Many people claim you can't, due to fact it's generally a mixture of U232 as well, which will decay rapidly and cook your detonator/firing mechanism, but a couple of interesting comments I found suggest a liquid fuel would be amenable to continuous separation:
Ah, thanks, I didn't notice the 'mixed U/Pu' bit originally.
On the second point, not only was it 1/3 of the equivalent (and specified) 235 design, it was also 1/3 less than predicted for itself, if I'm reading it correctly.
I wonder if that was an error in the prediction, or a fault of the weapon/design.
I've forgotten most of the nuclear chemistry involved in producing Pu, but iirc it can be done with natural uranium and a neutron source. But then again, if you have those, and can produce plutonium, why not just use that? Maybe it'd be helpful as a filler if you've got some, and only limited Pu resources, or to simplify weapon design (Any idea if the MET was gun-type or implosion-type, sources being unsurprisingly hard to find)?
The MET test was odd, the military wanted a highly dialed in yield to test effects of nuclear weapons but somehow there was a miscommunication and they ended up using U-233 as a replacement for U-235 as an experiment. We really don't have enough information about how well U-233 might work in a nuclear weapon to make a judgment on the subject.
Basically, proliferation concerns transition entirely to the honor system once someone is operating any sort of fission based reactor, even one powered by Thorium. Since it is quite easy to breed Plutonium merely by placing natural (and readily available) U-238 in a high neutron flux environment. Simply remove your Uranium samples every 90 days or so and chemically separate out the Plutonium.
The non-proliferation aspects of Thorium reactors are a bit overstated. Consider that the easiest way to create fissile fuel for nuclear weapons is to merely have access to an abundant neutron source, such as any fission reactor (Thorium or not). If you have an LFTR you can just place natural Uranium-238 near the reactor core and use the neutron flux to breed Pu-239, easy peasy.
As far as producing materials that could be stolen and used for nuclear weapons manufacture, that at least is a little bit better with a Thorium fuel cycle. Presumably the U-233 would simply be left to burn up in the reactor, rather than being separated out and kept in storage. If someone happened to obtain some used fuel they could potentially separate out the U-233 and use it to make weapons, although it would require substantial engineering. Also, since U-233 is very much more radioactive than U-235 or Pu-239 it requires handling with remote manipulators. Any organization that had the ability to separate U-233, engineer the appropriate modified bomb design, then process and machine the U-233 using only remote manipulation is extremely likely to have a sufficient level of technology and industry to build reactors or separators on their own (meaning, able to build nuclear weapons independent of having access to used Thorium fuel).
In theory you can, but nobody makes bombs with U233 because it's easier to make bombs in other ways.
It seems to me that the real question for proliferation is not "is it theoretically possible to make bombs with this," but rather "if you wanted to make bombs and you had this technology, would you use it to make your bombs, or would you still prefer other methods?"
It's easier to make them other ways at the moment, because U233 isn't common. But popularize and reduce the cost of this cycle, as well as claiming it's "proliferation resistant" and hence suitable for export to non-members of the we-got-nukes club, may well lead to weapons being made out of it.
Still, if you made a u233 device (and I'm thinking it would have to be a dirty bomb, an explosive seems unfeasible), it would difficult to get it to a target since it's so easy to spot the gamma rays.
u235 is still the way to go, since you can just hide it in a lead pipe in a crate of bananas.
