Many contemporary uses of uranium exploit its uniquenuclear properties. Uranium is used innuclear power plants andnuclear weapons because it is the only naturally occurring element with afissile isotope – uranium-235 – present in non-trace amounts. However, because of the low abundance of uranium-235 in natural uranium (which is overwhelmingly uranium-238), uranium needs to undergoenrichment so that enough uranium-235 is present. Uranium-238 is fissionable by fast neutrons and isfertile, meaning it can betransmuted to fissileplutonium-239 in anuclear reactor. Another fissile isotope,uranium-233, can be produced from naturalthorium and is studied for future industrial use in nuclear technology. Uranium-238 has a small probability forspontaneous fission or even induced fission with fast neutrons; uranium-235, and to a lesser degree uranium-233, have a much higher fission cross-section for slow neutrons. In sufficient concentration, these isotopes maintain a sustainednuclear chain reaction. This generates the heat innuclear power reactors and produces the fissile material for nuclear weapons. The primary civilian use for uranium harnesses the heat energy to produce electricity.Depleted uranium (238U) is used inkinetic energy penetrators andarmor plating.[10]
Uranium metal reacts with almost all non-metallic elements (exceptnoble gases) and theircompounds, with reactivity increasing with temperature.[17]Hydrochloric andnitric acids dissolve uranium, but non-oxidizing acids other than hydrochloric acid attack the element very slowly.[11] When finely divided, it can react with cold water; in air, uranium metal becomes coated with a dark layer ofuranium dioxide.[12] Uranium in ores is extracted chemically and converted intouranium dioxide or other chemical forms usable in industry.
In 1938,Otto Hahn andFritz Strassman discovered that barium was a product of bombardingUranium-235 with neutrons, and a year laterLise Meitner andOtto Robert Frisch developed the theory ofnuclear fission to explain this new phenomenon, making U-235 the firstfissile isotope to be discovered.[18] On bombardment with slow neutrons, uranium-235 most of the time splits into two smallernuclei, releasing nuclearbinding energy and more neutrons. If too many of these neutrons are absorbed by other uranium-235 nuclei, anuclear chain reaction occurs that results in a burst of heat or (in some circumstances) an explosion. In a nuclear reactor, such a chain reaction is slowed and controlled by aneutron poison, absorbing some of the free neutrons. Such neutron absorbent materials are often part of reactorcontrol rods (seenuclear reactor physics for a description of this process of reactor control). Other naturally occurring isotopes such asUranium-238 arefissionable, but not fissile, meaning that they only undergo fission when absorbing high energy (fast) neutrons.[19]
As little as 15 lb (6.8 kg) of uranium-235 can be used to make an atomic bomb.[20] The nuclear weapon detonated overHiroshima, calledLittle Boy, relied on uranium fission. However, the first nuclear bomb (theGadget used atTrinity) and the bomb that was detonated over Nagasaki (Fat Man) were both plutonium bombs.
β (tetragonal) stable from 668 to 775 °C (1,234 to 1,427 °F). Tetragonal, space groupP42/mnm,P42nm, orP4n2, lattice parametersa = 565.6 pm,b =c = 1075.9 pm.[22]
γ (body-centered cubic) from 775 °C (1,427 °F) to melting point—this is the most malleable and ductile state. Body-centered cubic, lattice parametera = 352.4 pm.[22]
Applications
Military
Various militaries use depleted uranium as high-density penetrators.
The major application of uranium in the military sector is in high-density penetrating projectiles. This ammunition consists ofdepleted uranium (DU) alloyed with 1–2% other elements, such astitanium ormolybdenum.[23] At high impact speed, the density, hardness, andpyrophoricity of the projectile enable the destruction of heavily armored targets. Tank armor and other removablevehicle 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 health questions concerning uranium compounds left in the soil (seeGulf War syndrome).[20]
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 thanlead in halting radiation from strong sources such asradium.[11] Other uses of depleted uranium include counterweights for aircraft control surfaces, as ballast for missilere-entry vehicles and as a shielding material.[12] Due to its high density, this material is found ininertial guidance systems and ingyroscopiccompasses.[12] 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.[24] The main risk of exposure to depleted uranium is chemical poisoning byuranium oxide rather than radioactivity (uranium being only a weakalpha emitter).
During the later stages ofWorld War II, the entireCold 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 usesplutonium-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 oftritium anddeuterium to undergonuclear 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 byfast neutrons from the nuclear fusion process.[25]
Civilian
The main use of uranium in the civilian sector is to fuelnuclear power plants. One kilogram of uranium-235 can theoretically produce about 20 terajoules of energy (2×1013joules), assuming complete fission; as muchenergy as 1.5 million kilograms (1,500tonnes) ofcoal.[10]
Commercial nuclear power plants use fuel that is typically enriched to around 3% uranium-235.[10] TheCANDU andMagnox designs are the only commercial reactors capable of using unenriched uranium fuel. Fuel used forUnited States Navy reactors is typically highly enriched inuranium-235 (the exact values areclassified). In abreeder reactor, uranium-238 can also be converted into plutonium-239 through the following reaction:[12]
Before (and, occasionally, after) the discovery of radioactivity, uranium was primarily used in small amounts for yellow glass and pottery glazes, such asuranium glass and inFiestaware.[26]
The discovery and isolation ofradium in uranium ore (pitchblende) byMarie 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.[27][28] 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 underUV light.Uranium glass used as lead-in seals in a vacuumcapacitor
The use ofpitchblende, uranium in its naturaloxide form, dates back to at least the year 79 AD, when it was used in theRoman Empire to add a yellow color toceramic glazes.[12] Yellow glass with 1% uranium oxide was found in a Roman villa on CapePosillipo in theGulf of Naples, Italy, by R. T. Gunther of theUniversity of Oxford in 1912.[31] Starting in the lateMiddle Ages, pitchblende was extracted from theHabsburg silver mines inJoachimsthal,Bohemia (now Jáchymov in the Czech Republic) in theOre Mountains, and was used as a coloring agent in the localglassmaking industry.[32] In the early 19th century, the world's only known sources of uranium ore were these mines.
Thediscovery of the element is credited to the German chemistMartin Heinrich Klaproth. While he was working in his experimental laboratory inBerlin in 1789, Klaproth was able to precipitate a yellow compound (likelysodium diuranate) by dissolvingpitchblende innitric acid and neutralizing the solution withsodium hydroxide.[32] Klaproth assumed the yellow substance was the oxide of a yet-undiscovered element and heated it withcharcoal to obtain a black powder, which he thought was the newly discovered metal itself (in fact, that powder was anoxide of uranium).[32][33] He named the newly discovered element "Uranit" after the planetUranus (named after the primordialGreek god of the sky), which had been discovered eight years earlier byWilliam Herschel.[34] He later renamed it "Uranium" to conform to the naming standard.[35]
Henri Becquerel discovered radioactivity by using uranium in 1896.[17] Becquerel made the discovery in Paris by leaving a sample of a uranium salt, K2UO2(SO4)2 (potassium uranyl sulfate), on top of an unexposedphotographic plate in a drawer and noting that the plate had become "fogged".[37] He determined that a form of invisible light or rays emitted by uranium had exposed the plate.
During World War I when theCentral Powers suffered a shortage of molybdenum to make artillery gun barrels and high speed tool steels, they routinely usedferrouranium 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,[38][39] until theManhattan Project and theCold War placed a large demand on uranium for fission research and weapon development.
Fission research
Cuboids of uranium produced during the Manhattan Project
A team led byEnrico Fermi in 1934 found that bombarding uranium with neutrons producesbeta rays (electrons orpositrons from the elements produced; seebeta particle).[40] The fission products were at first mistaken for new elements with atomic numbers 93 and 94, which the Dean of theSapienza University of Rome,Orso Mario Corbino, namedausenium and hesperium, respectively.[41][42][43][44] The experiments leading to the discovery of uranium's ability to fission (break apart) into lighter elements and releasebinding energy were conducted byOtto Hahn andFritz Strassmann[40] in Hahn's laboratory in Berlin.Lise Meitner and her nephew, physicistOtto Robert Frisch, published the physical explanation in February 1939 and named the process "nuclear fission".[45] Soon after, Fermi hypothesized that 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 uranium-235.[40] Fermi urgedAlfred 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 theUniversity of Minnesota to separate the world's firsturanium-235 sample in the Tate Laboratory. UsingColumbia University'scyclotron,John Dunning confirmed the sample to be the isolated fissile material on 1 March.[46] Further work found that the far more common uranium-238 isotope can betransmuted 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 andnuclear power. Despite fission having been discovered in Germany, theUranverein ("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 aneutron moderator than it is in reality. Germany's attempts to build anatural uranium /heavy water reactor had not come close to reaching criticality by the time the Americans reachedHaigerloch, the site of the last German wartime reactor experiment.[47]
On 2 December 1942, as part of theManhattan Project, another team led by Enrico Fermi was able to initiate the first artificial self-sustainednuclear chain reaction,Chicago Pile-1. An initial plan using enriched uranium-235 was abandoned as it was as yet unavailable in sufficient quantities.[48] Working in a lab below the stands ofStagg Field at theUniversity of Chicago, the team created the conditions needed for such a reaction by piling together 360 tonnes ofgraphite, 53 tonnes ofuranium oxide, and 5.5 tonnes of uranium metal, most of which was supplied byWestinghouse Lamp Plant in a makeshift production process.[40][49]
Two types of atomic bomb were developed by the United States duringWorld War II: a uranium-based device (codenamed "Little Boy") whose fissile material was highlyenriched uranium, and a plutonium-based device (seeTrinity test and "Fat Man") whose plutonium was derived from uranium-238. Little Boy became the first nuclear weapon used in war when it was detonated overHiroshima,Japan, on 6 August 1945. Exploding with a yield equivalent to 12,500 tonnes ofTNT, the blast and thermal wave of the bomb destroyed nearly 50,000 buildings and killed about 75,000 people (seeAtomic bombings of Hiroshima and Nagasaki).[37]
In 1943 theManhattan Project contracted two private companies,Union Carbide andChevron, to quietly compile a survey of uranium deposits around the world. As the survey results came in, two geology professors studied the results and suggested general guidelines for new sources, including uranium associated with gold mines inthe Rand area inSouth Africa.[50]
Initially it was believed that uranium was relatively rare, and thatnuclear 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.[51]
Reactors
Four light bulbs lit with electricity generated from the first artificial electricity-producing nuclear reactor,EBR-I (1951)
TheX-10 Graphite Reactor atOak 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'sExperimental Breeder Reactor I, located at the Atomic Energy Commission's National Reactor Testing Station nearArco, Idaho, became the first nuclear reactor to create electricity on 20 December 1951.[52] 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 itselectricity come from nuclear power generated byBORAX-III, another reactor designed and operated byArgonne National Laboratory).[53][54] The world's first commercial scale nuclear power station,Obninsk in theSoviet Union, began generation with its reactor AM-1 on 27 June 1954. Other early nuclear power plants wereCalder Hall in England, which began generation on 17 October 1956,[55] and theShippingport Atomic Power Station inPennsylvania, which began on 26 May 1958. Nuclear power was used for the first time for propulsion by asubmarine, theUSSNautilus, in 1954.[40][56]
In 1972, French physicistFrancis Perrin discovered fifteen ancient and no longer active natural nuclear fission reactors in three separate ore deposits at theOklo mine inGabon, Africa, collectively known as theOklo Fossil Reactors. The ore deposit is 1.7 billion years old; then, uranium-235 constituted about 3% of uranium on Earth.[57] This is high enough to permit a sustained chain reaction, if other supporting conditions exist. The capacity of the surrounding sediment to contain the health-threateningnuclear waste products has been cited by the U.S. federal government as supporting evidence for the feasibility to store spent nuclear fuel at theYucca Mountain nuclear waste repository.[57]
Contamination and the Cold War legacy
U.S. and USSR/Russian nuclear weapons stockpiles, 1945–2005
Above-groundnuclear tests by the Soviet Union and the United States in the 1950s and early 1960s and byFrance into the 1970s and 1980s[24] spread a significant amount offallout from uraniumdaughter isotopes around the world.[58] Additional fallout and pollution occurred from severalnuclear accidents.[59]
Uranium miners have a higher incidence ofcancer. An excess risk of lung cancer amongNavajo uranium miners, for example, has been documented and linked to their occupation.[60] TheRadiation 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.[61]
During theCold 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 thebreak-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 theRussian Federation and several other former Soviet states.[20] Police inAsia,Europe, andSouth America on at least 16 occasions from 1993 to 2005 haveintercepted shipments of smuggled bomb-grade uranium or plutonium, most of which was from ex-Soviet sources.[20] From 1993 to 2005 theMaterial Protection, Control, and Accounting Program, operated by thefederal government of the United States, spent about 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.[20]
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 theInternational Nuclear Event Scale, and this number dropped under four per year in 1995–2003. The number of employees receiving annual radiation doses above 20mSv, which is equivalent to a single full-bodyCT scan,[62] 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 billionUSD). Its key issue is "the deferred liabilities accumulated during the 70 years of the nuclear industry, particularly during the time of the Soviet Union". About 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.[63]
Occurrence
Uranium is anaturally occurring element found in low levels in all rock, soil, and water. It is the highest-numbered element found naturally in significant quantities on Earth and is almost always found combined with other elements.[12] Uranium is the48th most abundant element in the Earth's crust.[64] The decay of uranium,thorium, andpotassium-40 in Earth'smantle is thought to be the main source of heat[65][66] that keeps the Earth'souter core in the liquid state and drivesmantle convection, which in turn drivesplate tectonics.
Uranium'sconcentration in the Earth's crust is (depending on the reference) 2 to 4 parts per million,[11][24] or about 40 times as abundant assilver.[17] 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 theoceans may contain 1013 kg (2×1013 lb).[11] 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 phosphatefertilizers containing uranium impurities),[67] and its concentration in sea water is 3 parts per billion.[24]
Like all elements withatomic weights higher than that ofiron, uranium is only naturally formed by ther-process (rapid neutron capture) insupernovae andneutron star mergers.[68] Primordial thorium and uranium are only produced in the r-process, because thes-process (slow neutron capture) is too slow and cannot pass the gap of instability after bismuth.[69][70] Besides the two extant primordial uranium isotopes,235U and238U, the r-process also produced significant quantities of236U, which has a shorter half-life and so is anextinct radionuclide, having long since decayed completely to232Th. Further uranium-236 was produced by the decay of244Pu, accounting for the observed higher-than-expected abundance of thorium and lower-than-expected abundance of uranium.[71] While the natural abundance of uranium has been supplemented by the decay of extinct242Pu (half-life 375,000 years) and247Cm (half-life 16 million years), producing238U and235U respectively, this occurred to an almost negligible extent due to the shorter half-lives of these parents and their lower production than236U and244Pu, the parents of thorium: the247Cm/235U ratio at the formation of the Solar System was(7.0±1.6)×10−5.[72]
Uraninite, also known as pitchblende, is the most common ore mined to extract uranium.The evolution of Earth'sradiogenic heat flow over time: contribution from235U in red and from238U in green
Some bacteria, such asShewanella putrefaciens,Geobacter metallireducens and some strains ofBurkholderia fungorum, can use uranium for their growth and convert U(VI) to U(IV).[73][74] Recent research suggests that this pathway includes reduction of the soluble U(VI) via an intermediate U(V) pentavalent state.[75][76]Other organisms, such as thelichenTrapelia involuta ormicroorganisms such as thebacteriumCitrobacter, can absorb concentrations of uranium that are up to 300 times the level of their environment.[77]Citrobacter species absorburanyl ions when givenglycerol 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 inbioremediation todecontaminate uranium-polluted water.[32][78]The proteobacteriumGeobacter has also been shown to bioremediate uranium in ground water.[79] The mycorrhizal fungusGlomus intraradices increases uranium content in the roots of its symbiotic plant.[80]
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. ASphingomonas sp. strain BSAR-1 has been found to express a high activityalkaline 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 inE. coli.[81]
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.[32] Dry weight concentrations of uranium infood plants are typically lower with one to two micrograms per day ingested through the food people eat.[32]
Worldwide production of uranium in 2021 was 48,332tonnes, of which 21,819 t (45%) was mined inKazakhstan. Other important uranium mining countries areNamibia (5,753 t),Canada (4,693 t),Australia (4,192 t),Uzbekistan (3,500 t), andRussia (2,635 t).[82]
Uranium ore is mined in several ways:open pit,underground,in-situ leaching, andborehole mining.[10] 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.[83] High-grade ores found inAthabasca Basin deposits inSaskatchewan, Canada can contain up to 23% uranium oxides on average.[84] Uranium ore is crushed and rendered into a fine powder and then leached with either anacid oralkali. Theleachate is subjected to one of several sequences of precipitation, solvent extraction, and ion exchange. The resulting mixture, calledyellowcake, contains at least 75% uranium oxides U3O8. Yellowcake is thencalcined to remove impurities from the milling process before refining and conversion.[85]
It is estimated that 6.1 million tonnes of uranium exists in ores that are economically viable at US$130 per kg of uranium,[87] while 35 million tonnes are classed as mineral resources (reasonable prospects for eventual economic extraction).[88]
Some uranium also originates from dismantled nuclear weapons.[91] For example, in 1993–2013 Russia supplied the United States with 15,000 tonnes of low-enriched uranium within theMegatons to Megawatts Program.[92]
An additional 4.6 billion tonnes of uranium are estimated to be dissolved insea water (Japanese scientists in the 1980s showed that extraction of uranium from sea water usingion exchangers was technically feasible).[93][94] There have been experiments to extract uranium from sea water,[95] but the yield has been low due to the carbonate present in the water. In 2012,ORNL researchers announced the successful development of a new absorbent material dubbed HiCap which performs surface retention of solid or gas molecules, atoms or ions and also effectively removes toxic metals from water, according to results verified by researchers atPacific Northwest National Laboratory.[96][97]
Monthly uranium spot price in US$ per pound. The2007 price peak is clearly visible.[98]
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%), theUnited States (2.5%),Argentina (2.1%) andUkraine (1.9%).[99] In 2008, Kazakhstan was forecast to increase production and become the world's largest supplier of uranium by 2009;[100][101] Kazakhstan has dominated 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.[82] Most uranium was produced not by conventional underground mining of ores (29% of production), but byin situ leaching (66%).[82][102]
In the late 1960s, UN geologists discovered major uranium deposits and other rare mineral reserves inSomalia. 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.[103]
The ultimate available supply is believed to be sufficient for at least the next 85 years,[88] though some studies indicate underinvestment in the late twentieth century may produce supply problems in the 21st century.[104]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.[105]In other words, there is little high grade ore and proportionately much more low grade ore available.
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 toU 3O 8, which dates to the days of theManhattan Project whenU 3O 8 was used as an analytical chemistry reporting standard.[106]
The most common forms of uranium oxide aretriuranium octoxide (U 3O 8) andUO 2.[108] 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.[108] At ambient temperatures,UO 2 will gradually convert toU 3O 8. Because of their stability, uranium oxides are generally considered the preferred chemical form for storage or disposal.[108]
Aqueous chemistry
Uranium in its oxidation states III, IV, V, VI
Salts of manyoxidation states of uranium are water-soluble and may be studied inaqueous solutions. The most common ionic forms areU3+ (brown-red),U4+ (green),UO+ 2 (unstable), andUO2+ 2 (yellow), for U(III), U(IV), U(V), and U(VI), respectively.[109] A fewsolid and semi-metallic compounds such as UO andUS exist for the formal oxidation state uranium(II), but no simple ions are known to exist in solution for that state. Ions ofU3+ liberatehydrogen fromwater and are therefore considered to be highly unstable. TheUO2+ 2 ion represents the uranium(VI) state and is known to form compounds such asuranyl carbonate,uranyl chloride anduranyl sulfate.UO2+ 2 also formscomplexes with variousorganicchelating agents, the most commonly encountered of which isuranyl acetate.[109]
Unlike the uranyl salts of uranium andpolyatomic ion uranium-oxide cationic forms, theuranates, salts containing a polyatomic uranium-oxide anion, are generally not water-soluble.
Carbonates
The interactions of carbonate anions with uranium(VI) cause thePourbaix 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.
Uranium in a non-complexing aqueous medium (e.g.perchloric acid/sodium hydroxide).[110]
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).[110]
Relative concentrations of the different chemical forms of uranium in an aqueous carbonate solution.[110]
Effects of pH
The uranium fraction diagrams in the presence of carbonate illustrate this further: when the pH of a uranium(VI) solution increases, the uranium is converted to a hydrated uranium oxide hydroxide and at high pHs it becomes an anionic hydroxide complex.
