The Liquid Fluoride Thorium Reactors (LFTR) are purportedly safer (no harmful radioactive wastes produced), less expensive, smaller in size, does not use weapons-grade radioactive fuels and run on thorium-232, which is apparently an abundant heavy metal. The nuclear reactions in an LFTR can be started and stopped easily. The fuel does not have to be refined or enriched or made into pellet shapes. The reaction products are supposedly less harmful and have short half-lives.
Thus, I feel that the above qualities make LFTRs a potential fit for nuclear propulsion applications. LFTRs can be made smaller to fit an air/space vehicle. The controllability of the reactions gives us the ability to throttle power output on a vehicle. The reactors can be designed to withstand explosions. At present, aircraft black boxes are made tough and rugged, and designed to withstand crashes and submersion in water. So is the case with nuclear warheads on missiles. In the event of an explosion, the fuel is in such small quantities and the fission products are short-lived that there may not be the danger of contaminating radioactive fallout as is the case with Plutonium-239/241 or Uranium-235/238 reactors.
LFTRs could be used to power scramjets, since scramjets require massive amounts of energy to accelerate the working fluids to hypersonic speeds. But LFTRs could also find use in subsonic (bombers, freighters, fuel-tankers, etc.) and supersonic aircraft or even the single-stage-to-orbit (SSTO) aerospace planes.
More work needs to be done in this area. Here is a link to an article from How Stuff Works.com with embedded videos explaining how LFTRs work.
More links related to thorium and nuclear energy.
http://www.itheo.org/ International Thorium Energy Organization.
A good article on the history of LFTRs published in Mechanical Engineering magazine.
http://thoriummsr.wordpress.com/intro/ An excellent blog on Thorium reactors and nuclear energy. Excellent educational resources.
Energy from Thorium blog. This website has good links to websites and videos.
Thorium Energy Alliance Lots of educational material, links to websites and videos.