Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.
Uranium is the heaviest and last naturally occurring element in the periodic table. The periodic table is a chart that shows how chemical elements are related to each other. Uranium occurs near the beginning of the actinide family. The actinide family consists of elements with atomic numbers 90 through 103.
At one time, uranium was considered to be a relatively unimportant element. It had a few applications in the making of stains and dyes, in producing specialized steels, and in lamps. But annual sales before World War II (1939-45) amounted to no more than a few hundred metric tons of the metal and its compounds.
Then, a dramatic revolution occurred. Scientists discovered that one form of uranium will undergo nuclear fission. Nuclear fission is the process in which the nuclei of large atoms break apart. Large amounts of energy and smaller atoms are produced during fission. The first application of this discovery was in the making of nuclear weapons, such as the atomic bomb. After the war, nuclear power plants were built to make productive use of nuclear fission. Nuclear power plants convert the energy released by fission to electricity. Today, uranium is regarded as one of the most important elements for the future of the human race.
Discovery and naming
Credit for the discovery of uranium is usually given to German chemist Martin Klaproth (1743-1817). During the late 1780s, Klaproth was studying a common and well-known ore called pitchblende. At the time, scientists thought that pitchblende was an ore of iron and zinc.
During his research, however, Klaproth found that a small portion of the ore did not behave the way iron or zinc would be expected to behave. He concluded that he had found a new element and suggested the name uranium for the element. The name was given in honor of the Uranus, a planet that had been discovered only a few years earlier, in 1781.
For some time, scientists believed that Klaproth had isolated uranium. Eventually they realized he had found uranium oxide (UO2), a compound of uranium. It was not until a half century later, in fact, that the pure element was prepared. In 1841, French chemist Eugène-Melchior Peligot (1811-90) produced pure uranium from uranium oxide.
Early researchers did not know that uranium was radioactive. In fact, radioactivity was not discovered until 1898. Radioactivity is the tendency of an isotope or element to break down and give off radiation.
Uranium is a silvery, shiny metal that is both ductile and malleable. Ductile means capable of being drawn into thin wires. Malleable means capable of being hammered into thin sheets. Its melting point is 1,132.3°C (2,070.1°F) and its boiling point is about 3,818°C (6,904°F). Its density is about 19.05 grams per cubic centimeter.
Uranium is a relatively reactive element. It combines with nonmetals such as oxygen, sulfur, chlorine, fluorine, phosphorus, and bromine. It also dissolves in acids and reacts with water. It forms many compounds that tend to have yellowish or greenish colors.
Occurrence in nature
Uranium is a moderately rare element. Its abundance is estimated to be about 1 to 2 parts per million, making it about as abundant as bromine or tin. The most common ore of uranium is pitchblende, although it also occurs in other minerals, such as uraninite, carnotite, uranophane, and coffinite.
All isotopes of uranium are radioactive. Three of these occur naturally, uranium-234, uranium-235, uranium-238. By far the most common is uranium-238, making up about 99.276% of uranium found in the Earth's crust. Uranium-238 also has the longest half life, about 4,468,000,000 years.
Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.
The half life of a radioactive element is the time it takes for half of a sample of the element to break down. Imagine that the Earth's crust contains 100 million tonnes of uranium-238 today. Only half of a uranium-238 sample would remain 4,468,000,000 years from now (one half life). The remainder would have changed into other isotopes.
About a dozen other isotopes of uranium have been made artificially.
Uranium is mined in much the same way iron is. Ore is removed from the earth, then treated with nitric acid to make uranyl nitrate (UO2(NO3)2). This compound is converted to uranium
dioxide (UO2). Finally, this compound is converted to pure uranium metal with hydrogen gas:
Uses and compounds
Uranium compounds have been used to color glass and ceramics for centuries. Scientists have found that glass made in Italy as early as A.D. 79 was colored with uranium oxide. They have been able to prove that the coloring was done intentionally.
Some uranium compounds were used for this purpose until quite recently. In fact, a popular type of dishware known as "Fiesta Ware" made in the 1930s and 1940s sometimes used uranium oxide as a coloring material. Other glassware, ceramics, and glazes also contained uranium oxide as a coloring agent.
Uranium compounds have had other limited uses. For example, they have been used as mordants in dyeing operations. A mordant is a material that helps a dye stick to cloth. Uranium oxide has also found limited application as an attachment to filaments in lightbulbs. The compound reduces the speed at which an electric current enters the bulb. This reduces the likelihood of the filament heating too fast and breaking.
