alt Hacker NewsEnrichment requires feed stock, and active reactor fuel is much higher in fissionable isotopes than the uranium with which it was fed originally. The U238 naturally breeds up into stable-ish U/Th/Pu isotopes which you can totally turn into a bomb.
Obviously there are such things as "breeder reactors" that are deliberately designed for this. But there's really no such thing as a can't-be-used-for-bombs reactor.
If you're going for the enriched uranium route to a bomb, nobody is going to start with used nuclear fuel, because dealing with the highly radioactive spent fuel is such a huge PITA.
If you're going for the U233 (from Th) or Pu route, yes then you need a reactor and spent fuel reprocessing. But not enrichment of spent fuel.