92
  U  
238.028900
Uranium

Name: Uranium
Symbol: U
Atomic Number: 92
Atomic Weight: 238.028900
Family: Rare Earth Elements
CAS RN: 7440-61-1
Description: A silvery-white, dense, ductile and malleable, radioactive metal.
State (25C): Solid
Oxidation states: +3, +4, +5, +6

Molar Volume: 12.49 cm3/mole
Valence Electrons: 5f36d17s2

Boiling Point: 4091.15oK, 3818.0oC, 6904.4oF
Melting Point: 1405.15oK, 1132.0oC, 2069.6oF

Electrons Energy Level: 2, 8, 18, 32, 21, 9, 2
Isotopes: 26 + None Stable
Heat of Vaporization: 477 kJ/mol
Heat of Fusion: 15.48 kJ/mol

Density: 18.95 g/cm3 @ 300K
Specific Heat: 0.12 J/gK
Atomic Radius: 138.5 pm
Ionic Radius: 1.42
Electronegativity: 1.38 (Pauling), 1.2 (Allrod Rochow)
Vapor Pressure: 1.19E-06 Pa @ 1132C
89
Ac
227.0
90
Th
232.0
91
Pa
231.0
92
U
238.0
93
Np
237.0
94
Pu
(244)
95
Am
(243)
96
Cm
(247)
97
Bk
(247)
98
Cf
(251)
99
Es
(252)
100
Fm
(257)
101
Md
(258)
102
No
(259)
103
Lr
(260)

1s2 2s2p6 3s2p6d10 4s2p6d10f14 5s2p6d10f3 6s2p6d1 7s2

History

Pre-Discovery Use

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

Discovery

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

(Planet Uranus, named after the Greek Ouranos, the son and husband of Gaia).   The discovery of the element is credited to the German pharmacist 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.  Klaproth mistakingly 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).  He named the newly discovered element after the planet Uranus, which had been discovered eight years earlier by William Herschel.

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Martin Heinrich Klaproth

In 1841, Eugene-Melchior Peligot, who was Professor of Analytical Chemistry at the Central School of Arts and Manufactures in Paris, isolated the first sample of uranium metal by heating uranium tetrachloride (UCl4) with potassium in a platinum crucible.  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 before-mentioned but no longer secret coloring of pottery and glass.

Antoine Becquerel found uranium to be radioactive in 1896 and in the process discovered the concept of radioactivity itself.  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' as if it were partially exposed to light.   He determined that a form of invisible light or rays emitted by uranium had exposed the plate.

Fission Research

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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).  The ability of uranium to fission (break apart) into lighter elements and released binding energy when hit by thermal neutrons was discovered by Otto Hahn and Fritz Strassmann in late 1938.  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 2 1/2 neutrons are released by each fission of the rare uranium isotope uranium-235.  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 December 2, 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 of graphite, 58 tons of uranium oxide, and six tons of uranium metal.  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

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

Two types of atomic bombs were developed in the Manhatton Project during World War II; a plutonium-based (Fat-Man)  device whose plutonium was derived from the uranium-238 isotope, and a uranium-based device (Little Boy) whose fissile material was a blend of uranium isotopes that were highly enriched in uranium-235.   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 August 6th, 1945. Exploding with a yield equivalent to 12,500 tons of TNT (TriNitroToluene), the blast and thermal wave of the bomb destroyed nearly 50,000 buildings and killed approximately 75,000 people.

Initially it was believed that uranium was relatively rare, and that nuclear proliferation could be avoided by simply buying up all known uranium stocks, though within a decade large deposits of it were discovered in many places around the world.

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First four light bulbs lit with electricity generated from the first artificial nuclear reactor, EBRI (1951).

Electricity was generated for the first time by an artificial nuclear reactor on December 20, 1951 by Experimental Breeder Reactor I at the Idaho National Engineering Laboratory near Arco, Idaho.  Initially, only four 150-watt light bulbs were lit by the reactor but improvements in operating the reactor led to its use in powering the whole Argonne facility (later, the whole town of Arco became the first in the world to have all its electricity come from nuclear power).  The world's first commercial scale nuclear power station, Calder Hall, in England, began generation on October 17, 1956.   Another early power reactor was the Shippingport Reactor in Pennsylvania (1957).   Nuclear power was used for the first time in propulsion by the Uss Nautilus, which is a submarine that was set to sea in 1954.

Fifteen ancient and no longer active natural fission reactors were found in three separate ore deposits at the Okla 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 and at the time uranium-235 comprised about three percent of the total uranium on Earth.  This is high enough to permit nuclear fissions to occur, providing other conditions are right.  The concept of a natural nuclear reactor was theorized as early as 1956 by Paul Kuroda at the University of Arkansas.  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 from the nuclear power industry.

