Thorium is the 90th element in the Periodic Table, two places before uranium. The only naturally-occurring isotope is Th-232 which like U-238 is fertile, breeding fissile U-233 on irradiation with neutrons. In initial combination with a fissile component such as U-235 or plutonium it can thus serve as a nuclear fuel, with the advantage that even in a thermal system it can in principle breed slightly more fissile material than is consumed (with uranium this is possible only in a fast-neutron reactor). On the other hand, its chemistry is less convenient.
Latterly (2011 - 12) thorium has been promoted as a supposedly proliferation-resistant fuel safer than uranium in generating very little plutonium or other transuranic elements. It is true that U-233 would be unsuitable for stockpiled weapons, on account of the contaminating U-232 (half-life 70 years) with the dangerous (exceptionally energetic beta-gamma-emitting) daughter thallium-208 formed in significant amounts within a week or so; however, it is not certain that this would deter terrorist groups careless of personal safety, especially if the device could be prepared and used immediately after purification. As for waste, the U-233 generates much the same amount and range of fission products as does U-235 or plutonium, including the long-lived component.
India, with large thorium deposits and relatively little uranium, has long used thorium in its indigenous nuclear programme but is reticent about technical detail. In the 1990s there was a push for its adoption by other countries on grounds of proliferation-resistance since the thorium cycle generated no plutonium and could be used to consume existing civil or military stocks: Alvin Radkowsky, formerly chief scientist in the US nuclear submarine programme, devised an ingenious fuel structure and with his assistant Alex Galperin arranged for the Kurchatov Institute in Moscow to test it, although no results seem to have been published. Elsewhere the grounds for promoting it were disputed, its practicality was questioned, and there is a suggestion that the chief contribution of thorium to proliferation resistance has lain in providing innocent employment for Russian scientists.