THORIUM PROCESSING

preparation of the ore for use in various products. Thorium (Th) is a dense (11.7 grams per cubic centimetre), silvery metal that is softer than steel. It has a high melting temperature of approximately 1,750 C (3,180 F). Below about 1,360 C (2,480 F), the metal exists in the face-centred cubic (fcc) crystalline form; at higher temperatures up to its melting point, it takes on the body-centred cubic (bcc) form. Finely divided thorium metal will burn in air, but the massive metal is stable in air at ordinary temperatures (although it will react with oxygen to form a surface tarnish after prolonged exposure). Because of its reactivity, it is extracted from minerals only with difficulty. Almost all thorium found in nature is the isotope thorium-232 (several other isotopes exist in trace amounts or can be produced synthetically). This slightly radioactive material is not fissile itself, but it can be transformed in a nuclear reactor to the fissile uranium-233. Since thorium is present in the Earth's crust in about three times the quantity of uranium, its fertile quality represents a virtually unlimited source of nuclear energy. In order for this theoretical value to be realized, however, the barriers of costly extraction and conversion techniques would have to be overcome. Additional reading Comprehensive and up-to-date information on many aspects of metallurgy, individual metals, and alloys can be found in convenient reference-form arrangement in the following works: Metals Handbook, 9th ed., 17 vol. (197889), a massive and detailed source prepared under the direction of the American Society for Metals, with a 10th edition that began publication in 1990; Herman F. Mark et al. (eds.), Encyclopedia of Chemical Technology, 3rd ed., 31 vol. (197884), formerly known as Kirk-Othmer Encyclopedia of Chemical Technology, with a 4th edition begun in 1991; and its European counterpart, the first English-language edition of a monumental German work, Ullmann's Encyclopedia of Industrial Chemistry, 5th, completely rev. ed., edited by Wolfgang Gerhartz et al. (1985 ). The Editors of the Encyclopdia Britannica Joseph J. Katz, Glenn T. Seaborg, and Lester L. Morss, The Chemistry of the Actinide Elements, 2nd ed., 2 vol. (1986), presents both the theoretical and descriptive features of the chemistry of thorium, uranium, and plutonium in a logical and extremely well-written manner; the coverage is extensivefrom properties of individual isotopes to the technology for reprocessing irradiated reactor fuelsand is very lucid. Gmelins Handbuch der anorganischen Chemie, 8th ed., vol. A1 (1979), A3 (1981), and A4 (1982), is unquestionably the most comprehensive and authoritative source of information and data for all facets of thorium, uranium, and plutonium chemistry, physics, and metallurgy; compiled by recognized world-class experts and updated regularly, these volumes are the definitive reference in the field. Wallace W. Schulz