Cold War Legacy and Waste

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

During the "Cold War" between the Soviet Union and the United States, huge stockpiles of uranium were amassed and tens of thousands of nuclear weapons were created using enriched uranium.  Since the break-up of the Soviet Union in 1991, an estimated 600 tons of highly-enriched weapons grade uranium (enough to make 40,000 nuclear warheads) have been stored in poorly guarded facilities in the Russian Federation and several other former Soviet states.  Police in Asia, Europe, and South America on at least 16 occasions from 1993 to 2005 have intercepted shipments of smuggled bomb-grade uranium or plutonium, most of which was from ex-Soviet sources.  Since 1993, the Material Protection, Control, and Accounting Program, operated by the United States government, has spent approximately $550 million to help safeguard uranium and plutonium stockpiles in Russia.

Above-ground nuclear tests by the Soviet Union and the United States in the 1950s and early 1960s and by France and Israel into the 1970s and 1980s, spread a significant amount of fallout from uranium daughter isotopes around the world.  Additional fallout and pollution occurred from several nuclear accidents; the Windscale fire at the Sellafield nuclear plant in 1957 spread iodine-131 over much of Northern England, the Three Mile Island accident in 1979 released radon gas and some iodine-131, the Chernobyl disaster in 1986 released radon, iodine-131 and strontium-90 that spread over much of Europe.

Characteristics

From at least 79 CE, uranium was used as a colorant in ceramic glazes, producing colors that ranged from orange-red to lemon yellow .  

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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, strongly electropostive and a poor electrical conductor.   It is malleable, ductile, and slightly paramagnetic.  Uranium metal has very high density, 65% more dense than lead, but slightly less dense than gold.

1s2
2s2 2p6
3s2 3p6 3d10
4s2 4p6 4d10 4f14
5s2 5p6 5d10 5f3
6s2 6p6 6d1
7s2

Uranium metal reacts with nearly all nonmetallic elements and their compounds with reactivity increasing with temperature. 

Hydrochloric and nitric acids dissolve uranium but nonoxidizing acids attack the element very slowly.  When finely divided, it can react with cold water; in air, uranium metal becomes coated with a layer of uranium oxide.  Uranium in ores is extracted chemically and converted into uranium dioxide or other chemical forms usable in industry.

Uranium can be prepared by reducing uranium halides with alkali or alkaline earth metals or by reducing uranium oxides by calcium, aluminum, or carbon at high temperatures. The metal can also be produced by electrolysis of KUF5 or UF4, dissolved in a molten mixture of CaCl2 and NaCl.  High-purity uranium can be prepared by the thermal decomposition of uranium halides on a hot filament.

Uranium was the first element that was found to be fissile.  Upon bombardment with slow neutrons, its uranium-235 isotope becomes the very short lived uranium-236 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.  The first atomic bomb worked by this principle (nuclear fission). A more accurate name for both this and the hydrogen bomb (nuclear fusion) would be 'nuclear bomb' or "nuclear weapon", because only the nuclei participate.

Uranium metal has three allotropic forms:

Occurrence

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Uranium Ore

Uranium is a naturally occurring element found in low levels and always combined with other elements within all rock, soil, and water.  This is the highest-numbered element to be found naturally in significant quantities on earth.  It's average concentration in the Earth's crust is (depending on the reference) 2 to 4 parts per million, or about 40 times as abundant as silver.  The Earth's crust from the surface to 25 km (15 miles) down is calculated to contain 1017 kg (2 x 1017 lb) of uranium while theoceans may contain 1013 kg (2 x 1013 lb).   The concentration of uranium in soil ranges from 0.7 to 11 parts per million (up to 15 parts per million in farmland soil due to use of phosphate fertilizer) and 3 parts per billion of sea water is composed of the element.

It is more plentiful than antimony, tin, cadmium, mercury, or silver and is about as abundant as arsenic or molybdenum.  It is found in hundreds of minerals including uraninite (the most common uranium ore), autunite, uranophane, torbernite, and coffinite.   Significant concentrations of uranium occur in some substances such as phosphate rock deposits, and minerals such as lignite, and monazite sands in uranium-rich ores (it is recovered commercially from these sources with as little as 0.1% uranium.

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Four parts per million of the Earth's crust is composed of uranium.

The decay of uranium, thorium and potassium-40 in the Earth's mantle is thought to be the main source of heat that keeps the outer core liquid and drives mantle convection, which in turn drives plate tectonics.

Some micro-organisms, such as the lichen Trapelia involuta or the bacterium Citrobactor, can absorb concentrations of uranium that are up to 300 times higher than its environment.   Citrobactor species absorb uranyl ions when given glycerol phosphate (or other similar organic phosphates).  After one day, one gram of bacteria will encrust themselves with nine grams of uranyl phosphate crystals; creating the possibility that these organisms could be used to decontaminate uranium-polluted water.

