Thorium is a naturally-occurring radioactive element named for the Norse god Thor. Thorium has a 14 billion year half-life, and is almost as abundant as lead. Along with uranium decay, the radioactive decay of thorium is the primary source of thermal heat in the Earth's mantle. One non-nuclear use of thorium is as an impurity in high-performance camera lenses.
Although thorium shows promise as a nuclear fuel, its use in nuclear reactors is in the testing and development phases. India's plan to use thorium in nuclear power is the best-known, and as of 2012, China is researching thorium for use in generation IV reactor designs.
Thorium fuel cycle
Thorium has one naturally-occurring isotope, thorium-232. Thorium itself will not fission upon exposure to neutron radiation like uranium-235, but it will capture a neutron and quickly undergo beta decay (emit an electron) to become protactinium-233. Protactinium-233 has a half-life of about 27 days, and it decays into uranium-233, which is fissile (likely to undergo fission upon exposure to neutrons). Because it can be used to create or "breed" nuclear fuel, thorium is considered fertile.
If protactinium-233 is left exposed to neutron radiation, uranium-232 can be formed. Uranium-232, along with its decay products, emit high-energy gamma radiation. This property can be used to discourage uranium-233 proliferation and weaponization, by explicitly designing reactors to only be fueled with thorium, and to make the extraction of protactinium difficult.