Uranium

Uranium (pronounced /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. 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 (99.284%), uranium-235 (0.711%)^[1], 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,^[2] 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.^[3]

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.

Additional recommended knowledge

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Characteristics

  When refined, uranium is a silvery white, weakly radioactive metal, which is slightly softer than steel,^[4] strongly electropositive and a poor electrical conductor.^[5] It is malleable, ductile, and slightly paramagnetic.^[4] 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.^[6] Hydrochloric and nitric acids dissolve uranium, but nonoxidizing acids attack the element very slowly.^[5] When finely divided, it can react with cold water; in air, uranium metal becomes coated with a dark layer of uranium oxide.^[4] 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.^[7] The first atomic bomb worked by this principle (nuclear fission).

Applications

Military

  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).^[7]

Depleted uranium is also used as a shielding material in some containers used to store and transport radioactive materials.^[5] Other uses of DU include counterweights for aircraft control surfaces, as ballast for missile re-entry vehicles and as a shielding material.^[4] Due to its high density, this material is found in inertial guidance devices and in gyroscopic compasses.^[4] DU is preferred over similarly dense metals due to its ability to be easily machined and cast as well as its relatively low cost.^[8] 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.^[9]

Civilian

  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.^[3] 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,^[3] the CANDU reactor is the only commercial reactor capable of using 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:^{[4] 238}U(n, gamma) → ²³⁹U -(beta) → ²³⁹Np -(beta) → ²³⁹Pu.

    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),^[4] 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.^[4]

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.^[4] 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.^[10] 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.^[11] In the early 19th century, the world's only known source of uranium ores were these old mines.

Discovery

  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.^[11] 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).^[11][12] He named the newly discovered element after the planet Uranus, which had been discovered eight years earlier by William Herschel.^[13]

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.^[14][11] 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 Henri Becquerel discovered radioactivity by using uranium in 1896.^[6] Becquerel made the discovery in Paris by leaving a sample of a uranium salt on top of an unexposed photographic plate in a drawer and noting that the plate had become 'fogged'.^[15] He determined that a form of invisible light or rays emitted by uranium had exposed the plate.

Fission research

  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).^[16] 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.^{[17][18][19][20]} 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^[16] 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'.^[21] 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.^[16] 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, Chicago Pile-1. 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.^[16] 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

  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).^[15]

  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).^[22] The world's first commercial scale nuclear power station, Obninsk in the Soviet Union, began generation with its reactor AM-1 on 27 June 1954. Other early nuclear power plants were Calder Hall in England which began generation on 17 October 1956^[23] and the Shippingport Atomic Power Station in Pennsylvania which began on 26 May 1958. Nuclear power was used for the first time for propulsion by a submarine, the USS Nautilus, in 1954.^[16]

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.^[24] 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.^[24]

Cold War legacy and waste

  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 and plutonium made from uranium.

Since the break-up of the Soviet Union in 1991, an estimated 600 tons (540 tonnes) of highly-enriched weapons grade uranium (enough to make 40,000 nuclear warheads) have been stored in often inadequately guarded facilities in the Russian Federation and several other former Soviet states.^[7] 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.^[7] From 1993 to 2005 the Material Protection, Control, and Accounting Program, operated by the federal government of the United States, spent approximately US $550 million to help safeguard uranium and plutonium stockpiles in Russia.^[7] The improvements made provided repairs and security enhancements at research and storage facilities. Scientific American reported in February of 2006 that some of the facilities had been protected only by chain link fences which were in severe states of disrepair. According to an interview from the article, one facility had been storing samples of enriched (weapons grade) uranium in a broom closet prior to the improvement project; another had been keeping track of its stock of nuclear warheads using index cards kept in a shoe box.^[25]

Above-ground nuclear tests by the Soviet Union and the United States in the 1950s and early 1960s and by France into the 1970s and 1980s^[8] spread a significant amount of fallout from uranium daughter isotopes around the world.^[26] Additional fallout and pollution occurred from several nuclear accidents.