Plants absorb some uranium from the soil they are rooted in.  Dry weight concentrations of uranium in plants range from 5 to 60 parts per billion and ash from burnt wood can have concentrations up to 4 parts per million.  Dry weight concentrations of uranium in food plants are typically lower with one to two micrograms per day ingested through the food people eat.

Mining and Reserves

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Uraninite, also known as Pitchblende

Uranium is distributed worldwide and 21 countries export uranium ore, with Canada, Australia and Niger being the three largest exporters and the United States, Congo, South Africa, Gabon, Russia and China also having significant deposits.  Three million tons of uranium ore reserves are known to exist and an additional five billion tons of uranium are estimated to be in sea water (Japanese scientists in the 1980s proved that extraction of uranium from sea water using ion exchangers was feasible).

Uranium ore is mined in several ways; by open pit, underground or by leaching uranium from low-grade ores.

Australia has the world's largest uranium ore reserves — 40 percent of the planet's known supply.  In fact, the world's largest single uranium deposit is located at the Olymic Dam Mine in south Australia.  Almost all the uranium is exported, but under strict International Atomic Energy Agency safeguards to satisfy the Australian people and government that none of the uranium is used in nuclear weapons.   Australian uranium is used strictly for electricity production.  The Australian government is currently advocating an expansion of uranium mining, although issues with state governments and indigenous interests complicate the issue.

In spite of Australia's huge reserves, Canada remains the largest exporter of uranium ore, with mines located in the Athabasca Basin in northern Saskatchewan.  Cameco, the world’s largest, low-cost uranium producer accounting for 18% of the world's uranium production, operates three mines in the area.

Production/Enrichment

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Cascades of Gas Centrifuges are used to enrich uranium ore to concentrate its fissionable isotopes.

Commercial-grade uranium can be produced through the reduction of uranium halides with alkali or alkaline earth metals.  Uranium metal can also be made through electrolysi of KUF5 or UF4, dissolved in a molten calcium chloride (CaCl2) and sodium chloride (NaCl).  Very pure uranium can be produced through the thermal decomposition of uranium halides on a hot filament.

Owners and operators of U.S. civilian nuclear power reactors purchased from U.S. and foreign suppliers a total of 21,300 tons of uranium deliveries during 2001.  The average price paid was $26.39 per kilogram of uranium, a decrease of 16 percent compared with the 1998 price.  In 2001, the U.S. produced 1,018 tons of uranium from seven mining operations, all of which are west of the Mississippi River.

Enrichment of uranium ore to concentrate the fissionable uranium-235 is needed for use in power plants and nuclear weapons. A majority of neutrons released by a fissioning atom of uranium-235 must impact other uranium-235 atoms to sustain the nuclear chain reaction needed for these applications.  The concentration and amount of uranium-235 needed to achieve this is called a "critical mass".  The gas centrifuge process, where gaseous uranium hexafluoride (UF6) is separated by weight using high-speed centrifuges has become the cheapest and leading enrichment process (lighter UF6 concentrates in the center of the centrifuge).  The gaseous diffusion process was the previous leading method for enrichment and the one used in the Manhatton Project.  In this process, uranium hexafluoride is repeatedly diffused through a siver-zinc membrane and the different isotopes of uranium are separated by diffusion rate (uranium 238 is heavier and thus diffuses slightly slower than uranium-235).  The laser excitation method employs a laser beam of precise energy to sever the bond between uranium-235 and fluorine.  This leaves uranium-238 bonded to fluorine and allows uranium-235 metal to precipitate from the solution.  Another method is called liquide thermal diffusion.

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Powdered Yellowcake in a drum

Yellowcake is purified U-238.  It takes its name from the color and texture of the concentrates produced by early mining operations, despite the fact that modern mills using higher calcining temperatures produce "yellowcake" that is dull yellow to almost black.  Initially, the compounds formed in yellowcakes were not identified; in 1970, the U.S. Bureau of Mines still referred to yellowcakes as the final precipitate formed in the milling process and considered it to be ammonium diuranate or sodium diuranate.  The compositions were variable and depended upon precipitating conditions.  Among the compounds identified in yellowcakes include: uranyl hydroxide, uranyl sulfate, sodium para-uranate and uranyl peroxide, along with various uranium oxides.  Modern yellowcake typically contains 70 to 90 percent uranium oxide (U3O8) by weight.  (Other uranium oxides, such as UO2 and UO3, exist; the most stable oxide, U3O8, is actually considered to be a 1:2 molar mixture of these.)

