Alpha-emitter. Yeah, nasty shit, that Thorium. And really not fissionable per se until you get it to U-233 (aka, a thermal fuel) in a breeder process, has a half-life of about 12 BILLION years, and of course reprocessing hasn't been done in this country since the Carter administration.
--The fuel costs to separate the U-233 from the Th-232 are high.
--Impurities in the U-233 result in very high doses due to short-lived high-energy gamma emitters such as Thallium 208.
--Again, recycling issue with Th-228.
--Uranium is still more easily converted into nuclear fuel and still abundant enough to make Thorium impractical.