Thorium

Thorium is a chemical element in the periodic table that has the symbol Th and atomic number 90.

Notable characteristics

Thorium is a naturally occurring, slightly radioactive metal. When pure, thorium is a silvery white metal that retains its lustre for several months. However, when it is contaminated with the oxide, thorium slowly tarnishes in air, becoming grey and eventually black. Thorium oxide (ThO₂), also called thoria, has one of the highest boiling points of all oxides (3300°C). When heated in air, thorium metal turnings ignite and burn brilliantly with a white light.

Applications

Applications of thorium:

Mantles in portable gas lights. These mantles glow with a dazzling light when heated in a gas flame.
As an alloying element in magnesium, imparting high strength and creep resistance at elevated temperatures.
Thorium is used to coat tungsten wire used in electronic equipment.
Thorium has been used in welding electrodes and heat-resistant ceramics.
The oxide is used to control the grain size of tungsten used for electric lamps.
The oxide is used for high-temperature laboratory crucibles.
Thorium oxide added to glass helps create glasses of a high refactive index and with low dispersion. Consequently, they find application in high quality lenses for cameras and scientific instruments.
Thorium oxide has been used as a catalyst:
* In the conversion of ammonia to nitric acid.
* In petroleum cracking.
* In producing sulfuric acid.
Uranium-thorium age dating has been used to date hominid fossils.
As a fertile material for producing nuclear fuel. In particular, the proposed energy amplifier reactor design would employ thorium.
Thorium dioxide (ThO₂) is the active ingredient of Thorotrast, which was used as part of X-ray diagnostics. This use has been abandoned due to the carcinogenic nature of Thorotrast.
History

Thorium was discovered in 1828 by the Swedish chemist Jöns Jakob Berzelius, who named it after Thor, the Norse god of war. The metal had virtually no uses until the invention of the lantern mantle in 1885. The name Ionium was used for a thorium isotope early in the study of radioactivity.
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 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 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.

Isotopes

Naturally occurring thorium is composed of 1 isotope: 232-Th. 25 radioisotopes have been characterized with the most {abundant and/or stable} being 232-Th with a half-life of 14.05 billion years, 230-Th with a half-life of 75,380 years, 229-Th with a half-life of 7340 years, and 228-Th with a half-life of 1.92 years. All of the remaining radioactive isotopes have half-lifes that are less than 30 days and the majority of these have half lifes that are less than 10 minutes. This element also has 1 meta state.

The isotopes of thorium range in atomic weight from 212 amu (212-Th) to 236 amu (236-Th).

Precautions

Powdered thorium metal is often pyrophoric and should be handled carefully. Thorium disintegrates with the eventual production of "thoron", an isotope of radon (220-Rn). Radon gas is a radiation hazard. Good ventilation of areas where thorium is stored or handled is therefore essential.

Exposure to thorium in the air can lead to increased risk of cancers of the lung, pancreas and blood. Exposure to thorium internally leads to increased risk of liver diseases. This element has no known biological role. See also Thorotrast.