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uranium (yo͞orāˈnēəm), radioactive metallic chemical element; symbol U; at. no. 92; mass number of most stable isotope 238; m.p. 1,132℃; b.p. 3,818℃; sp. gr. 19.1 at 25℃; 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).

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The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



U, a radioactive chemical element of group III of Mendeleev’s periodic system; a member of the actinide series. Atomic number, 92; atomic weight, 238.029. A metal, uranium occurs in nature as a mixture of three isotopes: 238U (99.2739 percent; half-life, 4.51 × 109 years), 235U (0.7024 percent; half-life, 7.13 × 108 years), and 234U (0.0057 percent; half-life, 2.48 × 105years). Of the 11 artificial radioisotopes, whose mass numbers range from 227 to 240, the longest-lived is 233U (half-life, 1.62 × 105 years), which is produced by bombarding thorium with neutrons. The isotopes 238U and 235U are parents of radioactive series.

History. Uranium was discovered in 1789 by the German chemist M. S. Klaproth. Klaproth named the element for the planet Uranus, which had been discovered in 1781 by W. Herschel. Uranium was obtained in metallic form in 1841 by the French chemist E. Peligot, who reduced UCl4 with metallic potassium. The element was initially assigned an atomic weight of 120, and it was only in 1871 that D. I. Mendeleev concluded that the value should be doubled.

Uranium was long of interest to only a narrow group of chemists, and it was used only in producing dyes and glass. The discovery of the radioactivity of uranium in 1896 and of radium in 1898 led to the commercial processing of uranium ores in order to extract radium for use in research and medicine. Nuclear fission was discovered in 1939, and uranium has been a basic nuclear fuel since 1942.

Natural occurrence. Uranium is typically found in sedimentary rocks and in the granitic layer of the earth’s crust. The average abundance of uranium in the earth’s crust (clarke) is 2.5 × 10–4percent by weight. The figure is 3.5 × 10–4 percent for acidic igneous rocks, 3.2 × 10–4 percent for clays and shales, 5 × 10–5 percent for basic rocks, and 3 × 10–7 percent for ultrabasic rocks in the mantle. Uranium actively migrates in hot, cold, neutral, and alkaline water in the form of simple and complex ions, especially carbonate complexes. Oxidation-reduction reactions play an important role in the geochemistry of uranium, since uranium compounds as a rule are readily soluble in water containing an oxidizing medium and only sparingly soluble in water containing a reducing medium, for example, hydrogen sulfide solutions.

Approximately 100 uranium minerals are known, of which 12 have economic importance. During the earth’s history, the content of uranium in the crust has decreased as a result of radioactive decay, a process linked to the accumulation of Pb and He atoms in the crust. The radioactive decay of uranium plays an important role in the energy balance of the crust and is a significant source of the crust’s internal heat.

Physical properties. Uranium is similar in color to steel and can be easily worked. The phase transition temperatures of the three crystalline modifications of uranium (α, β, γ) are as follows: α → β, 668.8° ± 0.4°C; β → γ, 772.2° ± 0.4°C. The α form, is orthorhombic (a = 2.8538 angstroms [Å], b = 5.8662 Å, and c = 4.9557 Å); the β form has a tetragonal lattice (at 720°C, a = 10.759 Å and b = 5.656 Å); and the γ form is body-centered cubic (at 850°C, a = 3.538 Å).

The density of uranium in the α form at 25°C is 19.05±0.2 g/cm3; this form has a melting point of 1132° ± 1°C and a boiling point of 3818°C. The thermal conductivity of α-uranium is 28.05 watts/(m · °K) [0.067 calorie/(cm · sec · °C)] between 100° and 200°C and 29.72 watts/(m · °K) [0.071 calorie/(cm · sec · °C)] between 200° and 400°C. The α form has a specific heat capacity at 25°C of 27.67 kilojoules/(kg · °K) [6.612 calories/(g · °C)], and the specific resistivity is approximately 3 × 10–7 at room temperature and 5.5 × 10–7 at 600°C. At 0.68° ± 0.02°K, this form is superconductive; it is weakly paramagnetic, with a magnetic susceptibility of 1.72 × 10–6 at room temperature.

