Military

Various militaries use depleted uranium 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, such as titanium or molybdenum.^[18] At high impact speed, the density, hardness, and pyrophoricity of the projectile enable the destruction of heavily armored targets. Tank armor and other removable vehicle armor can also be hardened with depleted uranium plates. The use of depleted uranium became politically and environmentally contentious after the use of such munitions by the US, UK and other countries during wars in the Persian Gulf and the Balkans raised questions concerning uranium compounds left in the soil (see Gulf War syndrome).^[15]

Depleted uranium is also used as a shielding material in some containers used to store and transport radioactive materials. While the metal itself is radioactive, its high density makes it more effective than lead in halting radiation from strong sources such as radium.^[8] Other uses of depleted uranium include counterweights for aircraft control surfaces, as ballast for missile re-entry vehicles and as a shielding material.^[9] Due to its high density, this material is found in inertial guidance systems and in gyroscopic compasses.^[9] Depleted uranium is preferred over similarly dense metals due to its ability to be easily machined and cast as well as its relatively low cost.^[19] The main risk of exposure to depleted uranium 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 lesser extent afterwards, uranium-235 has been used as the fissile explosive material to produce nuclear weapons. Initially, two major types of fission bombs were built: a relatively simple device that uses uranium-235 and a more complicated mechanism that uses plutonium-239 derived from uranium-238. Later, a much more complicated and far more powerful type of fission/fusion bomb (thermonuclear weapon) was built, that uses a plutonium-based device to cause a mixture of tritium and deuterium to undergo nuclear fusion. Such bombs are jacketed in a non-fissile (unenriched) uranium case, and they derive more than half their power from the fission of this material by fast neutrons from the nuclear fusion process.^[20]

Civilian

The main use of uranium in the civilian sector is to fuel nuclear power plants. One kilogram of uranium-235 can theoretically produce about 20 terajoules of energy (2×10¹³ joules), assuming complete fission; as much energy as 1.5 million kilograms (1,500 tonnes) of coal.^[7]

Commercial nuclear power plants use fuel that is typically enriched to around 3% uranium-235.^[7] The CANDU and Magnox designs are the only commercial reactors 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:^[9]

²³⁸
₉₂U
+ n → ²³⁹
₉₂U
+ γ β⁻→  ²³⁹
₉₃Np
β⁻→  ²³⁹
₉₄Pu

Uranium glass glowing under UV light

Before (and, occasionally, after) the discovery of radioactivity, uranium was primarily used in small amounts for yellow glass and pottery glazes, such as uranium glass and in Fiestaware.^[21]

The discovery and isolation of radium in uranium ore (pitchblende) by Marie Curie sparked the development of uranium mining to extract the radium, which was used to make glow-in-the-dark paints for clock and aircraft dials.^[22][23] This left a prodigious quantity of uranium as a waste product, since it takes three tonnes of uranium to extract one gram of radium. This waste product was diverted to the glazing industry, making uranium glazes very inexpensive and abundant. Besides the pottery glazes, uranium tile glazes accounted for the bulk of the use, including common bathroom and kitchen tiles which can be produced in green, yellow, mauve, black, blue, red and other colors.

The uranium glaze on a Sencer Sarı ceramic glowing under UV light.

Uranium glass used as lead-in seals in a vacuum capacitor

Uranium was also used in photographic chemicals (especially uranium nitrate as a toner),^[9] in lamp filaments for stage lighting bulbs,^[24] to improve the appearance of dentures,^[25] 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 electron-dense "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 (4.47×10⁹ 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, uranium–lead dating and uranium–uranium dating. Uranium metal is used for X-ray targets in the making of high-energy X-rays.^[9]

Pre-discovery use

The use of uranium in its natural oxide form dates back to at least the year 79 CE, when it was used in the Roman Empire to add a yellow color to ceramic glazes.^[9] 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.^[26] 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.^[27] In the early 19th century, the world's only known sources of uranium ore were these mines. Mining for uranium in the Ore Mountains ceased on the German side after the Cold War ended and SDAG Wismut was wound down. On the Czech side there were attempts during the uranium price bubble of 2007 to restart mining, but those were quickly abandoned following a fall in uranium prices.^[28][29]

