It seems like production of a U-233 bomb is much simpler than production of a U-235 or Pu-239 weapon. There is the problem with production of U-232 which according to what I read can be solved by separating our Pa-233 from the Thorium and allowing it to decay to U-233 outside of a neutron flux. Given all that, it seems like U-233 production has a relatively low satellite signature and could potentially even be achieved by a well-funded non-state actor (no doubt in a Nehru jacket petting a fluffy, white cat). Since that isn’t happening (unless James Bond is really discreet), what is it that makes it hard? Is it especially difficult to separate Protactinium from Thorium? Does it require large facilities using exotic equipment?
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What would make it so much easier? The only thing it has going for it is that it’s bred from Thorium, which is extremely common in some parts of the world (India).
To use U233 as an implosion bomb material, the amount of U232 has to be below 50 ppm, which is a LOT lower than the 6.5% allowable max Pu240. U232 is also extraordinarily radioactive (gamma rays) even at such a low proportion and introduces serious handling issues- glove boxes and other normal U235/Pu239 aren’t sufficient, and remote handling equipment has to be used.
Plus, it still has to be bred from Thorium, and if you’re going to go to the trouble to breed fuel instead of separating the natural U235 from U238, you may as well breed plutonium which is a lot easier to work with, and has a slightly lower critical mass than U233.
In short, unless you have a lot of thorium and no way to get U235/Pu239, there’s no real reason to use U233 as fissile material in a nuclear bomb.
What made me think about that are the proliferation concerns that surround LFTR reactors. I understand that they take the protactinium out of the reactor and let it decay to very pure U-233. Another advantage is that U-233 can use the gun design which is much simpler to make. Indeed, you can find the plans on Wikipedia. There are also other neutron sources besides fission reactors, like Farnsworth fusors. I presume these don’t put out enough neutrons to produce sufficient quantities of protactinium.