The mechanical properties of uranium are sensitive to purity and to the modes of mechanical and thermal treatment. The average value of the modulus of elasticity for cast uranium is 20.5 × 10–2 meganewton/m2 (20.9 × 10–3 kilogram-force/mm2). The tensile strength at room temperature is 372–470 meganewtons/m2(38–48 kilograms-force/mm2); the strength is increased when the metal is quenched from the β and γ phases. The average Brinell hardness is 19.6–21.6 × 102 meganewtons/m2 (200–220 kilograms-force/mm2) .

Irradiation by a beam of neutrons (as in a nuclear reactor) alters the physical and mechanical properties of uranium: creep develops, brittleness increases, and articles made of uranium experience deformation. Uranium is therefore used in reactors in alloy form.

Uranium is a radioactive element. The nuclei of 235U and 233U undergo fission spontaneously; fission also results from the capture of both slow (thermal) and fast neutrons having fission cross sections of 508 × 10–24 cm2 (508 barns) and 533 × 10–24 cm2 (533 barns), respectively. The nuclei of 238U undergo fission only upon the capture of fast neutrons having an energy of not less than 1 million electron volts (MeV); upon the capture of slow neutrons, 238U is converted into 239Pu, whose nuclear properties are similar to those of 235U. The critical mass of uranium (93.5 percent 235U) in aqueous solutions is less than 1 kg; when the uranium is in the shape of a sphere, the critical mass is 50 kg without a reflector, or tamper, and 15–23 kg with a tamper. The critical mass of 233U is approximately one-third that of 235U.

Chemical properties. The configuration of the outer electron shells of the uranium atom is 7s26d15f3. Uranium is a reactive metal, manifesting oxidation states of +3, +4, +5, +6, and, sometimes, +2 in its compounds. The compounds of UIV and UVI are the most stable. Uranium is slowly oxidized in the air, and the film of dioxide formed on the surface does not protect the metal from further oxidation. In powder form, uranium is pyrophoric, burning with a bright flame.

Uranium reacts with oxygen to form a dioxide (UO2), a trioxide (UO3), and a large number of intermediate oxides, the most important of which is U3O8. The intermediates have properties similar to those of UO2 and UO3. At high temperatures, UO2 has homogeneous compositions ranging from UO1.60 to UO2.27. At temperatures of 500°–600°C, uranium reacts with fluorines to form a tetrafluoride (UF4, green acicular crystals only slightly soluble in water and acids) and a hexafluoride (UF6, a white crystalline substance that sublimes at 56.4°C).

Uranium reacts with sulfur to form a whole series of compounds, of which US (a nuclear fuel) is the most important. The hydride UH3 is obtained from the reaction of uranium with hydrogen at 220°C. The nitride U4N7 results from the reaction of uranium with nitrogen at temperatures from 450° to 700°C and atmospheric pressure; at higher pressures of nitrogen and the same temperatures, UN, U2N3, and UN2 may be obtained. At 750°–800°C, uranium reacts with carbon to form the monocarbide UC, the dicarbide UC2 and U2C3. The element forms various types of alloys with metals. Uranium slowly reacts with boiling water to form UO2 and H2; it reacts with steam in the temperature range 150°–250°C. Uranium dissolves in hydrochloric and nitric acids; to a lesser extent, it dissolves in hydrofluoric acid.

Formation of the uranyl ion (UO22+) is characteristic of UVI Uranyl salts are yellow and are readily soluble in water and mineral acids. Salts of UIV are green and are less soluble. The uranyl ion has a pronounced tendency toward complex formation in aqueous solutions with both inorganic and organic compounds; industrially, the most important are the carbonate, sulfate, fluoride, and phosphate complexes. Many uranates (salts of ur-anic acid, which has never been isolated in pure form) are known; the compositions of the salts depend on the conditions under which the salts are obtained. All uranates have low solubility in water.

