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Thorium

Thorium is much more abundant in nature than uranium. Thorium can also be used as a nuclear fuel through breeding to uranium-233 (U-233). When this thorium fuel cycle is used, much less plutonium and other transuranic elements are produced, compared with uranium fuel cycles. Several reactor concepts based on thorium fuel cycles are under consideration. Thorium is a naturally-occurring, slightly radioactive metal discovered in 1828 by the Swedish chemist Jons Jakob Berzelius, who named it after Thor, the Norse god of thunder. It is found in small amounts in most rocks and soils, where it is about three times more abundant than uranium. 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, but average 6-7%. There are substantial deposits in several countries (see table). Thorium-232 decays very slowly (its half-life is about three times the age of the earth) but other thorium isotopes occur in its and in uranium's decay chains. Most of these are short-lived and hence much more radioactive than Th-232, though on a mass basis they are negligible. The 2005 IAEA-NEA "Red Book" gives a figure of 4.5 million tonnes of reserves and additional resources, but points out that this excludes data from much of the world. Geoscience Australia confirms the above 300,000 tonne figure for Australia, but stresses that this is based on assumptions, not direct geological data in the same way as most mineral resources. 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 melting points of all oxides (3300

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