The ultimate supply of uranium is very large.  It is estimated that for a ten times increase in price, the supply of uranium that can be economically mined is increased 300 times.

Today, uranium is obtained from uranium ores such as pitchblende, uraninite (UO2), carnotite (K2(UO2)2VO41-3H2O) and autunite (Ca(UO2)2(PO4)210H2O) as well as from phosphate rock (Ca3(PO4)2), lignite (brown coal) and monazite sand ((Ce, La, Th, Nd, Y)PO4). Since there is little demand for uranium metal, uranium is usually sold in the form of sodium diuranate (Na2U2O76H2O), also known as yellow cake, or triuranium octoxide (U3O8).

Applications

Uranium-238, uranium's most common isotope, can be converted into plutonium-239, a fissionable material that can also be used as a fuel in nuclear reactors. To produce plutonium-239, atoms of uranium-238 are exposed to neutrons. Uranium-239 forms when uranium-238 absorbs a neutron. Uranium-239 has a half-life of about 23 minutes and decays into neptunium-239 through beta decay.  Neptunium-239 has a half-life of about 2.4 days and decays into plutonium-239, also through beta decay.

Although it does not occur naturally, uranium-233 is also a fissionable material that can be used as a fuel in nuclear reactors. To produce uranium-233, atoms of thorium-232 are exposed to neutrons. Thorium-233 forms when thorium-232 absorbs a neutron. Thorium-233 has a half-life of about 22 minutes and decays into protactinium-233 through beta decay. Protactinium-233 has a half-life of about 27 days and decays into uranium-233, also through beta decay. If completely fissioned, one pound (0.45 kilograms) of uranium-233 will provide the same amount of energy as burning 1,500 tons (1,350,000 kilograms) of coal.

Civilian

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Uranium glass glowing under UV light

Before radiation was discovered, uranium was primarily used in small amounts for yellow glass and pottery dyes (such as uranium glass and in Fiestaware.)  There was also some use in photographic chemicals (esp. uranium nitrate as a toner).  It was used in filaments for lamps and in the leather and wood industries for stains and dyes.  Uranium salts are mordants of silk or wool. Uranium was also used to improve the appearance of dentures.  After the discovery of uranium radiation, additional scientific and practical values of uranium were pursued.

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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 tons of coal.  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 typically enriched to around 3% uranium-235, though some reactor designs (such as the CANDU) reactors) can use unenriched uranium fuel. 

Military

After the discovery in 1939 that it could undergo nuclear fission, uranium gained importance with the development of practical uses of nuclear energy.  The first atomic bomb used in warfare, "Little Boy", was a uranium bomb.  This bomb contained enough of the uranium-235 isotope to start a runaway chain reaction which in a fraction of a second caused a large number of the uranium atoms to undergo fission, thereby releasing a fireball of energy.

Fuel used for United States Navy submarine reactors is typically highly enriched in uranium-235 (the exact values are classified information).  When uranium is enriched over 85% it is known as "weapons grade".  In a breeder reactor, uranium-238 can also be converted into plutonium through the following reactions:

238U(n, gamma)  rarrow.gif (63 bytes)  239U  --(beta)-->  239Np   --(beta)-->  239Pu.

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

Currently the major application of uranium in the U.S. military sector is in high-density penetrators.  This ammunition consists of depleted uranium alloyed with 1–2% other elements.  The applications of these armor-piercing rounds range from the 20 mm Phalanx gun of the U.S. Navy for piercing attacking missiles, through the 30 mm gun in A-10 aircraft, to 105mm and larger tank barrels.  At high impact speed, the density, hardness, and flammability of the projectile enable destruction of heavily armored targets.  Tank armour and the removable armour on combat vehicles are also hardened with depleted uranium (DU) plates.  The use of DU became a contentious political-environmental issue after US, UK and other countries' use of DU munitions in wars in the Persian Gulf and the Balkans raised questions of uranium compounds left in the soil.  Depleted uranium is also used as a shielding material in some containers used to store and transport radioactive materials.

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Uranyl Acetate is used in Analytical Chemistry

Other Uses

Compounds

Uranium compounds have been used for centuries to color glass.  A 2,000 year old sample of yellow glass found near Naples, Italy contains uranium oxide.  Uranium trioxide (UO3) is an orange powder and has been used in the manufacture of Fiestaware plates.  Other uranium compounds have also been used to make vaseline glass and glazes.  The uranium within these items is radioactive and should be treated with care.

Oxides & Uranates

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Uranium Dioxide: used to create fuel rods in nuclear power plants

Uranate is the chemical term for oxide anions of the element uranium.