Uranium and its compounds are toxic because of their chemical properties and radiation. The maximum permissible dosage of radiation for those whose work brings them into contact with uranium is 5 rem per year.

Production. Uranium is obtained from ores containing from 0.05 to 0.5 percent uranium. For the most part, the ores are not subjected to dressing; however, radiometric sorting based on the gamma radiation of radium, which always accompanies uranium, is used on a limited scale. In general, the ores are subjected to either acid leaching (sulfuric acid and, sometimes, nitric acid) or carbonate leaching. With the former, uranium enters the acid solution as UO2SO4 or as [UO2(SO4)3]4– anion complexes; with the latter, the element enters the sodium carbonate solution as [UO2(CO3)3]4–. Sorption on ion-exchange resins and extraction by organic solvents (tributyl phosphate, dialkylphosphoric acid, monoalkylphosphoric acid, amines) are used to remove uranium from solutions and slurries, to concentrate the uranium, and to eliminate impurities. Alkali is then added to precipitate either the hydroxide U(OH)4 or uranates of ammonium or sodium from solution.

To obtain compounds of high purity, the products obtained from the above processes are dissolved in nitric acid and subjected to refining operations that yield UO3 or U3O8 as final products; these oxides are reduced at 650°–800°C by hydrogen or dissociated ammonia to yield UO2, which is then converted into UF4 by treatment with gaseous hydrogen fluoride at 500°–600°C. UF4 may also be obtained from the precipitation of the crystal hydrate UF4 · nH2O from solutions by hydrofluoric acid; here, the precipitate is dehydrated at 450°C in a stream of hydrogen. The principal industrial method for obtaining uranium from UF4 is thermal reduction with calcium or magnesium; with this method, the uranium metal is obtained in the form of ingots weighing as much as 1.5 tons. The ingots are refined in vacuum furnaces.

Because 235U is a basic nuclear fuel, the enrichment and/or separation of this isotope are extremely important processes in the production of uranium. They are achieved through the gaseous thermal diffusion process, centrifugal isotopic separation, and other methods based on the difference in the masses of 238U and 235U. In the separation processes, uranium is used in the form of the volatile hexafluoride UF6. In producing highly enriched uranium or uranium with a high concentration of certain isotopes, the critical masses must be taken into consideration. The most suitable method in this case is the reduction of uranium oxides by calcium; here, the CaO slag formed can be easily separated from uranium by dissolution in acids.

The methods of powder metallurgy are employed in producing powdered uranium, uranium dioxide, and carbides and nitrides of uranium, as well as certain other high-melting uranium compounds in powder form.

Use. Metallic uranium and its compounds are used chiefly as fuel in nuclear reactors. A natural or slightly enriched mixture of uranium isotopes is used in steady-state reactors of electric power plants. Highly enriched uranium is used in nuclear power plants and fast reactors. The isotope 235U is used in nuclear weapons. As a fertile material, 238U serves as a source of fissionable plutonium, which is also a nuclear fuel.


In organisms. Uranium is present in extremely minute quantities (10–5–10–8 percent) in the tissues of plants, animals, and humans. The uranium concentration in plant ash is 1.5 × 10–5 percent (with a uranium content in the soil of approximately 10–4 percent). The highest concentrations of the element are found in certain fungi and algae (the latter figuring prominently in the biogenic migration of uranium through the chain linking water, aquatic plants, fish, and humans). With animals and humans, uranium enters the gastrointestinal tract with food and water and the respiratory tract with air; it also enters the body through the skin and mucosa. In the gastrointestinal tract, approximately 1 percent of the quantity of soluble uranium compounds ingested is absorbed; for the compounds of low solubility, the amount absorbed is at most 0.1 percent. In the lungs, the corresponding figures are 50 percent and 20 percent.