None of these applications is of very much importance today, however. By far the most important application is in nuclear weapons and nuclear power plants. The reason for this importance is that one isotope of uranium, uranium-235, undergoes nuclear fission.
Nuclear fission is the process by which neutrons are fired at a target. The target is usually made of uranium atoms. When neutrons hit the target, they cause the nuclei of uranium atoms to break apart. Smaller elements are formed and very Large amounts of energy are given off.
When this reaction is carried out with no attempt to capture or control the energy, an enormous explosion takes place. This release of nuclear energy accounts for the power of a nuclear weapon such as an atomic bomb. In reactors, the energy released during fission is used to boil water. Steam is produced and is converted to electricity. The controlled release of nuclear energy takes place in a nuclear power plant.
Separating twins to make energy
S uppose neutrons are fired at a big block of uranium metal. Would nuclear fission occur? Would this be the way to make an atomic bomb? Could this process be used in a nuclear power plant?
The answer to all these questions is no. Only one isotope of uranium undergoes nuclear fission, uranium-235. The most common isotope, uranium-238, does not undergo fission. There is no way to make a bomb or a nuclear power plant with a chunk of natural uranium metal.
It is necessary is to increase the percentage of uranium-235 in the metal. As a chunk of uranium metal contains more uranium-235, it is more likely to undergo nuclear fission.
In making a bomb or a power plant, then, the first step is to separate the isotopes of uranium from each other. The goal is to produce more uranium-235 and less uranium-238.
hat goal sounds easy, but it is very difficult to do. All isotopes of uranium behave very much alike. They have the same chemical properties. The only way they differ from each other is by weight. An atom of uranium-238, for example, weighs about 1 percent more than an atom of uranium-235. That's not much of a difference.
Scientists separate these isotopes in a centrifuge. A centrifuge is a machine that spins containers of materials at very high speeds. They are like some of the rides at an amusement park. A person sits in a compartment at the end of a long arm. When the ride is turned on, the compartment spins around faster and faster.
In a centrifuge, heavier objects spin farther out than do lighter objects. A mixture of uranium-235 and uranium-238 can be separated slightly in a centrifuge. But the separation is not very good because the isotopes weigh almost the same amount.
In practice, a mixture of isotopes must be centrifuged many times. Each time, the separation gets better.
Scientists prepare enriched uranium by this method. Enriched uranium contains more uranium-235 and less uranium-238. Enriched uranium was used to make atomic bombs and is now used in nuclear power plants. It contains enough uranium-235 to allow nuclear fission to occur.
Today, over 400 nuclear power plants exist worldwide, producing about 17 percent of all electricity. Many people believe nuclear power will be more important in the future as the world's supply of coal, oil, and natural gas eventually runs out.
Other people are concerned about the dangers of nuclear power. The radiation released and radioactive wastes produced by nuclear power plants have made them unpopular in the United States. No new nuclear generators have been built for over ten years. It is not clear what the future of nuclear power plants in the United States will be.
Uranium poses an exceptional risk in powdered form. In this form, it tends to catch fire spontaneously.
Since it is a radioactive element, uranium must be handled with great care. In addition, it poses an exceptional risk in powdered form. In this form, it tends to catch fire spontaneously.
"Uranium (revised)." Chemical Elements: From Carbon to Krypton. . Encyclopedia.com. (August 23, 2017). http://www.encyclopedia.com/science/news-wires-white-papers-and-books/uranium-revised
"Uranium (revised)." Chemical Elements: From Carbon to Krypton. . Retrieved August 23, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/news-wires-white-papers-and-books/uranium-revised
█ LARRY GILMAN
Uranium is a radioactive, metallic element with 92 protons and a variable number of neutrons in the nucleus of each atom. There are 16 isotopes of uranium, the most common being uranium-238 (238U). The second-commonest isotope of uranium, 235U, is used for building nuclear weapons, generating electricity, and propelling some submarines, aircraft carriers, and other vessels. Heat released by uranium decay also keeps Earth's interior hot, providing the energy for continental drift and volcanic eruptions.
Uranium was discovered in 1789 by German chemist Martin Heinrich Klaproth (1743–1817), and its property of radioactivity was discovered by French physicist Henri Becquerel (1852–1908) in 1896. 235U was first isolated in kilogram quantities by the United States during World War II, and was used in war by the United States in the bomb that destroyed the city of Hiroshima, Japan in 1945. Since that time uranium has been mined in many countries and purified in large quantities for both bombs and fuel. Worldwide, several hundred nuclear reactors produce electricity from uranium, while tens of thousands of nuclear weapons (mostly held by the United States and the Russian Federation) rely on uranium either as their primary explosive (in fission bombs) or as a trigger explosive (in fusion bombs).
Uranium atoms are unstable; that is, their nuclei tend spontaneously to fission or break down into smaller nuclei, fast particles (including neutrons), and high-energy photons. The fission of an isolated uranium nucleus is a randomly timed event; however, collision with a neutron may trigger a uranium nucleus to fission immediately. Crowding large numbers of uranium atoms together can enable the neutrons emitted by a few nuclei undergoing fission to cause other nuclei to fission, whose released neutrons in turn trigger still other nuclei, and so on. If this chain reaction proceeds at a constant rate, it may be used to generate electricity; if it proceeds at an exponentially increasing rate, a nuclear explosion results.
Only 0.71% of natural uranium is 235U, the major isotope directly useful for nuclear power and weapons. Many tons of ore must therefore be refined to produce a single kilogram of 235U. The amount of 235U needed to make a bomb, however, is not great: about 15 lb (7 kg). Quantities of uranium sufficient for many thousands of bombs are thus available around the world; some 21 countries export uranium, with Canada, Australia, and Niger being the three largest producers.
The most common isotope of uranium, 238U, comprises 99.28% of the uranium in the Earth's crust. 238U is comparatively stable, with a half-life of 4.5 billion years, and so is not directly useful for power and nuclear weapons. It is added to some antitank and antiaircraft ammunition to increase their density and thus their penetrating power. Depleted-uranium munitions, as these weapons are termed, were used extensively by the United States during the Gulf War of 1991 and in the Kosovo conflict of 1999. Because of their slight radioactivity, there is ongoing debate about whether they may cause long-term health problems in areas where they have been used.
238U is also a major ingredient of most reactor fuel. In reactor cores, this 238U is bombarded by neutrons, which transmute some of it into the element plutonium. Plutonium can be used directly for power and weapons; the first and third nuclear weapons ever exploded were produced by the United States using plutonium transmuted from 238U, and a number of other countries, including India, Israel, Pakistan, and North Korea, have developed the capability to obtain plutonium for bombs by the same means.
Both 235U and plutonium must be in fairly concentrated form for use in bomb manufacture. Alloys that have been diluted by 238U or other substances result in bulkier explosive devices; at sufficiently great dilution, a nuclear explosion is not obtainable. (However, some experts say that a nuclear explosion might be obtainable from an alloy that is as little as 10% 235U.) It follows that any organization that wishes to build an atomic weapon must either obtain fairly concentrated 235U or plutonium by purchase or theft, or obtain them in dilute form and then concentrate them.
These obstacles have been surmounted by a number of governments, and may eventually be surmounted by terrorist organizations. Illegal traffic in weapons-grade 235U and plutonium has accelerated since the breakup of the Soviet Union in 1991, because its successor states have been too poor and disorganized to keep nuclear material secure. Some 600 tons, or enough for about 40,000 bombs, of raw weapons-grade fissionables are
stored in poorly guarded stockpiles in the Russian Federation and other states; small quantities have already entered the black market. On over 16 occasions since 1993, police in Asia, Europe, or South America have intercepted illegally held bomb-grade uranium or plutonium, most of it from ex-Soviet sources. In 1994, police seized a metal briefcase when a civilian jetliner from Moscow landed in Munich, Germany; the briefcase contained 363.4 grams of weapons-grade plutonium. In April 2000, almost a kilogram of bomb-grade uranium was seized in the Republic of Georgia. In 2001, police in Bogota, Colombia seized some 600 grams of bomb-grade 235U from the house of an animal feed salesman, the enrichment level of which corresponded to that of Russian fuel for submarines and icebreakers. And on September 11, 2001, four men were arrested in the ex-Soviet republic of Georgia in possession of almost 2 kilograms of bomb-grade 235U—a large fraction of the amount required for a bomb. Since that day, the idea that stolen uranium might be used for terrorist acts has gained increased attention.
Through its Material Protection, Control, and Accounting Program, the United States has spent about $550 million since 1993 to help safeguard uranium and plutonium stocks in Russia, supplying complete security systems or partial protection for about a third of the material considered most vulnerable by the U.S. Department of Energy.
█ FURTHER READING:
Ladika, Susan. "Tracing the Shadowy Origins of Nuclear Contraband." Science no. 5522 (2001): 1634.
Stone, Richard. "Nuclear Trafficking: 'A Real and Dangerous Threat'." Science no. 5522 (2001): 1632–36.
