[Steely Tom]

I find it provocative that the Iranians are still using uranium hexafluoride and centrifuges to separate isotopes. The US declassified that process in 1954, assuming that on one would ever try to build a nuclear weapon that way again. If true, the Iranians will probably never have a sizable nuclear arsenal, though even on nuclear bomb can ruin your whole day.

It's a lot easier to make a functional bomb from U235 than from Pu239. You can test your bomb geometry and be pretty sure it will work without actually exploding one. The bomb that was dropped on Hiroshima was a U235 device, and the design was tested at full scale for the first time on the day it was used in combat.

Pu239 requires much testing at full scale. Kind of hard to keep it a secret these days.

U235 is a lot harder to make, and a lot more expensive, but you can keep your program under wraps until the day you use it. Especially if the organization that's supposed to be keeping an eye on you is as stupid and feckless as the IAEA. Pu239 is what you want if you are trying to build an arsenal for long-term defense, but everyone will know what you're up to, no way to make excuses for huge flashes and obvious glass-lined craters and mushroom clouds visible from hundreds of miles away.

[Lonesome in Massachussets]

The US uses laser chemistry for isotope separation, it’s orders of magnitude cheaper than gaseous diffusion via centrifuges. These guys are going at it Robinson Cursoe style.