uranium (yoorāʹnēəm), radioactive metallic chemical element; symbol U; at. no. 92; mass number of most stable isotope 238; m.p. 1,132°C; b.p. 3,818°C; sp. gr. 19.1 at 25°C; valence +3, +4, +5, or +6.
Uranium is a hard, dense, malleable, ductile, silver-white, radioactive metal of the actinide series in Group 3 of the periodic table . Uranium has three distinct forms (see allotropy ); the orthorhombic crystalline structure occurs at room temperature. It is a highly reactive metal and reacts with almost all the nonmetallic elements and their compounds, especially at elevated temperatures. It dissolves readily in nitric and hydrochloric acids but resists attack by alkalies. It forms solid solutions and intermetallic compounds with many of the metals. Metallic uranium tarnishes in air and when finely divided ignites spontaneously.
Isotopes and Radioactive Decay
Naturally occurring uranium is a mixture of three isotopes. The most abundant (greater than 99%) and most stable is uranium-238 (half-life 4.5×109 years); also present are uranium-235 (half-life 7×108 years) and uranium-234 (half-life 2.5×105 years). There are 16 other known isotopes. Uranium-238 is the parent substance of the 18-member radioactive decay series known as the uranium series (see radioactivity ). Some relatively long-lived members of this series include uranium-234, thorium-230, and radium-226; the final stable member of the series is lead-206. Uranium-235, also called actinouranium, is the parent substance of the so-called actinium series, a 15-member radioactive decay series ending in stable lead-207; protactinium-231 and actinium-227 are the relatively stable members of this series. Because the rate of decay in these series is constant, it is possible to estimate the age of uranium samples (e.g., minerals) from the relative amounts of parent substance and final product (see dating ).
Natural Occurrence and Processing
Uranium is widely distributed in its ores but is not found uncombined in nature. It is a fairly abundant element in the earth's crust, being about 40 times as abundant as silver. Several hundred uranium-containing minerals have been found but only a few are commercially significant. The most common are uraninite (essentially uranium dioxide) and pitchblende ; other commercially important uranium-containing minerals include carnotite (a potassium uranate-vanadate) and brannerite (a uranium titanate). Ores with as little as 0.1% uranium are mined and processed. Most ores are processed by chemical methods including leaching and solvent extraction. Leaching produces the material known as yellowcake, which is largely triuranium octoxide (U3O8); it is then processed with nitric acid. The uranium is obtained as pure uranyl nitrate, UO2(NO3)2·6H2O, which is typically decomposed to the trioxide, UO3, by heating and reduced to the dioxide, UO2, with hydrogen. The dioxide is chemically and physically stable at high temperatures, and is the form most often used as nuclear reactor fuel. The dioxide may be converted to the tetrafluoride, UF4, by treatment with hydrogen fluoride gas, HF. The pure metal is obtained by electrolysis or chemical reduction of the tetrafluoride, or by chemical reduction of the dioxide. Uranium is further processed, or enriched, to increase the percentage of uranium-235 so that the uranium can be used in a reactor or, with much greater enrichment, a weapon. In enrichment, uranium-238 is separated from uranium-235 by a diffusion or centrifuge process using the gaseous hexafluoride, UF6, which is produced when the metal reacts with fluorine.
Discovery and Uses
The discovery of uranium is commonly credited to Martin H. Klaproth , who in 1789, while experimenting with pitchblende, concluded that it contained a new element, which he named after the planet Uranus, discovered only eight years earlier. However, the substance that Klaproth identified was not pure uranium but an oxide. Eugene M. Péligot isolated the element in 1841. Antoine H. Becquerel discovered its radioactivity in 1896. Before the discovery of nuclear fission by Otto Hahn and Fritz Strassmann in 1939, the principal use of uranium (chiefly as the oxides) was in pigments, ceramic glazes, and a yellow-green fluorescent glass and as a source of radium for medical purposes. It has also been added to steels to increase their strength and toughness. However, because of the high toxicity (both chemical and radiological) of uranium and its compounds, and because of their importance as nuclear fuel, these earlier uses have been largely curtailed.
Uranium gained importance with the development of practical uses of nuclear energy . Uranium-235 is the only naturally occurring nuclear fission fuel, but this isotope is only about 1 part in 140 of natural uranium; the balance is mostly uranium-238. Because the supply of uranium-235 is limited, countries have worked to develop fast breeder reactors that convert nonfissionable uranium-238 to fissionable plutonium-239 (see nuclear reactor ).