Long story short, Th isn't actually fissable. It is fertile. You have to use something else (like U235/Pu/etc.) to create a stream of neutrons to turn Th into P

Long story short, Th isn't actually fissable. It is fertile. You have to use something else (like U235/Pu/etc.) to create a stream of neutrons to turn Th into Pa which decays into U233 (iirc). which is then fissable. (and strangely enough, can be used to convert more Th...) The problem is if the Pa absorbs another neutron, it no longer produces U233 and so it drops the yeild (and leads to other nasties, etc.). This is, iirc. why the molten salt reactors are so appealing. You can run the reactor, continuously remove the Pa and let it decay into U233 which you then reintroduce into the reactor. http://hal.archives-ouvertes.fr/docs/00/04/14/97/PDF/document_IAEA.pdf http://hal.archives-ouvertes.fr/docs/00/04/14/97/PDF/documen... is an interesting link discussing the feasibility of various reactor designs.