238Np and 239Np decay within days and 236Np actually has less of a spontaneous-fission problem), but it's not as easy to produce in large quantities; it doesn't occur naturally and it takes either two neutron captures on the rare 235U, five neutron captures on 232Th, or a neutron knockout on 238U (highly unusual in a moderated reactor, and fairly unlikely even in a fast one) to produce it.
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