Uranium Dioxide (UO2), an oxide of uranium, also known as urania or uranic oxide is a black crystalline powder that occurs naturally in the mineral urannite.   If produced from enriched uranium it is used in nuclear fuel rods in nuclear reactors.  A mixture of uranium and plutonium dioxides is used as MOX fuel.   Prior to 1960 it was used as yellow and black color in ceramic glazes and glass.

Uranium Trioxide (UO3), also called uranyl oxide, uranium(VI) oxide, and uranic oxide, is the hexavalent oxide of uranium. The solid may be obtained by heating uranyl nitrate to 400 C. Its most commonly encountered polymorph, ?-UO3, is a yellow-orange powder.

Triiuranium Octaoxide (U3O8) is an olive green to black, odorless solid compound of uranium.  In spite of its color, it is one of the more popular constituents of yellowcake and is shipped between mills and refineries in this form.

Uranyl Peroxide or Uranium Peroxide Hydrate (UO4nH2O) is a pale-yellow, soluble peroxide of uranium.  It is found present at one stage of the enriched uranium fuel cycle and in yellowcake prepared via the in situ leaching and resin ion exchange system.  This compound, also expressed as: UO3(H2O2)(H2O), is very similar to uranium trioxide hydrate UO3nH2O.  The dissolution behaviour of both compounds are very sensitive to the hydration state (n can vary between 0 and 4).  One main characteristic of uranium peroxide is that it consists of small needles with an average AMAD of about 1.1 m.

The uranyl minerals Studitite, UO44H2O, and metastudtite, UO42H2O, are the only minerals discovered to date found to contain peroxide.

Sodium Diuranate (Na2U2O76H2O) is a uranium salt also known as the yellow oxide of uranium.   Along with ammonium diurante ((NH4)2U2O7) it was a component in early yellowcakes, the ratio of the two species determined by process conditions; yellowcake is now largely a mix of various uranium oxides.

Hydrides, Carbides & Nitrides

Uranium metal heated to 250 to 300 C reacts with hydrogen to form uranium hydride.   Yet higher temperatures will reversibly remove the hydrogen.  This property makes uranium hydrides convenient starting materials to create reactive uranium powder along with various uranium carbide, nitride and halide compounds.  Two crystal modifications of uranium hydride exist: an a form that is obtained at low temperatures and a form that is created when the formation temperature is above 250C.

Uranium carbides and uranium nitrides are both relatively inert semimetallic compounds that are minimally soluble in acids, react with water, and can ignite in air to form U3O8.   Carbides of uranium include uranium monocarbide (UC), uranium dicarbide (UC2), and diuranium tricarbide (U2C3).  Both UC and UC2 are formed by adding carbon to molten uranium or by exposing the metal to carbon monoixide at high temperatures.  Stable below 1800C, U2C3 is prepared by subjecting a heated mixture of UC and UC2 to mechanical stress.   Uranium nitrides obtained by direct exposure of the metal to nitrogen include uranium mononitride (UN), uranium dinitride (UN2), and diuranium trinitride (U2N3).

Halides

Uranium Tetrafluoride (UF4) is known as "green salt" and is an intermediate product in the production of uranium hexafluoride. It has the appearance of an emerald-green solid.

Uranium Hexafluoride (UF6) is a colorless crystalline solid which forms a vapor at temperatures above 56.4 C. UF6 is the compound of uranium used for the two most common enrichment processes, gaseous diffusion enrichment, and gas centrifuge enrichment.  It is simply called "hex" in the industry. It is corrosive to many metals and reacts violently with water and oils.

Other Compounds

Uranyl Nitrate (UO2(NO3)2) is an extraordinarily toxic, soluble uranium salt.  It appears as a yellow crystalline solid.

Uranium Carbonate (UO2(CO3)) is found in both the mineral and organic fractions of coal and its fly ash and is the main component of uranium in mine tailing water.

Uranium Trihydride (UH3) appears as a black powder, is highly reactive, and pyrophoric.

Uranium Rhodium Germainium (URhGe) is the first discovered alloy that becomes superconducting in the presence of an extremely strong electromagnetic field.

Uranyl Sulfate (UO2SO4) a sulfate of uranium that presents as an odorless lemon-yellow sand-like solid in its pure crystalline form. I t has found use as a negative stain in mocroscopy and tracer in biology.   The Aqueous Homogeneous Reactor experiment, constructed in 1951, circulated a fuel composed of 565 grams of U-235 enriched to 14.7% in the form of uranyl sulfate.   The acid process of milling uranium ores involves precipitating uranyl sulfate from the pregnant leaching solution to produce yellowcake.

