[DennisP]

Yes, but you have to separate the protactinium shortly after it's created, or you end up with enough U232 to make a bomb impractical. And in a solid-fuel reactor, what are you going to do, shut down the reactor every day so you can melt the fuel rods, pull out the protactinium, and refabricate them?

A liquid-fueled reactor could be another matter, and we'd want to keep an eye on them in non-nuclear states. But another factor is the breeding ratio, which is barely over one for thermal thorium. If someone were to pull out much fissile, the reactor would shut down, and they'd have to go begging for more fissile to start it up again.

Another advantage for liquid fuel is very high burnup, so pretty much all you're taking out of the reactor is fission products, not leftover fissile that could theoretically be reprocessed.