Discovery

The planet Uranus, which uranium is named after

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.^[27] Klaproth 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).^[27][30] He named the newly discovered element after the planet Uranus (named after the primordial Greek god of the sky), which had been discovered eight years earlier by William Herschel.^[31]

In 1841, Eugène-Melchior Péligot, 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.^[27][32]

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

Henri Becquerel discovered radioactivity by using uranium in 1896.^[14] Becquerel made the discovery in Paris by leaving a sample of a uranium salt, K₂UO₂(SO₄)₂ (potassium uranyl sulfate), on top of an unexposed photographic plate in a drawer and noting that the plate had become "fogged".^[33] He determined that a form of invisible light or rays emitted by uranium had exposed the plate.

During World War I when the Central Powers suffered a shortage of molybdenum to make artillery gun barrels and high speed tool steels, they routinely used ferrouranium alloy as a substitute, as it presents many of the same physical characteristics as molybdenum. When this practice became known in 1916 the US government requested several prominent universities to research the use of uranium in manufacturing and metalwork. Tools made with these formulas remained in use for several decades,^[34][35] until the Manhattan Project and the Cold War placed a large demand on uranium for fission research and weapon development.

Fission research

Cubes and cuboids of uranium produced during the Manhattan project

A team led by Enrico Fermi in 1934 observed that bombarding uranium with neutrons produces the emission of beta rays (electrons or positrons from the elements produced; see beta particle).^[36] The fission products were at first mistaken for new elements with atomic numbers 93 and 94, which the Dean of the Faculty of Rome, Orso Mario Corbino, christened ausonium and hesperium, respectively.^{[37][38][39][40]} 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^[36] in Hahn's laboratory in Berlin. Lise Meitner and her nephew, the physicist Otto Robert Frisch, published the physical explanation in February 1939 and named the process "nuclear fission".^[41] 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 2.5 neutrons are released by each fission of the rare uranium isotope uranium-235.^[36] Fermi urged Alfred O. C. Nier to separate uranium isotopes for determination of the fissile component, and on 29 February 1940, Nier used an instrument he built at the University of Minnesota to separate the world's first uranium-235 sample in the Tate Laboratory. Using Columbia University's cyclotron, John Dunning confirmed the sample to be the isolated fissile material on 1 March.^[42] Further work found that the far more common uranium-238 isotope can be transmuted into plutonium, which, like uranium-235, is also fissile by thermal neutrons. These discoveries led numerous countries to begin working on the development of nuclear weapons and nuclear power. Despite fission having been discovered in Germany, the Uranverein ("uranium club") Germany's wartime project to research nuclear power and/or weapons was hampered by limited resources, infighting, the exile or non-involvement of several prominent scientists in the field and several crucial mistakes such as failing to account for impurities in available graphite samples which made it appear less suitable as a neutron moderator than it is in reality. Germany's attempts to build a natural uranium / heavy water reactor had not come close to reaching criticality by the time the Americans reached Haigerloch, the site of the last German wartime reactor experiment.^[43]

On 2 December 1942, as part of the Manhattan Project, another team led by Enrico Fermi was able to initiate the first artificial self-sustained nuclear chain reaction, Chicago Pile-1. An initial plan using enriched uranium-235 was abandoned as it was as yet unavailable in sufficient quantities.^[44] 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 360 tonnes of graphite, 53 tonnes of uranium oxide, and 5.5 tonnes of uranium metal, a majority of which was supplied by Westinghouse Lamp Plant in a makeshift production process.^[36][45]

Nuclear weaponry

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

Two major types of atomic bombs were developed by the United States during World War II: a uranium-based device (codenamed "Little Boy") whose fissile material was highly enriched uranium, and a plutonium-based device (see Trinity test and "Fat Man") whose plutonium was derived from uranium-238. 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 trinitrotoluene, 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).^[33] Initially it was believed that uranium was relatively rare, and that nuclear proliferation could be avoided by simply buying up all known uranium stocks, but within a decade large deposits of it were discovered in many places around the world.^[46]