Uranium Ore
Pitchblende, Uraninite, UO2 Carnotite, K2(UO2)2VO41-3H2O
Autunite, Ca(UO2)2(PO4)210H2O Triuranium Octoxide, U3O8
Yellow Cake, Sodium Diuranate, Na2U2O76H2O

Isotopes

Its two principal isotopes are uranium-235 and uranium-238.  Naturally-occurring uranium also contains a small amount of the uranium-234 isotope, which is a decay product of uranium-238.  The isotope uranium-235 or enriched uranium is important for both nuclear reactors and nuclear weapons because it is the only isotope existing in nature to any appreciable extent that is fissile, that is, fissionable by thermal neutrons.  The isotope uranium-238 is also important because it absorbs neutrons to produce a radioactive isotope that subsequently decays to the isotope plutonium-239, which also is fissile.

Uranium's most stable isotope, uranium-238, has a half-life of about 4,468,000,000 years.  It decays into thorium-234 through alpha decay or decays through spontaneous fission.

Natural Concentrations

Naturally occurring uranium is composed of three major isotopes, uranium-238 (99.28% natural abundance), uranium-235 (0.71%), and uranium-234 (0.0054%.  All three isotopes are radioactive, creating radioisotopes, with the most abundant and stable being uranium-238 with a half-life of 4.5 109 years (close to the age of the Earth), uranium-235 with a half-life of 7 108 years, and uranium-234 with a half-life of 2.5 105 years. 

Uranium-238 is an a emitter, decaying through the 18-member uranium natural decay series into lead#Isotopes-206.  The decay series of uranium-235 (also called actinouranium) has 15 members that ends in lead-207, protactinium-231 and actinium-227.  The constant rates of decay in these series makes comparison of the ratios of parent to daughter elements useful in radiometric dating.  Uranium-233 is made from thorium-232 by neutron bombardment.

The isotope uranium-235 or enriched uranium is important for both nuclear reactors and nuclear weapons because it is the only isotope existing in nature to any appreciable extent that is fissile, that is, can be broken apart by thermal neutrons.  The isotope uranium-238 is also important because it absorbs neutrons to produce a radioactive isotope that subsequently decays to the isotope plutonium-239, which also is fissile.

atom.gif (700 bytes)

Isotope  
Atomic Mass
 
Half-Life
217U 217.02437) 26 ms
218U 218.02354 6 ms
219U 219.02492 55 s
220U 220.02472 ~60 ns
221U 221.02640 ~700 ns
222U 222.02609 1.4 s
223U 223.02774 21 s
224U 224.027605 940 s
225U 225.029391 61 ms
226U 226.029339 269 ms
227U 227.031156 1.1 minutes
228U 228.031374 9.1 minutes
229U 229.033506 58 minutes
230U 230.033940 20.8 days
231U 231.036294 4.2 days
232U 232.0371562 68.9 years
233U 233.0396352 1.592 x 105 years
234U 234.0409521 2.455 x 105 years
235U 235.0439299 7.04 x 108 years
236U 236.045568 2.342 x 107 years
237U 237.0487302 6.75 days
238U 238.0507882 4.468 x 109 years
239U 239.0542933 23.45 minutes
240U 240.056592 14.1 hours
241U 241.06033 ~5 minutes
242U 242.06293 16.8 minutes

Enrichment

Enrichment of uranium ore through isotoope separation to concentrate the fissionable uranium-235 is needed for use in nuclear power plants and nuclear weapons. A majority of neutrons released by a fissioning atom of uranium-235 must impact other uranium-235 atoms to sustain the nuclear chain reaction needed for these applications.  The concentration and amount of uranium-235 needed to achieve this is called a 'critical mass.'

To be considered 'enriched' the uranium-235 fraction has to be increased to significantly greater than its concentration in naturally-occurring uranium. Enriched uranium typically has a uranium-235 concentration of between 3 and 5%.  The process produces huge quantities of uranium that is depleted of uranium-235 and with a correspondingly increased fraction of uranium-238, called depleted uranium or 'DU'.   To be considered 'depleted', the uranium-235 isotope concentration has to have been decreased to significantly less than  0.711% (by weight).  Typically the amount of uranium-235 left in depleted uranium is 0.2% to 0.3%.  This represents anywhere from 28% to 42% of the original fraction of uranium-235.

The gas centrifuge process, where gaseous uranium hexafluoride (UF6) is separated by weight using high-speed centrifuges, has become the cheapest and leading enrichment process (lighter UF6 concentrates in the center of the centrifuge).   The gaseous diffusion process was the previous leading method for enrichment and the one used in the Manhattan Project.  In this process, uranium hexafluoride is repeatedly diffused through a silver-zinc membrane and the different isotopes of uranium are separated by diffusion rate (uranium 238 is heavier and thus diffuses slightly slower than uranium-235).  The laser excitation method employs a laser beam of precise energy to sever the bond between uranium-235 and fluorine. T his leaves uranium-238 bonded to fluorine and allows uranium-235 metal to precipitate from the solution.  Another method is called liquid thermal diffusion.

