I think one fear is about the proliferation of reprocessing technology. So currently, with a traditional LWR, you buy X fuel rods, then later IAEA inspectors come by and check that those X rods are in the spent fuel pool. Reprocessing, to the very little extent it's done at all, is done by countries that already have nuclear weapons (except maybe Japan?), so the horse has already left the barn.
But a MSR, at least for some of the more advanced concepts, would have online reprocessing on site (continually siphoning off fission products and adding small amounts of fresh fuel). Additionally, since the fuel wouldn't be in discrete bundles, I think there's a fear that small amounts of fuel could be diverted to a clandestine weapons program.
But a MSR, at least for some of the more advanced concepts, would have online reprocessing on site (continually siphoning off fission products and adding small amounts of fresh fuel). Additionally, since the fuel wouldn't be in discrete bundles, I think there's a fear that small amounts of fuel could be diverted to a clandestine weapons program.