Reactors

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

The X-10 Graphite Reactor at Oak Ridge National Laboratory (ORNL) in Oak Ridge, Tennessee, formerly known as the Clinton Pile and X-10 Pile, was the world's second artificial nuclear reactor (after Enrico Fermi's Chicago Pile) and was the first reactor designed and built for continuous operation. Argonne National Laboratory's Experimental Breeder Reactor I, located at the Atomic Energy Commission's National Reactor Testing Station near Arco, Idaho, became the first nuclear reactor to create electricity on 20 December 1951.^[47] Initially, four 150-watt light bulbs were lit by the reactor, but improvements eventually enabled it to power the whole facility (later, the town of Arco became the first in the world to have all its electricity come from nuclear power generated by BORAX-III, another reactor designed and operated by Argonne National Laboratory).^[48][49] 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,^[50] 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.^[36][51]

Prehistoric naturally occurring fission

In 1972, the French physicist Francis Perrin discovered fifteen ancient and no longer active natural nuclear fission reactors in three separate ore deposits at the Oklo mine in Gabon, West Africa, collectively known as the Oklo Fossil Reactors. The ore deposit is 1.7 billion years old; then, uranium-235 constituted about 3% of the total uranium on Earth.^[52] This is high enough to permit a sustained nuclear fission chain reaction to occur, provided other supporting conditions exist. The capacity of the surrounding sediment to contain the health-threatening nuclear waste products has been cited by the U.S. federal government as supporting evidence for the feasibility to store spent nuclear fuel at the Yucca Mountain nuclear waste repository.^[52]

Contamination and the Cold War legacy

U.S. and USSR/Russian nuclear weapons stockpiles, 1945–2005

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^[19] spread a significant amount of fallout from uranium daughter isotopes around the world.^[53] Additional fallout and pollution occurred from several nuclear accidents.^[54]

Uranium miners have a higher incidence of cancer. An excess risk of lung cancer among Navajo uranium miners, for example, has been documented and linked to their occupation.^[55] The Radiation Exposure Compensation Act, a 1990 law in the US, required $100,000 in "compassion payments" to uranium miners diagnosed with cancer or other respiratory ailments.^[56]

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. After the break-up of the Soviet Union in 1991, an estimated 600 short tons (540 metric tons) of highly enriched weapons grade uranium (enough to make 40,000 nuclear warheads) had been stored in often inadequately guarded facilities in the Russian Federation and several other former Soviet states.^[15] 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.^[15] 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. This money was used for improvements and security enhancements at research and storage facilities.^[15]

Safety of nuclear facilities in Russia has been significantly improved since the stabilization of political and economical turmoil of the early 1990s. For example, in 1993 there were 29 incidents ranking above level 1 on the International Nuclear Event Scale, and this number dropped under four per year in 1995–2003. The number of employees receiving annual radiation doses above 20 mSv, which is equivalent to a single full-body CT scan,^[57] saw a strong decline around 2000. In November 2015, the Russian government approved a federal program for nuclear and radiation safety for 2016 to 2030 with a budget of 562 billion rubles (ca. 8 billion dollars). Its key issue is "the deferred liabilities accumulated during the 70 years of the nuclear industry, particularly during the time of the Soviet Union". Approximately 73% of the budget will be spent on decommissioning aged and obsolete nuclear reactors and nuclear facilities, especially those involved in state defense programs; 20% will go in processing and disposal of nuclear fuel and radioactive waste, and 5% into monitoring and ensuring of nuclear and radiation safety.^[58]

Origin

Along with all elements having atomic weights higher than that of iron, uranium is only naturally formed by the r-process (rapid neutron capture) in supernovae and neutron star mergers.^[59] Primordial thorium and uranium are only produced in the r-process, because the s-process (slow neutron capture) is too slow and cannot pass the gap of instability after bismuth.^[60][61] Besides the two extant primordial uranium isotopes, ²³⁵U and ²³⁸U, the r-process also produced significant quantities of ²³⁶U, which has a shorter half-life and so is an extinct radionuclide, having long since decayed completely to ²³²Th. Uranium-236 was itself enriched by the decay of ²⁴⁴Pu, accounting for the observed higher-than-expected abundance of thorium and lower-than-expected abundance of uranium.^[62] While the natural abundance of uranium has been supplemented by the decay of extinct ²⁴²Pu (half-life 0.375 million years) and ²⁴⁷Cm (half-life 16 million years), producing ²³⁸U and ²³⁵U respectively, this occurred to an almost negligible extent due to the shorter half-lives of these parents and their lower production than ²³⁶U and ²⁴⁴Pu, the parents of thorium: the ²⁴⁷Cm:²³⁵U ratio at the formation of the Solar System was (7.0±1.6)×10⁻⁵.^[63]

Biotic and abiotic

Uraninite, also known as pitchblende, is the most common ore mined to extract uranium.