The Windscale fire at the Sellafield nuclear plant in 1957 spread iodine-131, a short lived radioactive isotope, over much of Northern England.

The Three Mile Island accident in 1979 released a small amount of iodine-131. The amounts released by the partial meltdown of the Three Mile Island power plant were minimal, and an environmental survey only found trace amounts in a few field mice dwelling nearby. As I-131 has a half life of slightly more than eight days, any danger posed by the radioactive material has long since passed for both of these incidents.

The Chernobyl disaster in 1986, however, was a complete core breach meltdown and partial detonation of the reactor, which ejected iodine-131 and strontium-90 over a large area of Europe. The 28 year half-life of strontium-90 means that only recently has some of the surrounding countryside around the reactor been deemed safe enough to be habitable.^[8]

Occurrence

Biotic and abiotic

Main article: Uranium in the environment

  Uranium is a naturally occurring element that can be found in low levels within all rock, soil, and water. Uranium is also the highest-numbered element to be found naturally in significant quantities on earth and is always found combined with other elements.^[4] Along with all elements having atomic weights higher than that of iron, it is only naturally formed in supernova explosions.^[27] The decay of uranium, thorium and potassium-40 in the Earth's mantle is thought to be the main source of heat^[28][29] that keeps the outer core liquid and drives mantle convection, which in turn drives plate tectonics.

Its average concentration in the Earth's crust is (depending on the reference) 2 to 4 parts per million,^[5][8] or about 40 times as abundant as silver.^[6] The Earth's crust from the surface to 25 km (15 mi) down is calculated to contain 10¹⁷ kg (2×10¹⁷ lb) of uranium while the oceans may contain 10¹³ kg (2×10¹³ lb).^[5] 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 fertilizers), and 3 parts per billion of sea water is composed of the element.^[8]

It is more plentiful than antimony, tin, cadmium, mercury, or silver, and it is about as abundant as arsenic or molybdenum.^[4][8] It is found in hundreds of minerals including uraninite (the most common uranium ore), autunite, uranophane, torbernite, and coffinite.^[4] 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^[4] (it is recovered commercially from these sources with as little as 0.1% uranium^[6]).

  Some organisms, such as the lichen Trapelia involuta or microorganisms such as the bacterium Citrobacter, can absorb concentrations of uranium that are up to 300 times higher than in their environment.^[30] Citrobacter 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; this creates the possibility that these organisms could be used in bioremediation to decontaminate uranium-polluted water.^[11][31]

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.^[11] Dry weight concentrations of uranium in food plants are typically lower with one to two micrograms per day ingested through the food people eat.^[11]

Production and mining

  Uranium ore is mined in several ways: by open pit, underground, in-situ leaching, and borehole mining (see uranium mining).^[3] Low-grade uranium ore typically contains 0.1 to 0.25% of actual uranium oxides, so extensive measures must be employed to extract the metal from its ore.^[32] High-grade ores found in Athabasca Basin deposits in Saskatchewan, Canada can contain up to 70% uranium oxides, and therefore must be diluted with waste rock prior to milling. Uranium ore is crushed and rendered into a fine powder and then leached with either an acid or alkali. The leachate is then subjected to one of several sequences of precipitation, solvent extraction, and ion exchange. The resulting mixture, called yellowcake, contains at least 75% uranium oxides. Yellowcake is then calcined to remove impurities from the milling process prior to refining and conversion.