Another way to look at this is as follows: Pressurized Heavy Water Reactors (PHWR) use natural uranium (0.71% fissile material).  From Pressurized Water Reactors (PWRs) of typical design (most USA reactors are PWR) we note the fuel goes in with about 4% uranium-235 and 96% uranium-238 and comes out with about 1% uranium-235, 1% plutonium-239 and 95% uranium-238.  If the plutonium-239 were removed (fuel reprocessing is not allowed in the USA) and this were added to the depleted uranium then we would have 1.2% fissile material in the reprocessed depleted uranium and at the same time have 1% fissile material in the left over spent fuel. Both of these would be considered enriched fuels for a PHWR style reactor.

Uranium-233, an artificial isotope, is used as a reactor fuel in India.  It has also been tested in nuclear weapons, but the results were unpromising.  It is made from thorium-232 by neutron bombardment.

Precautions

All isotopes and compounds of uranium are radioactive, chemically poisonous and teratogenic.

Uranium salts could cause irreversible renal damage because they accumulate in kidney tubules, but no conclusive evidence has yet been produced.

No deaths have been associated with prolonged occupational exposure to inhaled uranium compounds.  Although accidental inhalation exposure to a high concentration of uranium hexafluoride has resulted in human fatalities, those deaths were not associated with uranium itself.

Exposure to environmental uranium or to uranium at levels found at hazardous waste sites will not be lethal to humans but exposure to some of its decay products, especially radon, strontium-90, and iodine-131 does pose a significant health threat.  Radon can collect in confined spaces, such as basements, and when inhaled over long periods, can lead to the wasting of lung tissue and lung cancer.  Strontium-90, a component of nuclear fallout, is a highly radioactive isotope that is chemically similar to calcium and is therefore readily absorbed by bones and bone marrow.  Its presence in these tissues can cause bone cancer, cancer of nearby tissues, and leukemia.  Iodine-131, also a component of radioactive fallout, it is absorbed by the body and can cause damage to the thyroid and even cause thyroid cancer.

180px-UO3_gamma_lattice.jpg (7784 bytes)

Uranium Trioxide

Radiological effects are generally local because this is the nature of alpha radiation, the primary form from U-238 decay. Uranium compounds in general are poorly absorbed by the lining in the lungs and may remain a radiological hazard indefinitely. Uranyl (UO2+) ions, such as from uranium trixode or uranyl nitrate and other hexavalent uranium compounds have been shown to cause birth defects and immune system damage in laboratory animals.

Finely-divided uranium metal presents a fire hazard because uranium is pyrophoric, so small grains will ignite spontaneously in air at room temperature.

A person can be exposed to uranium (or its radioactive daughters such as radon) by inhaling dust in air or from smoking tobacco which have been grown using certain phosphate fertilizers, or ingesting water and food.

Almost all uranium that is ingested is excreted during digestion,  but up to 5% is absorbed by the body when the soluble uranyl ion is ingested while only 0.5% is absorbed when insoluble forms of uranium, such as its oxide, are ingested.  However, soluble uranium compounds tend to quickly pass through the body whereas insoluble uranium compounds, especially when ingested via dust into the lungs, pose a more serious exposure hazard.  After entering the bloodstream, the absorbed uranium tends to bioaccumulate and stay for many years in bone tissue because of uranium's affinity for phosphates.   Uranium does not absorb through the skin, and alpha particles released by uranium cannot penetrate the skin.

The amount of uranium in air is usually very small; however, people who live near government facilities that made or tested nuclear weapons, or facilities that mine or process uranium ore or enrich uranium for reactor fuel, may have increased exposure to uranium.  Houses or structures which are over uranium deposits (either natural or man-made slag deposits) may have an increased incidence of exposure to radon gas.

Uranium mining carries the danger of airborne radioactive dust and the release of radioactive radon gas and its daughter products (an added danger to the already dangerous activity of all hard rock mining.  As a result, without proper ventilation,   uranium miners have a dramatically increased risk of later development of lung cancer  and other pulmonary diseases.  There is also the possible danger of groundwater contamination with the toxic chemicals used in the separation of the uranium ore.