The evolution of Earth's radiogenic heat flow over time: contribution from ²³⁵U in red and from ²³⁸U in green

Uranium is a naturally occurring element that can be found in low levels within all rock, soil, and water. Uranium is the 51st element in order of abundance in the Earth's crust. Uranium is also the highest-numbered element to be found naturally in significant quantities on Earth and is almost always found combined with other elements.^[9] The decay of uranium, thorium, and potassium-40 in the Earth's mantle is thought to be the main source of heat^[64][65] that keeps the Earth's outer core in the liquid state and drives mantle convection, which in turn drives plate tectonics.

Uranium's average concentration in the Earth's crust is (depending on the reference) 2 to 4 parts per million,^[8][19] or about 40 times as abundant as silver.^[14] 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).^[8] 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),^[66] and its concentration in sea water is 3 parts per billion.^[19]

Uranium is more plentiful than antimony, tin, cadmium, mercury, or silver, and it is about as abundant as arsenic or molybdenum.^[9][19] Uranium is found in hundreds of minerals, including uraninite (the most common uranium ore), carnotite, autunite, uranophane, torbernite, and coffinite.^[9] 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^[9] (it is recovered commercially from sources with as little as 0.1% uranium^[14]).

Some bacteria, such as Shewanella putrefaciens, Geobacter metallireducens and some strains of Burkholderia fungorum, use uranium for their growth and convert U(VI) to U(IV).^[67][68] Recent research suggests that this pathway includes reduction of the soluble U(VI) via an intermediate U(V) pentavalent state.^[69][70] Other 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 the level of their environment.^[71] Citrobacter species absorb uranyl ions when given glycerol phosphate (or other similar organic phosphates). After one day, one gram of bacteria can 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.^[27][72] The proteobacterium Geobacter has also been shown to bioremediate uranium in ground water.^[73] The mycorrhizal fungus Glomus intraradices increases uranium content in the roots of its symbiotic plant.^[74]

In nature, uranium(VI) forms highly soluble carbonate complexes at alkaline pH. This leads to an increase in mobility and availability of uranium to groundwater and soil from nuclear wastes which leads to health hazards. However, it is difficult to precipitate uranium as phosphate in the presence of excess carbonate at alkaline pH. A Sphingomonas sp. strain BSAR-1 has been found to express a high activity alkaline phosphatase (PhoK) that has been applied for bioprecipitation of uranium as uranyl phosphate species from alkaline solutions. The precipitation ability was enhanced by overexpressing PhoK protein in E. coli.^[75]

Plants absorb some uranium from soil. 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.^[27] Dry weight concentrations of uranium in food plants are typically lower with one to two micrograms per day ingested through the food people eat.^[27]

Production and mining

Worldwide production of uranium in 2021 amounted to 48,332 tonnes, of which 21,819 t (45%) was mined in Kazakhstan. Other important uranium mining countries are Namibia (5,753 t), Canada (4,693 t), Australia (4,192 t), Uzbekistan (3,500 t), and Russia (2,635 t).^[76]

Uranium ore is mined in several ways: by open pit, underground, in-situ leaching, and borehole mining (see uranium mining).^[7] Low-grade uranium ore mined typically contains 0.01 to 0.25% uranium oxides. Extensive measures must be employed to extract the metal from its ore.^[77] High-grade ores found in Athabasca Basin deposits in Saskatchewan, Canada can contain up to 23% uranium oxides on average.^[78] Uranium ore is crushed and rendered into a fine powder and then leached with either an acid or alkali. The leachate is subjected to one of several sequences of precipitation, solvent extraction, and ion exchange. The resulting mixture, called yellowcake, contains at least 75% uranium oxides U₃O₈. Yellowcake is then calcined to remove impurities from the milling process before refining and conversion.^[79]