Commercial-grade uranium can be produced through the reduction of uranium halides with alkali or alkaline earth metals.^[4] Uranium metal can also be made through electrolysis of KUF₅ or UF₄, dissolved in a molten calcium chloride (CaCl₂) and sodium chloride (NaCl) solution.^[4] Very pure uranium can be produced through the thermal decomposition of uranium halides on a hot filament.^[4]

Resources and reserves

It is estimated that 4.7 million tonnes of uranium ore reserves are economically viable, while 35 million tonnes are classed as mineral resources (reasonable prospects for eventual economic extraction).^[33] An additional 4.6 billion tonnes of uranium are estimated to be in sea water (Japanese scientists in the 1980s showed that extraction of uranium from sea water using ion exchangers was feasible).^[34][35]

Exploration for uranium is continuing to increase with US$200 million being spent world wide in 2005, a 54% increase on the previous year.^[33]

Australia has 40% of the world's uranium ore reserves^[36] and the world's largest single uranium deposit, located at the Olympic Dam Mine in South Australia.^[37] Almost all of the uranium production is exported, under strict International Atomic Energy Agency safeguards against use in nuclear weapons.

The largest single source of uranium ore in the United States was the Colorado Plateau located in Colorado, Utah, New Mexico, and Arizona. The U.S. federal government paid discovery bonuses and guaranteed purchase prices to anyone who found and delivered uranium ore, and was the sole legal purchaser of the uranium. The economic incentives resulted in a frenzy of exploration and mining activity throughout the Colorado Plateau from 1947 through 1959 that left thousands of miles of crudely graded roads spider-webbing the remote deserts of the Colorado Plateau, and thousands of abandoned uranium mines, exploratory shafts, and tailings piles. The frenzy ended as suddenly as it had begun, when the U.S. government stopped purchasing the uranium.

Supply

  In 2005, seventeen countries produced concentrated uranium oxides, with Canada (27.9% of world production) and Australia (22.8%) being the largest producers and Kazakhstan (10.5%), Russia (8.0%), Namibia (7.5%), Niger (7.4%), Uzbekistan (5.5%), the United States (2.5%), Ukraine (1.9%) and China (1.7%) also producing significant amounts.^[38] The ultimate supply of uranium is believed to be very large and sufficient for at least the next 85 years^[33] although some studies indicate underinvestment in the late twentieth century may produce supply problems in the 21st century.^[39] It is estimated that for a ten times increase in price, the supply of uranium that can be economically mined is increased 300 times.^[40]

Compounds

Oxidation states and oxides

Oxides

Calcined uranium yellowcake as produced in many large mills contains a distribution of uranium oxidation species in various forms ranging from most oxidized to least oxidized. Particles with short residence times in a calciner will generally be less oxidized than particles that have long retention times or are recovered in the stack scrubber. While uranium content is referred to for U₃O₈ content, to do so is inaccurate and dates to the days of the Manhattan project when U₃O₈ was used as an analytical chemistry reporting standard.

Phase relationships in the uranium-oxygen system are highly complex. The most important oxidation states of uranium are uranium(IV) and uranium(VI), and their two corresponding oxides are, respectively, uranium dioxide (UO₂) and uranium trioxide (UO₃).^[41] Other uranium oxides such as uranium monoxide (UO), diuranium pentoxide (U₂O₅), and uranium peroxide (UO₄•2H₂O) are also known to exist.

The most common forms of uranium oxide are triuranium octaoxide (U₃O₈) and the aforementioned UO₂.^[42] Both oxide forms are solids that have low solubility in water and are relatively stable over a wide range of environmental conditions. Triuranium octaoxide is (depending on conditions) the most stable compound of uranium and is the form most commonly found in nature. Uranium dioxide is the form in which uranium is most commonly used as a nuclear reactor fuel.^[42] At ambient temperatures, UO₂ will gradually convert to U₃O₈. Because of their stability, uranium oxides are generally considered the preferred chemical form for storage or disposal.^[42]