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Uranium Data
 

Atomic Structure

  • Atomic Radius: 138.5 pm
  • Atomic Volume: 12.59cm3/mol
  • Covalent Radius: 1.42
  • Cross Section (Thermal Neutron Capture) Barns: 7.57
  • Crystal Structure: Orthorhombic
  • Electron Configuration:
    1s2 2s2p6 3s2p6d10 4s2p6d10f14 5s2p6d10f3 6s2p6d1 7s2
  • Electrons per Energy Level: 2, 8, 18, 32, 21, 9, 2
  • Ionic Radius: 0.52
  • Filling Orbital: 5f3
  • Number of Electrons (with no charge): 92
  • Number of Neutrons (most common/stable nuclide): 146
  • Number of Protons: 92
  • Oxidation States: 6, 5, 4, 3
  • Valence Electrons: 5f3 6d1 7s2

Chemical Properties

  • Electrochemical Equivalent: 1.4801 g/amp-hr
  • Electron Work Function: 3.63eV
  • Electronegativity: 1.38 (Pauling); 1.2 (Allrod Rochow)
  • Heat of Fusion: 8.52 kJ/mol
  • Incompatibilities:
    Carbon dioxide, carbon tetrachloride, nitric acid, fluorine
  • Ionization Potential
    • First: 6.05
  • Valence Electron Potential (-eV): 170

Physical Properties

  • Atomic Mass Average: 238.0289
  • Boiling Point: 4407K, 4134C, 7473F
  • Coefficient of Lineal Thermal Expansion/K-1: 0.0000134E-6
  • Conductivity
    Electrical: 0.038 106/cm
    Thermal: 0.276 W/cmK
  • Density: 18.95 g/cm3 @ 300K
  • Description:
    A very heavy silvery-white radioactive metal.
  • Elastic Modulus:
    • Bulk: 100/GPa
    • Rigidity: 111/GPa
    • Youngs: 208/GPa
  • Enthalpy of Atomization: 489.5 kJ/mole @ 25C
  • Enthalpy of Fusion: 15.48 kJ/mole
  • Enthalpy of Vaporization: 477 kJ/mole
  • Flammablity Class: Combustible Solid
  • Freezing Point: see melting point
  • Hardness Scale
    • Brinell: 2400 MN m-2
    • Mohs: 6
    • Vickers: 1960 MN m-2
  • Heat of Vaporization: 477 kJ/mol
  • Melting Point: 1405K, 1132C, 2070F
  • Molar Volume: 12.49 cm3/mole
  • Physical State (at 20C & 1atm): Solid
  • Specific Heat: 0.12 J/gK
  • Vapor Pressure: 1.19E-06 Pa @ 1132C

Regulatory / Health

  • CAS Number
    • 7440-61-1 metal, pyrophoric
  • UN/NA ID and ERG Guide Number
    • 2979  / 162 metal, pyrophoric
  • RTECS: YR3490000
  • NFPA 704
    • Health:
    • Fire:
    • Reactivity:
    • Special Hazard: Radioactive<
    • OSHA Permissible Exposure Limit (PEL)
      • TWA: 0.25 mg/m3
    • OSHA PEL Vacated 1989
      • TWA: 0.2 mg/m3
      • STEL: 0.6 mg/m3
    • NIOSH Recommended Exposure Limit (REL)
      • TWA: 0.2 mg/m3
      • STEL: 0.6 mg/m3
      • IDLH: 10 mg/m3 (Potential NIOSH carcinogen)
    • Routes of Exposure: Inhalation; Ingestion; Skin and/or eye contact
    • Target Organs: Skin, kidneys, bone marrow, lymphatic system
    • Levels In Humans:
      Note: this data represents naturally occuring levels of elements in the typical human, it DOES NOT represent recommended daily allowances.
      • Blood/mg dm-3: N/A
      • Bone: N/A
      • Liver: N/A
      • Muscle: N/A
      • Daily Dietary Intake: n/a
      • Total Mass In Avg. 70kg human: n/a

    Who / Where / When / How

    • Discoverer: Martin J. Klaproth
    • Discovery Location: Berlin Germany
    • Discovery Year: 1789
    • Name Origin:
      From planet Uranus.
    • Abundance:
      • Estimated Crustal: 2.7 milligrams per kilogram
      • Estimated Oceanic: 3.210-3 milligrams per liter
      • Atmosphere: N/A
      • Sun (Relative to H=1E12): N/A
    • Sources:
      Primary uranium bearing ores are uraninite, autunite, carnotite, samarskite (some varieties of samarskite contain up to 23% uranium) and torbernite. Annual world wide production is around 35,000 tons. Primary mining areas are in canada, Zaire, Czech Republic and USA.
    • Uses:
      For many centuries it was used as a pigment for glass. Now it is used as a fuel in nuclear reactors and in nuclear bombs. Depleted Uranium is used in casings of armor piercing arterial shells, armor plating on tanks and as ballast in the wings of some large aircraft.

Miscellaneous

Fusion Heat (kJ/mol): 12.6 kJ/mol and/or 8.52 kJ/mol
Evaporation Heat (kJ/mol): 417 kJ/mol
Lattice Constant (): 2.850
1st. Ionization Energy: 686.4 kJ/mol
Van der Waals Forces: 168 pm