Commercial-grade uranium can be produced through the reduction of uranium halides with alkali or alkaline earth metals.^[9] Uranium metal can also be prepared through electrolysis of KUF
₅ or UF
₄, dissolved in molten calcium chloride (CaCl
₂) and sodium chloride (NaCl) solution.^[9] Very pure uranium is produced through the thermal decomposition of uranium halides on a hot filament.^[9]

• 
  World uranium production (mines) and demand^[76]
• 
  Yellowcake is a concentrated mixture of uranium oxides that is further refined to extract pure uranium.
• 
  Uranium production 2015, in tonnes^[80]

Resources and reserves

Uranium price 1990–2022.

It is estimated that 6.1 million tonnes of uranium exists in ore reserves that are economically viable at US$130 per kg of uranium,^[81] while 35 million tonnes are classed as mineral resources (reasonable prospects for eventual economic extraction).^[82]

Australia has 28% of the world's known uranium ore reserves^[81] and the world's largest single uranium deposit is located at the Olympic Dam Mine in South Australia.^[83] There is a significant reserve of uranium in Bakouma, a sub-prefecture in the prefecture of Mbomou in the Central African Republic.^[84]

Some uranium also originates from dismantled nuclear weapons.^[85] For example, in 1993–2013 Russia supplied the United States with 15,000 tonnes of low-enriched uranium within the Megatons to Megawatts Program.^[86]

An additional 4.6 billion tonnes of uranium are estimated to be dissolved in sea water (Japanese scientists in the 1980s showed that extraction of uranium from sea water using ion exchangers was technically feasible).^[87][88] There have been experiments to extract uranium from sea water,^[89] but the yield has been low due to the carbonate present in the water. In 2012, ORNL researchers announced the successful development of a new absorbent material dubbed HiCap which performs surface retention of solid or gas molecules, atoms or ions and also effectively removes toxic metals from water, according to results verified by researchers at Pacific Northwest National Laboratory.^[90][91]

Supplies

Monthly uranium spot price in US$ per pound. The 2007 price peak is clearly visible.^[92]

In 2005, ten countries accounted for the majority of the world's concentrated uranium oxides: Canada (27.9%), Australia (22.8%), Kazakhstan (10.5%), Russia (8.0%), Namibia (7.5%), Niger (7.4%), Uzbekistan (5.5%), the United States (2.5%), Argentina (2.1%) and Ukraine (1.9%).^[93] In 2008 Kazakhstan was forecast to increase production and become the world's largest supplier of uranium by 2009.^[94][95] The prediction came true, and Kazakhstan does dominate the world's uranium market since 2010. In 2021, its share was 45.1%, followed by Namibia (11.9%), Canada (9.7%), Australia (8.7%), Uzbekistan (7.2%), Niger (4.7%), Russia (5.5%), China (3.9%), India (1.3%), Ukraine (0.9%), and South Africa (0.8%), with a world total production of 48,332 tonnes.^[76] Most of uranium was produced not by conventional underground mining of ores (29% of production), but by in situ leaching (66%).^[76][96]

In the late 1960s, UN geologists also discovered major uranium deposits and other rare mineral reserves in Somalia. The find was the largest of its kind, with industry experts estimating the deposits at over 25% of the world's then known uranium reserves of 800,000 tons.^[97]

The ultimate available supply is believed to be sufficient for at least the next 85 years,^[82] although some studies indicate underinvestment in the late twentieth century may produce supply problems in the 21st century.^[98] Uranium deposits seem to be log-normal distributed. There is a 300-fold increase in the amount of uranium recoverable for each tenfold decrease in ore grade.^[99] In other words, there is little high grade ore and proportionately much more low grade ore available.

