URANIUM PROCESSING

preparation of the ore for use in various products. Uranium (U), although very dense (19.1 grams per cubic centimetre), is a relatively weak, nonrefractory metal. Indeed, the metallic properties of uranium appear to be intermediate between those of silver and other true metals and those of the nonmetallic elements, so that it is not valued for structural applications. The principal value of uranium is in the radioactive and fissionable properties of its isotopes. In nature, almost all (99.27 percent) of the metal consists of uranium-238; the remainder consists of uranium-235 (0.72 percent) and uranium-234 (0.006 percent). Of these naturally occurring isotopes, only uranium-235 is directly fissionable by neutron irradiation. However, uranium-238, upon absorbing a neutron, forms uranium-239, and this latter isotope eventually decays into plutonium-239a fissile material of great importance in nuclear power and nuclear weapons. Another fissile isotope, uranium-233, can be formed by neutron irradiation of thorium-232. Even at room temperature, finely divided uranium metal reacts with oxygen and nitrogen. At higher temperatures it reacts with a wide variety of alloying metals to form intermetallic compounds. Solid-solution formation with other metals occurs only rarely, owing to the singular crystalline structures formed by uranium atoms. Between room temperature and its melting point of 1,132 C (2,070 F), uranium metal exists in three crystalline forms known as the alpha (a), beta (b), and gamma (g) phases. Transformation from the alpha to the beta phase occurs at 668 C (1,234 F) and from the beta to the gamma phase at 775 C (1,427 F). Gamma uranium has a body-centred cubic (bcc) crystal structure, while beta uranium has a tetragonal structure. The alpha phase, however, consists of corrugated sheets of atoms in a highly asymmetrical orthorhombic structure. This anisotropic, or distorted, structure makes it difficult for the atoms of alloying metals to substitute for uranium atoms or to occupy spaces between uranium atoms in the crystal lattice. Only molybdenum and niobium have been observed to form solid-solution alloys with uranium. Wallace W. Schulz 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. Walter D. Wilkinson, Uranium Metallurgy, vol. 1, Uranium Process Metallurgy (1962); and Robert C. Merritt, The Extractive Metallurgy of Uranium (1971), although older references, still contain a wealth of timely information concerning the mining and milling of uranium ores as well as the extractive metallurgical procedures used to process ores to purified uranium metal; both works should be consulted for in-depth discussion of the metallurgy of uranium metal. Wallace W. Schulz