[DennisP]

The question though is whether the fissionable material is usable for weapons in any practical sense.

In the Integral Fast Reactor, for example, you end up with a mixture of the four isotopes of plutonium. It's impossible to use that mixture for weapons, and it's much harder to isolate the Pu239 than it is to enrich natural uranium.

In some thorium designs, the fissionable U233 (bred from thorium) is mixed with U232, which is also very hard to separate and makes the material unworkable for weapons. (However, this is not true of all thorium designs. Chemically separate the protactinium and it will decay to pure U233.)

If you had one of the nonproliferating designs and were silly enough to attempt using it for weapons production, you would need much larger and more sophisticated enrichment facilities than if you just enriched natural uranium. If you get your startup fuel from other countries, you can forgo enrichment facilities entirely, making it very clear that you don't have a weapons program.