Thorium: Encyclopedia II - Thorium - Occurrence

Thorium - Occurrence

Thorium is found in small amounts in most rocks and soils, where it is about three times more abundant than uranium, and is about as common as lead. Soil commonly contains an average of around 6 parts per million (ppm) of thorium. Thorium occurs in several minerals, the most common being the rare earth-thorium-phosphate mineral, monazite, which contains up to about 12% thorium oxide. There are substantial deposits in several countries. Thorium-232 decays very slowly (its half-life is about three times the age of the earth) but other thorium isotopes occur in the thorium and uranium decay chains. Most of these are short-lived and hence much more radioactive than Th-232, though on a mass basis they are negligible.

Thorium - Thorium as a nuclear fuel

Thorium, as well as uranium, can be used as fuel in a nuclear reactor. Although not fissile itself, thorium-232 (Th-232) will absorb slow neutrons to produce uranium-233 (U-233), which is fissile. Hence, like uranium-238 (U-238), it is fertile.

In one significant respect U-233 is better than uranium-235 and plutonium-239, because of its higher neutron yield per neutron absorbed. Given a start with some other fissile material (U-235 or Pu-239), a breeding cycle similar to but more efficient than that with U-238 and plutonium (in slow-neutron reactors) can be set up. The Th-232 absorbs a neutron to become Th-233 which normally decays to protactinium-233 and then U-233. The irradiated fuel can then be unloaded from the reactor, the U-233 separated from the thorium, and fed back into another reactor as part of a closed nuclear fuel cycle.

Problems include the high cost of fuel fabrication due partly to the high radioactivity of U-233 which is always contaminated with traces of U-232; the similar problems in recycling thorium due to highly radioactive Th-228; some weapons proliferation risk of U-233; and the technical problems (not yet satisfactorily solved) in reprocessing. Much development work is still required before the thorium fuel cycle can be commercialised, and the effort required seems unlikely while (or where) abundant uranium is available.

Nevertheless, the thorium fuel cycle, with its potential for breeding fuel without the need for fast neutron reactors, holds considerable potential long-term. Thorium is significantly more abundant than uranium, so it is a key factor in the sustainability of nuclear energy.

India has particularly large reserves of thorium, and so have planned their nuclear power program to eventually use it exclusively, phasing out uranium as an input material. This ambitious plan uses both fast and thermal breeder reactors.

The current thorium mineral reserve estimates (in tons)[1]:

• 340,000 Australia
• 300,000 India
• 300,000 United States
• 180,000 Norway
• 100,000 Canada
• 39,000 South Africa
• 18,000 Brazil
• 4,500 Malaysia
• 100,000 Other

Adapted from the Wikipedia article "Occurrence", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki