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uranium

  (yʊ-rā'nē-əm) pronunciation
n. (Symbol U)

A heavy silvery-white metallic element, radioactive and toxic, easily oxidized, and having 14 known isotopes of which U 238 is the most abundant in nature. The element occurs in several minerals, including uraninite and carnotite, from which it is extracted and processed for use in research, nuclear fuels, and nuclear weapons. Atomic number 92; atomic weight 238.03; melting point 1,132°C; boiling point 3,818°C; specific gravity 18.95; valence 2, 3, 4, 5, 6.

[New Latin ūranium, after Ūranus, Uranus. See Uranus.]

WORD HISTORY   Some chemical elements, such as ytterbium and berkelium, derive their names from the places they were discovered, but the element uranium owes its name to an earlier scientific discovery, that of the planet Uranus. Sir William Herschel, who discovered Uranus in 1781, wanted to name the planet Georgium sidus, “the Georgian planet,” in honor of George III; others called it Herschel. Eventually convention prevailed and the planet came to be called Uranus, like Mercury and Pluto the name of a heavenly deity in classical mythology. This god, called Ouranos in Greek (Latinized as Uranus), was chosen because he was the father of Saturn (Greek Kronos), the deity of the planet next in line, who himself was the father of Jupiter (Greek Zeus), the deity of the next planet. The name of this new planet Uranus was then used in the name of a new chemical element discovered eight years later by M.H. Klaproth. Klaproth, a German scientist, gave it the Latin name uranium in honor of the discovery of Uranus. Uranium passed into English shortly thereafter, being first recorded in the third edition of the Encyclopedia Britannica, published in 1797.


 
 

A chemical element, symbol U, atomic number 92, atomic weight 238.03. The melting point is 1132°C (2070°F) and the boiling point is 3818°C (6904°F). Uranium is one of the actinide series. See also Actinide elements; Periodic table.

Uranium in nature is a mixture of three isotopes: 234U, 235U, and 238U. Uranium is believed to be concentrated largely in the Earth's crust, where the average concentration is 4 parts per million (ppm). The total uranium content of the Earth's crust to a depth of 15 mi (25 km) is calculated to be 2.2 × 1017 lb (1017 kg); the oceans may contain 2.2 × 1013 lb (1013 kg) of uranium. Several hundred uranium-containing minerals have been identified, but only a few are of commercial interest. See also Radioactive minerals; Uraninite.

Because of the great importance of the fissile isotope 235U, rather sophisticated industrial methods for its separation from the natural isotope mixture have been devised. The gaseous diffusion process, which in the United States is operated in three large plants (at Oak Ridge, Tennessee; Paducah, Kentucky; and Portsmouth, Ohio) has been the established industrial process. Other processes applied to the separation of uranium include the centrifuge process, in which gaseous uranium hexafluoride is separated in centrifuge cascades, the liquid thermal diffusion process, the separation nozzle, and laser excitation.

Uranium is a very dense, strongly electropositive, reactive metal; it is ductile and malleable, but a poor conductor of electricity. Many uranium alloys are of great interest in nuclear technology because the pure metal is chemically active and anisotropic and has poor mechanical properties. However, cylindrical rods of pure uranium coated with silicon and canned in aluminum tubes (slugs) are used in production reactors. Uranium alloys can also be useful in diluting enriched uranium for reactors and in providing liquid fuels. Uranium depleted of the fissile isotope 235U has been used in shielded containers for storage and transport of radioactive materials. See also Nuclear fuels; Nuclear reactor.

Uranium reacts with nearly all nonmetallic elements and their binary compounds. Uranium dissolves in hydrochloric acid and nitric acid, but nonoxidizing acids, such as sulfuric, phosphoric, or hydrofluoric acid, react very slowly. Uranium metal is inert to alkalies, but addition of peroxide causes formation of water-soluble peruranates. See also Uranium metallurgy.

Uranium reacts reversibly with hydrogen to form UH3 at 250°C (482°F). Correspondingly, the hydrogen isotopes form uranium deuteride, UD3, and uranium tritide, UT3. The uranium-oxygen system is extremely complicated. Uranium monoxide, UO, is a gaseous species which is not stable below 1800°C (3270°F). In the range UO2 to UO3, a large number of phases exist. The uranium halides constitute an important group of compounds. Uranium tetrafluoride is an intermediate in the preparation of the metal and the hexafluoride. Uranium hexafluoride, which is the most volatile uranium compound, is used in the isotope separation of 235U and 238U. The halides react with oxygen at elevated temperatures to form uranyl compounds and ultimately U3O8.


 

n
U

A heavy, radioactive metallic element. Its atomic number is 92, and its atomic weight is 238.0289. Uranium is the heaviest of the natural elements. Isotopes of uranium are used in nuclear power plants to provide neutrons for nuclear reactions that result in release of energy.

 

Chemical element of the actinide series (with many transition element properties), chemical symbol U, atomic number 92. A dense, hard, silvery white metal that tarnishes in air, it is isolated from such ores as pitchblende. Until the discovery of the first transuranium element in 1940, uranium was believed to be the heaviest element. Radioactivity was discovered in uranium by A.-H. Becquerel. All its isotopes are radioactive; several have half-lives long enough to permit determination of the age of the Earth by uranium-thorium-lead dating and uranium-234 – uranium-238 dating. Nuclear fission was discovered in 1938 in uranium bombarded with neutrons, and the self-sustaining nuclear chain reaction, the atomic bomb, and the generation of nuclear power followed. Uranium has various valences in compounds, some of which have been used as colours in ceramic glazes, in lightbulb filaments, in photography, and as dyes and mordants.

For more information on uranium, visit Britannica.com.

 
(yūrā'nēəm) , radioactive metallic chemical element; symbol U; at. no. 92; at. wt. 238.0289; m.p. 1,132°C; b.p. 3,818°C; sp. gr. 19.1 at 25°C; valence +3, +4, +5, or +6.

Properties

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 important is pitchblende, mined in the Congo River basin and NW Canada. Coffinite (a uranium silicate) and carnotite (a potassium uranate-vanadate) are important minerals found in Colorado and Utah. Ores with as little as 0.1% uranium are mined and processed. Most ores are processed by chemical methods including leaching and solvent extraction. 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.

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, the use of fast breeder reactors that convert nonfissionable uranium-238 to fissionable plutonium-239 is becoming increasingly important (see nuclear reactor). Uranium-235 can be separated from uranium-238 by a diffusion process using the gaseous hexafluoride, UF6; the compound of the lighter isotope diffuses faster.


 

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

Periodicals

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.

 

A chemical element that is naturally radioactive. An isotope of uranium, uranium 235, is the main fuel for nuclear reactors and atomic bombs. Its symbol is U. (See fission and chain reaction.)

 

A chemical element, atomic number 92, atomic weight 238.03, symbol U.

 
Word Tutor: uranium
pronunciation

IN BRIEF: A heavy metal that is radioactive.

pronunciation Uranium is used to produce energy for some power plants that make electricity.

 
Wikipedia: uranium
92 protactiniumuraniumneptunium
Nd

U

(Uqb)
U-TableImage.png
General
Name, symbol, number uranium, U, 92
Chemical series actinides
Group, period, block n/a7, f
Appearance silvery gray metallic;
corrodes to a spalling
black oxide coat in air
HEUraniumC.jpg
Standard atomic weight 238.02891(3) g·mol−1
Electron configuration [Rn] 5f3 6d1 7s2
Electrons per shell 2, 8, 18, 32, 21, 9, 2
Physical properties
Phase solid
Density (near r.t.) 19.1 g·cm−3
Liquid density at m.p. 17.3 g·cm−3
Melting point 1405.3 K
(1132.2 °C, 2070 °F)
Boiling point 4404 K
(4131 °C, 7468 °F)
Heat of fusion 9.14 kJ·mol−1
Heat of vaporization 417.1 kJ·mol−1
Heat capacity (25 °C) 27.665 J·mol−1·K−1
Vapor pressure
P/Pa 1 10 100 1 k 10 k 100 k
at T/K 2325 2564 2859 3234 3727 4402
Atomic properties
Crystal structure orthorhombic
Oxidation states 3+,4+,5+,6+[57]
(weakly basic oxide)
Electronegativity 1.38 (Pauling scale)
Ionization energies 1st: 597.6 kJ/mol
2nd: 1420 kJ/mol
Atomic radius 175 pm
Van der Waals radius 186 pm
Miscellaneous
Magnetic ordering paramagnetic
Electrical resistivity (0 °C) 0.280 µΩ·m
Thermal conductivity (300 K) 27.5 W·m−1·K−1
Thermal expansion (25 °C) 13.9 µm·m−1·K−1
Speed of sound (thin rod) (20 °C) 3155 m/s
Young's modulus 208 GPa
Shear modulus 111 GPa
Bulk modulus 100 GPa
Poisson ratio 0.23
CAS registry number 7440-61-1
Selected isotopes
Main article: Isotopes of uranium
iso NA half-life DM DE (MeV) DP
232U syn 68.9 y α & SF 5.414 228Th
233U syn 159,200 y SF & α 4.909 229Th
234U 0.0054% 245,500 y SF & α 4.859 230Th
235U 0.7204% 7.038×108 y SF & α 4.679 231Th
236U syn 2.342×107 y SF & α 4.572 232Th
238U 99.2742% 4.468×109 y SF & α 4.270 234Th
References

Uranium (IPA: /jʊˈreɪniəm/)is a white/black metallic chemical element in the actinide series of the periodic table that has the symbol U and atomic number 92. It has 92 protons and electrons, 6 of them valence electrons. It can have between 141 and 146 neutrons, with 143 and 146 in its most common isotopes. Uranium has the highest atomic weight of the naturally occurring elements (see plutonium). Uranium is approximately 70% more dense than lead and is weakly radioactive. It occurs naturally in low concentrations (a few parts per million) in soil, rock and water, and is commercially extracted from uranium-bearing minerals such as uraninite (see uranium mining).

In nature, uranium atoms exist as uranium-238 (99.275%), uranium-235 (0.711%), and a very small amount of uranium-234 (0.0058%). Uranium decays slowly by emitting an alpha particle. The half-life of uranium-238 is about 4.47 billion years and that of uranium-235 is 704 million years,[1] making them useful in dating the age of the Earth (see uranium-thorium dating, uranium-lead dating and uranium-uranium dating). Along with thorium and plutonium, uranium is one of the three fissile elements, meaning it can easily break apart to become lighter elements. While uranium-238 has a small probability to fission spontaneously or when bombarded with fast neutrons, the much higher probability of uranium-235 and to a lesser degree uranium-233 to fission when bombarded with slow neutrons generates the heat in nuclear reactors used as a source of power, and provides the fissile material for nuclear weapons. Both uses rely on the ability of uranium to produce a sustained nuclear chain reaction. Depleted uranium (uranium-238) is used in kinetic energy penetrators and armor plating.[2]

Uranium is used as a colorant in uranium glass, producing orange-red to lemon yellow hues. It was also used for tinting and shading in early photography. The 1789 discovery of uranium in the mineral pitchblende is credited to Martin Heinrich Klaproth, who named the new element after the planet Uranus. Eugène-Melchior Péligot was the first person to isolate the metal, and its radioactive properties were uncovered in 1896 by Antoine Becquerel. Research by Enrico Fermi and others starting in 1934 led to its use as a fuel in the nuclear power industry and in Little Boy, the first nuclear weapon used in war. An ensuing arms race during the Cold War between the United States and the Soviet Union produced tens of thousands of nuclear weapons that used enriched uranium and uranium-derived plutonium. The security of those weapons and their fissile material following the breakup of the Soviet Union in 1991 along with the legacy of nuclear testing and nuclear accidents is a concern for public health and safety.

Characteristics

An induced nuclear fission event involving uranium-235
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An induced nuclear fission event involving uranium-235

When refined, uranium is a silvery white, weakly radioactive metal, which is slightly softer than steel,[3] strongly electropositive and a poor electrical conductor.[4] It is malleable, ductile, and slightly paramagnetic.[3] Uranium metal has very high density, being approximately 70% more dense than lead, but slightly less dense than gold.

Uranium metal reacts with nearly all nonmetallic elements and their compounds, with reactivity increasing with temperature.[5] Hydrochloric and nitric acids dissolve uranium, but nonoxidizing acids attack the element very slowly.[4] When finely divided, it can react with cold water; in air, uranium metal becomes coated with a dark layer of uranium oxide.[3] Uranium in ores is extracted chemically and converted into uranium dioxide or other chemical forms usable in industry.

Uranium was the first element that was found to be fissile. Upon bombardment with slow neutrons, its uranium-235 isotope becomes a very short-lived uranium-236 isotope, which immediately divides into two smaller nuclei, releasing nuclear binding energy and more neutrons. If these neutrons are absorbed by other uranium-235 nuclei, a nuclear chain reaction occurs and, if there is nothing to absorb some neutrons and slow the reaction, the reaction is explosive. As little as 15 lb (7 kg) of uranium-235 can be used to make an atomic bomb.[6] The first atomic bomb worked by this principle (nuclear fission).

Applications

Military

Depleted uranium is used by various militaries as high-density penetrators.
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Depleted uranium is used by various militaries as high-density penetrators.

The major application of uranium in the military sector is in high-density penetrators. This ammunition consists of depleted uranium (DU) alloyed with 1–2% other elements. At high impact speed, the density, hardness, and flammability of the projectile enable destruction of heavily armored targets. Tank armor and the removable armor on combat vehicles are also hardened with depleted uranium (DU) plates. The use of DU became a contentious political-environmental issue after the use of DU munitions by the US, UK and other countries during wars in the Persian Gulf and the Balkans raised questions of uranium compounds left in the soil (see Gulf War Syndrome).[6]

Depleted uranium is also used as a shielding material in some containers used to store and transport radioactive materials.[4] Other uses of DU include counterweights for aircraft control surfaces, as ballast for missile re-entry vehicles and as a shielding material.[3] Due to its high density, this material is found in inertial guidance devices and in gyroscopic compasses.[3] DU is preferred over similarly dense metals due to its ability to be easily machined and cast as well as its relatively low cost.[7] Counter to popular belief, the main risk of exposure to DU is chemical poisoning by uranium oxide rather than radioactivity (uranium being only a weak alpha emitter).

During the later stages of World War II, the entire Cold War, and to a much lesser extent afterwards, uranium was used as the fissile explosive material to produce nuclear weapons. Two major types of fission bombs were built: a relatively simple device that uses uranium-235 and a more complicated mechanism that uses uranium-238-derived plutonium-239. Later, a much more complicated and far more powerful fusion bomb that uses a plutonium-based device in a uranium casing to cause a mixture of tritium and deuterium to undergo nuclear fusion was built.[8]

Civilian

The most visible civilian use of uranium is as the thermal power source used in nuclear power plants.
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The most visible civilian use of uranium is as the thermal power source used in nuclear power plants.

The main use of uranium in the civilian sector is to fuel commercial nuclear power plants; by the time it is completely fissioned, one kilogram of uranium can theoretically produce about 20 trillion joules of energy (20×1012 joules); as much electricity as 1500 tonnes of coal.[2] Generally this is in the form of enriched uranium, which has been processed to have higher-than-natural levels of uranium-235 and can be used for a variety of purposes relating to nuclear fission.

Commercial nuclear power plants use fuel that is typically enriched to around 3% uranium-235,[2] though some reactor designs (such as the CANDU reactors) can use unenriched uranium fuel. Fuel used for United States Navy reactors is typically highly enriched in uranium-235 (the exact values are classified). In a breeder reactor, uranium-238 can also be converted into plutonium through the following reaction:[3] 238U(n, gamma) → 239U -(beta) → 239Np -(beta) → 239Pu.

Uranium glass glowing under UV light
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Uranium glass glowing under UV light
Uranium glass used as lead-in seals in a vacuum capacitor
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Uranium glass used as lead-in seals in a vacuum capacitor

Prior to the discovery of radiation, uranium was primarily used in small amounts for yellow glass and pottery dyes (such as uranium glass and in Fiestaware). Uranium was also used in photographic chemicals (esp. uranium nitrate as a toner),[3] in lamp filaments, to improve the appearance of dentures, and in the leather and wood industries for stains and dyes. Uranium salts are mordants of silk or wool. Uranyl acetate and uranyl formate are used as stains in transmission electron microscopy, to increase the contrast of biological specimens in ultrathin sections and in negative staining of viruses, isolated cell organelles and macromolecules.

The discovery of the radioactivity of uranium ushered in additional scientific and practical uses of the element. The long half-life of the isotope uranium-238 (4.51×109 years) makes it well-suited for use in estimating the age of the earliest igneous rocks and for other types of radiometric dating (including uranium-thorium dating and uranium-lead dating). Uranium metal is used for X-ray targets in the making of high-energy X-rays.[3]

History

Pre-discovery use

The use of uranium in its natural oxide form dates back to at least the year 79, when it was used to add a yellow color to ceramic glazes.[3] Yellow glass with 1% uranium oxide was found in a Roman villa on Cape Posillipo in the Bay of Naples, Italy by R. T. Gunther of the University of Oxford in 1912.[9] Starting in the late Middle Ages, pitchblende was extracted from the Habsburg silver mines in Joachimsthal, Bohemia (now Jáchymov in the Czech Republic) and was used as a coloring agent in the local glassmaking industry.[10] In the early 19th century, the world's only known source of uranium ores were these old mines.

Discovery

Antoine Becquerel discovered the phenomenon of radioactivity by exposing a photographic plate to uranium (1896).
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Antoine Becquerel discovered the phenomenon of radioactivity by exposing a photographic plate to uranium (1896).

The discovery of the element is credited to the German chemist Martin Heinrich Klaproth. While he was working in his experimental laboratory in Berlin in 1789, Klaproth was able to precipitate a yellow compound (likely sodium diuranate) by dissolving pitchblende in nitric acid and neutralizing the solution with sodium hydroxide.[10] Klaproth mistakenly assumed the yellow substance was the oxide of a yet-undiscovered element and heated it with charcoal to obtain a black powder, which he thought was the newly discovered metal itself (in fact, that powder was an oxide of uranium).[10][11] He named the newly discovered element after the planet Uranus, which had been discovered eight years earlier by William Herschel.[12]

In 1841, Eugène-Melchior Péligot, who was Professor of Analytical Chemistry at the Conservatoire National des Arts et Métiers (Central School of Arts and Manufactures) in Paris, isolated the first sample of uranium metal by heating uranium tetrachloride with potassium.[13][10] Uranium was not seen as being particularly dangerous during much of the 19th century, leading to the development of various uses for the element. One such use for the oxide was the aforementioned but no longer secret coloring of pottery and glass.

Antoine Becquerel discovered radioactivity by using uranium in 1896.[5] Becquerel made the discovery in Paris by leaving a sample of uranium on top of an unexposed photographic plate in a drawer and noting that the plate had become 'fogged'.[14] He determined that a form of invisible light or rays emitted by uranium had exposed the plate.

Fission research

Enrico Fermi (bottom left) and the rest of the team that initiated the first artificial nuclear chain reaction (1942).
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Enrico Fermi (bottom left) and the rest of the team that initiated the first artificial nuclear chain reaction (1942).