Nuclear Power Plants, Security
"Uranium." Encyclopedia of Espionage, Intelligence, and Security. . Encyclopedia.com. (August 23, 2017). http://www.encyclopedia.com/politics/encyclopedias-almanacs-transcripts-and-maps/uranium
"Uranium." Encyclopedia of Espionage, Intelligence, and Security. . Retrieved August 23, 2017 from Encyclopedia.com: http://www.encyclopedia.com/politics/encyclopedias-almanacs-transcripts-and-maps/uranium
uranium (yōōrā´nēəm), 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).
"uranium." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. (August 23, 2017). http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/uranium
"uranium." The Columbia Encyclopedia, 6th ed.. . Retrieved August 23, 2017 from Encyclopedia.com: http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/uranium
melting point: 1,408°C
boiling point: 4,404°C
density: 19.04 g/cm3
most common ions: U3+, U4+, UO2+, UO22+
Uranium is a very dense, highly reactive, metallic element that has the highest atomic mass of the naturally occurring elements. Natural uranium consists of two long-lived radioactive isotopes : 238U (99.28%) and 235U (0.72%). A very small amount of 234U (0.005%) occurs in secular equilibrium with 238U. Uranium was discovered in 1789 by Martin Klaproth, who named it after the planet Uranus (which had just been discovered). In 1841 Eugène Melchior Péligot prepared uranium metal and proved that Klaproth had actually isolated uranium dioxide.
Uranium is found in Earth's crust at an average concentration of about 2 ppm, and is more abundant than silver or mercury. The most common uranium-containing mineral is uraninite, a complex uranium oxide. Other uranium-containing minerals are autunite, a hydrated calcium uranium phosphate, and carnotite, a hydrated potassium uranium vanadate.
The most prevalent form of uranium in aqueous solution is the light yellow, fluorescent uranyl ion UO22+. The U4+ cation (green in solution) can be obtained by strong reduction of U(VI), but readily oxidizes back to UO22+ in air. The pentavalent ion UO2+ can be reversibly formed by reduction of UO22+, but it readily disproportionates into U(IV) and U(VI). The trivalent U3+ can be formed by reduction of U(IV) but is unstable to oxidation in aqueous solution.
After the discovery of uranium radioactivity by Henri Becquerel in 1896, uranium ores were used primarily as a source of radioactive decay products such as 226Ra. With the discovery of nuclear fission by Otto Hahn and Fritz Strassman in 1938, uranium became extremely important as a source of nuclear energy. Hahn and Strassman made the experimental discovery; Lise Meitner and Otto Frisch provided the theoretical explanation. Enrichment of the spontaneous fissioning isotope 235U in uranium targets led to the development of the atomic bomb, and subsequently to the production of nuclear-generated electrical power. There are considerable amounts of uranium in nuclear waste throughout the world.
see also Actinium; Berkelium; Einsteinium; Fermium; Lawrencium; Mendelevium; Neptunium; Nobelium; Plutonium; Protactinium; Rutherfordium; Thorium.
W. Frank Kinard
Katz, Joseph J.; Seaborg, Glenn T.; and Morss, Lester T. (1986). The Chemistry of the Actinide Elements, 2nd edition. New York: Chapman and Hall.
Lide, David R., ed. (1995–1996). The CRC Handbook of Chemistry and Physics, 76th edition. Boca Raton, FL: CRC Press.
"Uranium." Chemistry: Foundations and Applications. . Encyclopedia.com. (August 23, 2017). http://www.encyclopedia.com/science/news-wires-white-papers-and-books/uranium
"Uranium." Chemistry: Foundations and Applications. . Retrieved August 23, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/news-wires-white-papers-and-books/uranium
"uranium." World Encyclopedia. . Encyclopedia.com. (August 23, 2017). http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/uranium
"uranium." World Encyclopedia. . Retrieved August 23, 2017 from Encyclopedia.com: http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/uranium
u·ra·ni·um / yoŏˈrānēəm/ • n. the chemical element of atomic number 92, a gray, dense radioactive metal used as a fuel in nuclear reactors. (Symbol: U)
"uranium." The Oxford Pocket Dictionary of Current English. . Encyclopedia.com. (August 23, 2017). http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/uranium-0
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"uranium." The Concise Oxford Dictionary of English Etymology. . Encyclopedia.com. (August 23, 2017). http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/uranium-1
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"uranium." Oxford Dictionary of Rhymes. . Encyclopedia.com. (August 23, 2017). http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/uranium
"uranium." Oxford Dictionary of Rhymes. . Retrieved August 23, 2017 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/uranium