Oxidation states and oxides

Oxides

Triuranium octoxide (left) and uranium dioxide (right) are the two most common uranium 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 those with long retention times or particles recovered in the stack scrubber. Uranium content is usually referenced to U
₃O
₈, which dates to the days of the Manhattan Project when U
₃O
₈ was used as an analytical chemistry reporting standard.^[100]

Phase relationships in the uranium-oxygen system are complex. The most important oxidation states of uranium are uranium(IV) and uranium(VI), and their two corresponding oxides are, respectively, uranium dioxide (UO
₂) and uranium trioxide (UO
₃).^[101] Other uranium oxides such as uranium monoxide (UO), diuranium pentoxide (U
₂O
₅), and uranium peroxide (UO
₄·2H
₂O) also exist.

The most common forms of uranium oxide are triuranium octoxide (U
₃O
₈) and UO
₂.^[102] Both oxide forms are solids that have low solubility in water and are relatively stable over a wide range of environmental conditions. Triuranium octoxide 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.^[102] At ambient temperatures, UO
₂ will gradually convert to U
₃O
₈. Because of their stability, uranium oxides are generally considered the preferred chemical form for storage or disposal.^[102]

Aqueous chemistry

Uranium in its oxidation states III, IV, V, VI

Salts of many oxidation states of uranium are water-soluble and may be studied in aqueous solutions. The most common ionic forms are U³⁺
(brown-red), U⁴⁺
(green), UO⁺
₂ (unstable), and UO²⁺
₂ (yellow), for U(III), U(IV), U(V), and U(VI), respectively.^[103] 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 UO²⁺
₂ ion represents the uranium(VI) state and is known to form compounds such as uranyl carbonate, uranyl chloride and uranyl sulfate. UO²⁺
₂ also forms complexes with various organic chelating agents, the most commonly encountered of which is uranyl acetate.^[103]

Unlike the uranyl salts of uranium and polyatomic ion uranium-oxide cationic forms, the uranates, salts containing a polyatomic uranium-oxide anion, are generally not water-soluble.

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. 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 because a U(VI) cation is able to bind two terminal oxides and three or more carbonates to form anionic complexes.

Pourbaix diagrams^[104]

Uranium in a non-complexing aqueous medium (e.g. perchloric acid/sodium hydroxide).[104]	Uranium in carbonate solution
Relative concentrations of the different chemical forms of uranium in a non-complexing aqueous medium (e.g. perchloric acid/sodium hydroxide).[104]	Relative concentrations of the different chemical forms of uranium in an aqueous carbonate solution.[104]

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.^[105] 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.^[105]

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 U
₃O
₈.^[105] Carbides of uranium include uranium monocarbide (UC), uranium dicarbide (UC
₂), and diuranium tricarbide (U
₂C
₃). Both UC and UC
₂ are formed by adding carbon to molten uranium or by exposing the metal to carbon monoxide at high temperatures. Stable below 1800 °C, U
₂C
₃ is prepared by subjecting a heated mixture of UC and UC
₂ to mechanical stress.^[106] Uranium nitrides obtained by direct exposure of the metal to nitrogen include uranium mononitride (UN), uranium dinitride (UN
₂), and diuranium trinitride (U
₂N
₃).^[106]

Halides

Uranium hexafluoride is the feedstock used to separate uranium-235 from natural uranium.

All uranium fluorides are created using uranium tetrafluoride (UF
₄); UF
₄ itself is prepared by hydrofluorination of uranium dioxide.^[105] Reduction of UF
₄ with hydrogen at 1000 °C produces uranium trifluoride (UF
₃). Under the right conditions of temperature and pressure, the reaction of solid UF
₄ with gaseous uranium hexafluoride (UF
₆) can form the intermediate fluorides of U
₂F
₉, U
₄F
₁₇, and UF
₅.^[105]

At room temperatures, UF
₆ has a high vapor pressure, making it useful in the gaseous diffusion process to separate the rare uranium-235 from the common uranium-238 isotope. This compound can be prepared from uranium dioxide and uranium hydride by the following process:^[105]

UO
₂ + 4 HF → UF
₄ + 2 H
₂O (500 °C, endothermic)

UF
₄ + F
₂ → UF
₆ (350 °C, endothermic)

The resulting UF
₆, a white solid, is highly reactive (by fluorination), easily sublimes (emitting a vapor that behaves as a nearly ideal gas), and is the most volatile compound of uranium known to exist.^[105]

One method of preparing uranium tetrachloride (UCl
₄) is to directly combine chlorine with either uranium metal or uranium hydride. The reduction of UCl
₄ by hydrogen produces uranium trichloride (UCl
₃) while the higher chlorides of uranium are prepared by reaction with additional chlorine.^[105] 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 UH
₃ to those element's acids.^[105] Known examples include: UBr
₃, UBr
₄, UI
₃, and UI
₄. UI
₅ has never been prepared. Uranium oxyhalides are water-soluble and include UO
₂F
₂, UOCl
₂, UO
₂Cl
₂, and UO
₂Br
₂. Stability of the oxyhalides decrease as the atomic weight of the component halide increases.^[105]

Enrichment

Cascades of gas centrifuges are used to enrich uranium ore to concentrate its fissionable isotopes.

In nature, uranium is found as uranium-238 (99.2742%) and uranium-235 (0.7204%). Isotope separation concentrates (enriches) the fissile uranium-235 for nuclear weapons and most nuclear power plants, except for gas cooled reactors and pressurised heavy water reactors. Most neutrons released by a fissioning atom of uranium-235 must impact other uranium-235 atoms to sustain the nuclear chain reaction. The concentration and amount of uranium-235 needed to achieve this is called a 'critical mass'.

To be considered 'enriched', the uranium-235 fraction should be between 3% and 5%.^[120] This 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 should be no more than 0.3%.^[121] The price of uranium has risen since 2001, so enrichment tailings containing more than 0.35% uranium-235 are being considered for re-enrichment, driving the price of depleted uranium hexafluoride above $130 per kilogram in July 2007 from $5 in 2001.^[121]

The gas centrifuge process, where gaseous uranium hexafluoride (UF
₆) is separated by the difference in molecular weight between ²³⁵UF₆ and ²³⁸UF₆ using high-speed centrifuges, is the cheapest and leading enrichment process.^[33] The gaseous diffusion process had been the leading method for enrichment and was 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 (since uranium-238 is heavier it diffuses slightly slower than uranium-235).^[33] 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.^[7] An alternative laser method of enrichment is known as atomic vapor laser isotope separation (AVLIS) and employs visible tunable lasers such as dye lasers.^[122] Another method used is liquid thermal diffusion.^[8]

The only significant deviation from the ²³⁵U to ²³⁸U ratio in any known natural samples occurs in Oklo, Gabon, where natural nuclear fission reactors consumed some of the ²³⁵U some two billion years ago when the ratio of ²³⁵U to ²³⁸U was more akin to that of low enriched uranium allowing regular ("light") water to act as a neutron moderator akin to the process in humanmade light water reactors. The existence of such natural fission reactors which had been theoretically predicted beforehand was proven as the slight deviation of ²³⁵U concentration from the expected values were discovered during uranium enrichment in France. Subsequent investigations to rule out any nefarious human action (such as stealing of ²³⁵U) confirmed the theory by finding isotope ratios of common fission products (or rather their stable daughter nuclides) in line with the values expected for fission but deviating from the values expected for non-fission derived samples of those elements.

Effects and precautions

Normal functioning of the kidney, brain, liver, heart, and other systems can be affected by uranium exposure, because, besides being weakly radioactive, uranium is a toxic metal.^{[27][131][132]} Uranium is also a reproductive toxicant.^[133][134] Radiological effects are generally local because alpha radiation, the primary form of ²³⁸U decay, has a very short range, and will not penetrate skin. Alpha radiation from inhaled uranium has been demonstrated to cause lung cancer in exposed nuclear workers.^[135] While the CDC has published one study that no human cancer has been seen as a result of exposure to natural or depleted uranium,^[136] exposure to uranium and its decay products, especially radon, is a significant health threat.^[137] 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.^[138]

Although accidental inhalation exposure to a high concentration of uranium hexafluoride has resulted in human fatalities, those deaths were associated with the generation of highly toxic hydrofluoric acid and uranyl fluoride rather than with uranium itself.^[139] Finely divided uranium metal presents a fire hazard because uranium is pyrophoric; small grains will ignite spontaneously in air at room temperature.^[9]

Uranium metal is commonly handled with gloves as a sufficient precaution.^[140] Uranium concentrate is handled and contained so as to ensure that people do not inhale or ingest it.^[140]