A team led by Enrico Fermi in 1934 observed that bombarding uranium with neutrons produces the emission of beta rays (electrons or positrons; see beta particle).[15] The fission products were at first mistaken for new elements of atomic numbers 93 and 94, which the Dean of the Faculty of Rome, Orso Mario Corbino, christened ausonium and hesperium, respectively.[16][17][18][19] The experiments leading to the discovery of uranium's ability to fission (break apart) into lighter elements and release binding energy were conducted by Otto Hahn and Fritz Strassmann[15] in Hahn's laboratory in Berlin. Lise Meitner and her nephew, physicist Otto Robert Frisch, published the physical explanation in February 1939 and named the process 'nuclear fission'.[20] Soon after, Fermi hypothesized that the fission of uranium might release enough neutrons to sustain a fission reaction. Confirmation of this hypothesis came in 1939, and later work found that on average about 2 1/2 neutrons are released by each fission of the rare uranium isotope uranium-235.[15] Further work found that the far more common uranium-238 isotope can be transmuted into plutonium, which, like uranium-235, is also fissionable by thermal neutrons.

On 2 December 1942, another team led by Enrico Fermi was able to initiate the first artificial nuclear chain reaction. Working in a lab below the stands of Stagg Field at the University of Chicago, the team created the conditions needed for such a reaction by piling together 400 tons (360 tonnes) of graphite, 58 tons (53 tonnes) of uranium oxide, and six tons (five and a half tonnes) of uranium metal.[15] Later researchers found that such a chain reaction could either be controlled to produce usable energy or could be allowed to go out of control to produce an explosion more violent than anything possible using chemical explosives.

Bombs and reactors

The mushroom cloud over Hiroshima after the dropping of the uranium-based atomic bomb nicknamed 'Little Boy' (1945)
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The mushroom cloud over Hiroshima after the dropping of the uranium-based atomic bomb nicknamed 'Little Boy' (1945)

Two major types of atomic bomb were developed in the Manhattan Project during World War II: a plutonium-based device (see Trinity test and 'Fat Man') whose plutonium was derived from uranium-238, and a uranium-based device (nicknamed 'Little Boy') whose fissile material was highly enriched uranium. The uranium-based Little Boy device became the first nuclear weapon used in war when it was detonated over the Japanese city of Hiroshima on 6 August 1945. Exploding with a yield equivalent to 12,500 tonnes of TNT, the blast and thermal wave of the bomb destroyed nearly 50,000 buildings and killed approximately 75,000 people (see Atomic bombings of Hiroshima and Nagasaki).[14]

Four light bulbs lit with electricity generated from the first artificial electricity-producing nuclear reactor, EBR-I (1951)
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Four light bulbs lit with electricity generated from the first artificial electricity-producing nuclear reactor, EBR-I (1951)

Experimental Breeder Reactor I at the Idaho National Laboratory(INL) near Arco, Idaho became the first functioning artificial nuclear reactor on 20 December 1951. Initially, four 150-watt light bulbs were lit by the reactor, but improvements eventually enabled it to power the whole facility (later, the whole town of Arco became the first in the world to have all its electricity come from nuclear power).[21] The world's first commercial scale nuclear power station, Calder Hall in England, began generation on 17 October 1956.[22] Another early power reactor was the Shippingport Reactor in Pennsylvania, which began electricity production in 1957. Nuclear power was used for the first time for propulsion by a submarine, the USS Nautilus, in 1954.[15]

Fifteen ancient and no longer active natural nuclear fission reactors were found in three separate ore deposits at the Oklo mine in Gabon, West Africa in 1972. Discovered by French physicist Francis Perrin, they are collectively known as the Oklo Fossil Reactors. The ore they exist in is 1.7 billion years old; at that time, uranium-235 constituted about three percent of the total uranium on Earth.[23] This is high enough to permit a sustained nuclear fission chain reaction to occur, providing other conditions are right. The ability of the surrounding sediment to contain the nuclear waste products in less than ideal conditions has been cited by the U.S. federal government as evidence of their claim that the Yucca Mountain facility could safely be a repository of waste for the nuclear power industry.[23]

Cold War legacy and waste

U.S. and USSR/Russian nuclear weapons stockpiles, 1945–2006
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U.S. and USSR/Russian nuclear weapons stockpiles, 1945–2006

During the Cold War</