Aqueous chemistry

Ions that represent the four different oxidation states of uranium are soluble and therefore can be studied in aqueous solutions. They are: U³⁺ (red), U⁴⁺ (green), UO₂⁺ (unstable), and UO₂²⁺ (yellow).^[43] A few solid and semi-metallic compounds such as UO and US exist for the formal oxidation state uranium(II), but no simple ions are known to exist in solution for that state. Ions of U³⁺ liberate hydrogen from water and are therefore considered to be highly unstable. The UO₂²⁺ ion represents the uranium(VI) state and is known to form compounds such as the carbonate, chloride and sulfate. UO₂²⁺ also forms complexes with various organic chelating agents, the most commonly-encountered of which is uranyl acetate.^[43]

Carbonates

The interactions of carbonate anions with uranium(VI) cause the Pourbaix diagram to change greatly when the medium is changed from water to a carbonate containing solution. It is interesting to note that while the vast majority of carbonates are insoluble in water (students are often taught that all carbonates other than those of alkali metals are insoluble in water), uranium carbonates are often soluble in water. This is due to the fact that a U(VI) cation is able to bind two terminal oxides and three or more carbonates to form anionic complexes.

The fraction digrams explain this further, it can be seen that when the pH of a uranium(VI) solution is increased that the uranium is converted to a hydrated uranium oxide hydroxide and then at high pHs to an anionic hydroxide complex.

On addition of carbonate to the system the uranium is converted to a series of carbonate complexes when the pH is increased, one important overall effect of these reactions is to increase the solubility of the uranium in the range pH 6 to 8. This is important when considering the long term stability of used uranium dioxide nuclear fuels.

Hydrides, carbides and nitrides

Uranium metal heated to 250 to 300 °C (482 to 572 °F) reacts with hydrogen to form uranium hydride. Even 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.^[45] Two crystal modifications of uranium hydride exist: an α form that is obtained at low temperatures and a β form that is created when the formation temperature is above 250 °C.^[45]

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.^[45] 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 monoxide at high temperatures. Stable below 1800 °C, U₂C₃ is prepared by subjecting a heated mixture of UC and UC₂ to mechanical stress.^[46] Uranium nitrides obtained by direct exposure of the metal to nitrogen include uranium mononitride (UN), uranium dinitride (UN₂), and diuranium trinitride (U₂N₃).^[46]

Halides

  All uranium fluorides are created using uranium tetrafluoride (UF₄); UF₄ itself is prepared by hydrofluorination of uranium dioxide.^[45] Reduction of UF₄ with hydrogen at 1000 °C produces uranium trifluoride (UF₃). Under the right conditions of temperature and pressure, the reaction of solid UF₄ with gaseous uranium hexafluoride (UF₆) can form the intermediate fluorides of U₂F₉, U₄F₁₇, and UF₅.^[45]

At room temperatures, UF₆ has a high vapor pressure, making it useful in the gaseous diffusion process to separate highly valuable uranium-235 from the far more common uranium-238 isotope. This compound can be prepared from uranium dioxide and uranium hydride by the following process:^[45]

UO₂ + 4HF + heat (500 °C) → UF₄ + 2H₂O
UF₄ + F₂ + heat (350 °C) → UF₆

The resulting UF₆ white solid is highly reactive (by fluorination), easily sublimes (emitting a nearly perfect gas vapor), and is the most volatile compound of uranium known to exist.^[45]

One method of preparing uranium tetrachloride (UCl₄) is to directly combine chlorine with either uranium metal or uranium hydride. The reduction of UCl₄ by hydrogen produces uranium trichloride (UCl₃) while the higher chlorides of uranium are prepared by reaction with additional chlorine.^[45] All uranium chlorides react with water and air.

