THORIUM

Background |  Name |   Sources |  Uses |  Substitutes and Alternative Sources

Thorium is a silver-gray, radioactive, metallic element. Its atomic number is 90 and its symbol is Th. It was discovered in 1829 by the Swedish chemist Jöns Jakob Berzelius. It is the most common of a group of elements called the actinides. It is the 39th most abundant element in the Earth’s crust at 7.2 parts per million (ppm). Other elements in the actinide group are the natural elements uranium and plutonium. However, most of the actinide elements are not naturally occurring and have the atomic numbers from 89, actinium, through 103, lawrencium.

It is also radioactive. The radioactive breakdown of uranium and thorium create the energy that heats the interior of the Earth. Based on the estimates of the abundance of thorium in the Earth’s crust, there is more energy in thorium than in the fossil fuels and uranium combined! In the 1980’s, 45 tons of thorium was used every year. However, it is used less and less because of the state and federal laws about the handling, transportation and disposal of radioactive materials. Its use will most likely continue to decline unless less expensive methods of disposal are developed.

The principal isotope of thorium has a half-life of 14,000,000,000 years.

Thorium and uranium are the only two actinide elements that are found in large enough quantities to mine. Thorium is found in the minerals monazite (rare earth-thorium phosphate) and thorianite (thorium dioxide).

Thorium was named after Thor, the mythical Scandinavian god of war.

The most important source of thorium is the mineral monazite. The largest reserves of thorium are in placer deposits. (A placer deposit is a deposit of heavy-mineral sands deposited by moving water.) Some thorium has been recovered from igneous veins and igneous carbonate deposits called carbonatites. It is estimated that the thorium in these deposits totals approximately 500,000 tons. In addition, some igneous deposits contain smaller concentrations of thorium, but may contain resources of more than 2 million tons of thorium.

Significant deposits of thorium are found in Australia, Brazil, Canada, Greenland, India, South Africa, and the United States. Even though the U.S. has significant thorium resources, nearly all the thorium used in the U.S. is imported. Thorium is imported as processed thorium compounds, usually from France, Canada, the United Kingdom, Japan, and other nations.

Thorium and thorium compounds, especially the oxide, have the interesting property of having very high melting temperatures. As a result, it is used for high temperature applications such as coatings on tungsten filaments in light bulbs and for high temperature laboratory equipment. It is also used to make specialized lenses in optical equipment. Thorium is alloyed with magnesium to create lightweight, high strength metals. Such alloys were used in the aerospace industry. Thorium use is being studied as a non-prolific fuel source by the nuclear energy industry.

Other elements can be used in place of thorium in magnesium-thorium alloys, including zirconium and yttrium.