The distribution of uranium in the organism is uneven. The major sites for deposition and accumulation are the spleen, kidneys, skeleton, and liver; when uranium compounds of low solubility are present in the air, there will also be accumulations in the lungs and bronchopulmonary lymph nodes. Uranium (in the form of carbonates and complexes with proteins) does not circulate for prolonged periods in the blood.

The uranium content in the organs and tissues of animals and humans does not exceed 10–7 g per g (g/g) of tissue. Thus, in cattle the concentration is 1 × 10–8 g per milliliter (g/ml) in the blood, 8 × 10–8 g/g in the liver, 4 × 10–11 g/g in the muscles, and 9 × 10–8 g/g in the spleen.

In humans, the uranium concentration is 6 × 10–9 g/g in the liver, 6–9 × 10–9 g/g in the lungs, 4.7 × 10–7 g/g in the spleen, and 4 × 10–10 g/ml in the blood. In human kidneys, it is 5.3 × 10–9 g/g in the cortex and 1.3 × 10–8 g/g in the medulla. The concentration is 1 × 10–9 g/g in the bones, 1 × 10–8 g/g in bone marrow, and 1.3 × 10–7 g/g in the hair.

The uranium present in bony tissue causes a constant irradiation of this tissue (the half-life of uranium in the skeleton being approximately 300 days). The lowest uranium concentrations in humans are in the brain and heart (10–10 g/g). The daily intake of the element is 1.9 × 10–6 g from foods and liquids and 7 × 10–9 g from the air. The amounts eliminated each day are 0.5–5.0 × 10–7 g in the urine, 1.4–1.8 × 10–6 g in the feces, and 2 × 10–8 g through the hair.

According to data from the International Radiation Protection Association, the content of uranium in humans averages 9 × 10–5 g, with variations depending on location. It is believed that uranium is necessary for the vital activities of plants, animals, and humans, but the exact physiological role of the element has not yet been determined.


Toxic effects. The toxic effects of uranium derive from the element’s chemical properties and depend on solubility, uranyl compounds, and other soluble uranium compounds having a higher degree of toxicity. Poisoning by uranium and its compounds can occur at plants for the extraction and production of uranium and at industrial installations that use uranium.

Upon entering the organism, uranium acts on all organs and tissues as a generalaction cell poison. The symptoms are the result primarily of injury to the kidneys (appearance of protein and sugar in the urine, eventual oliguria); the liver and gastrointestinal tract are also affected. A distinction is made between acute and chronic poisoning; with the latter, the symptoms develop gradually and are less pronounced. Blood production and the nervous system may also be affected in cases of chronic poisoning. It is believed that the molecular mechanism by which uranium acts is related to the element’s capacity to depress enzyme activity.

To prevent uranium poisoning, continuous production processes and hermetically sealed equipment are used in industry. Uranium is not allowed to contaminate the air, and water leaving installations where uranium is produced or used is purified. The health of those who work with uranium is monitored, and all norms pertaining to the allowable content of uranium in the environment are adhered to.



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A metallic element in the actinide series, symbol U, atomic number 92, atomic weight 238.03; highly toxic and radioactive; ignites spontaneously in air and reacts with nearly all nonmetals; melts at 1132°C, boils at 3818°C; used in nuclear fuel and as the source of 235U and plutonium.
A dense, silvery, ductile, strongly electropositive metal.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


a radioactive silvery-white metallic element of the actinide series. It occurs in several minerals including pitchblende, carnotite, and autunite and is used chiefly as a source of nuclear energy by fission of the radioisotope uranium-235. Symbol: U; atomic no.: 92; atomic wt.: 238.0289; half-life of most stable isotope, 238U:451 × 109 years; valency: 2-6; relative density: 18.95 (approx.); melting pt.: 1135?C; boiling pt.: 4134?C
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
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