Bromides and iodides of uranium are formed by direct reaction of, respectively, bromine and iodine with uranium or by adding UH₃ to those element's acids.^[45] Known examples include: UBr₃, UBr₄, UI₃, and UI₄. Uranium oxyhalides are water-soluble and include UO₂F₂, UOCl₂, UO₂Cl₂, and UO₂Br₂. Stability of the oxyhalides decrease as the atomic weight of the component halide increases.^[45]

Isotopes

Natural concentrations

Main article: Isotopes of uranium

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 4.51×10⁹ years (close to the age of the Earth), uranium-235 with a half-life of 7.13×10⁸ years, and uranium-234 with a half-life of 2.48×10⁵ years.^[47]

Uranium-238 is an α emitter, decaying through the 18-member uranium natural decay series into lead-206.^[6] The decay series of uranium-235 (also called actino-uranium) has 15 members that ends in lead-207, protactinium-231 and actinium-227.^[6] 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.^[4]

The isotope uranium-235 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.^[6] 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.^[16]

Enrichment

Main article: Enriched uranium

  Enrichment of uranium ore through isotope separation to concentrate the fissionable uranium-235 is needed for use in nuclear weapons and most nuclear power plants with the exception of pressurised heavy water reactors. 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%.^[48] 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 its natural concentration. Typically the amount of uranium-235 left in depleted uranium is 0.2% to 0.3%.^[49] As the price of uranium has risen since 2001, some enrichment tailings containing more than 0.35% uranium-235 are being considered for re-enrichment, driving the price of these depleted uranium hexafluoride stores above $130 per kilogram in July, 2007 from just $5 in 2001.^[49]

The gas centrifuge process, where gaseous uranium hexafluoride (UF₆) is separated by the difference in molecular weight between ²³⁵UF₆ and ²³⁸UF₆ using high-speed centrifuges, has become the cheapest and leading enrichment process (lighter UF₆ concentrates in the center of the centrifuge).^[15] 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).^[15] The molecular laser isotope separation 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.^[3] Another method is called liquid thermal diffusion.^[5]

Precautions

Exposure

A person can be exposed to uranium (or its radioactive daughters such as radon) by inhaling dust in air or by ingesting contaminated water and food. The amount of uranium in air is usually very small; however, people who work in factories that process phosphate fertilizers, live near government facilities that made or tested nuclear weapons, live or work near a modern battlefield where depleted uranium weapons have been used, or live or work near a coal-fired power plant, facilities that mine or process uranium ore, or enrich uranium for reactor fuel, may have increased exposure to uranium.^[50][51] Houses or structures that are over uranium deposits (either natural or man-made slag deposits) may have an increased incidence of exposure to radon gas.

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.^[11] 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.^[11] Uranium does not absorb through the skin, and alpha particles released by uranium cannot penetrate the skin.

Effects

The greatest health risk from large intakes of uranium is toxic damage to the kidneys, because, in addition to being weakly radioactive, uranium is a toxic metal.^[52][53][11] Uranium is a reproductive toxicant.^[54] Radiological effects are generally local because this is the nature of alpha radiation, the primary form from U-238 decay. No human cancer has been seen as a result of exposure to natural or depleted uranium,^[55] but exposure to some of its decay products, especially radon, does pose a significant health threat.^[8] Exposure to strontium-90, iodine-131, and other fission products is unrelated to uranium exposure, but may result from medical procedures or exposure to spent reactor fuel or fallout from nuclear weapons.^[56] Although accidental inhalation exposure to a high concentration of uranium hexafluoride has resulted in human fatalities, those deaths were not associated with uranium itself.^[57] Finely-divided uranium metal presents a fire hazard because uranium is pyrophoric, so small grains will ignite spontaneously in air at room temperature.^[4]

See also

• Nuclear engineering
• Nuclear fuel cycle
• Nuclear physics
• K-65 residues
• List of uranium mines
• Isotopes of uranium

References

Full reference information for multi-page works cited

• John Emsley (2001). "Uranium", Nature's Building Blocks: An A to Z Guide to the Elements. Oxford: Oxford University Press, 476–82. ISBN 0-19-850340-7. 
• Glenn T. Seaborg (1968). "Uranium", The Encyclopedia of the Chemical Elements. Skokie, Illinois: Reinhold Book Corporation, 773–86. LCCCN 68-29938. be-x-old:Уран (хімічны элемент)