When carbonate is added, uranium is converted to a series of carbonate complexes if the pH is increased. One effect of these reactions is increased solubility of uranium in the pH range 6 to 8, a fact that has a direct bearing on the long term stability of spent uranium dioxide nuclear fuels.
Hydrides, carbides and nitrides
Uranium metal heated to 250 to 300 °C (482 to 572 °F) reacts withhydrogen to formuranium 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 uraniumcarbide,nitride, andhalide compounds.[111] 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.[111]
Uranium carbides anduranium nitrides are both relativelyinertsemimetallic compounds that are minimally soluble inacids, react with water, and can ignite inair to formU 3O 8.[111] Carbides of uranium include uranium monocarbide (UC), uranium dicarbide (UC 2), and diuranium tricarbide (U 2C 3). Both UC andUC 2 are formed by adding carbon to molten uranium or by exposing the metal tocarbon monoxide at high temperatures. Stable below 1800 °C,U 2C 3 is prepared by subjecting a heated mixture of UC andUC 2 to mechanical stress.[112] Uranium nitrides obtained by direct exposure of the metal tonitrogen include uranium mononitride (UN), uranium dinitride (UN 2), and diuranium trinitride (U 2N 3).[112]
Halides
Uranium hexafluoride is the feedstock used to separate uranium-235 from natural uranium.
All uranium fluorides are created usinguranium tetrafluoride (UF 4);UF 4 itself is prepared by hydrofluorination of uranium dioxide.[111] Reduction ofUF 4 with hydrogen at 1000 °C producesuranium trifluoride (UF 3). Under the right conditions of temperature and pressure, the reaction of solidUF 4 with gaseousuranium hexafluoride (UF 6) can form the intermediate fluorides ofU 2F 9,U 4F 17, andUF 5.[111]
At room temperatures,UF 6 has a highvapor pressure, making it useful in thegaseous 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:[111]
UO 2 + 4 HF →UF 4 + 2H 2O (500 °C, endothermic)
UF 4 +F 2 →UF 6 (350 °C, endothermic)
The resultingUF 6, a white solid, is highlyreactive (by fluorination), easilysublimes (emitting a vapor that behaves as a nearlyideal gas), and is the most volatile compound of uranium known to exist.[111]
One method of preparinguranium tetrachloride (UCl 4) is to directly combinechlorine with either uranium metal or uranium hydride. The reduction ofUCl 4 by hydrogen producesuranium trichloride (UCl 3) while the higher chlorides of uranium are prepared by reaction with additional chlorine.[111] All uranium chlorides react with water and air.
Bromides andiodides of uranium are formed by direct reaction of, respectively,bromine andiodine with uranium or by addingUH 3 to those element's acids.[111] Known examples include:UBr 3,UBr 4,UI 3, andUI 4.UI 5 has never been prepared. Uranium oxyhalides are water-soluble and includeUO 2F 2,UOCl 2,UO 2Cl 2, andUO 2Br 2. Stability of the oxyhalides decrease as theatomic weight of the component halide increases.[111]
Natural uranium consists of three major isotopes: uranium-238 (99.28% natural abundance), uranium-235 (0.71%), and uranium-234 (0.0054%). There are also five other trace isotopes: uranium-240, a decay product ofplutonium-244;[115] uranium-239, which is formed when238U undergoes spontaneous fission, releasing neutrons that are captured by another238U atom; uranium-237, which is formed when238U captures a neutron but emits two more, which then decays toneptunium-237;uranium-236, which occurs in trace quantities due to neutron capture on235U and as a decay product of plutonium-244;[115] and finally,uranium-233, which is formed in thedecay chain of neptunium-237. Additionally,uranium-232 would be produced by thedouble beta decay of naturalthorium-232, though this energetically possible process has never been observed.[118]
Uranium-238 is the most stable isotope of uranium, with a half-life of about4.463×109 years,[8] roughly theage of the Earth. Uranium-238 is predominantly an alpha emitter, decaying to thorium-234. It ultimately decays through theuranium series, which has 18 members, intolead-206.[17] Uranium-238 is not fissile, but is a fertile isotope, because afterneutron activation it can be converted to plutonium-239, another fissile isotope. Indeed, the238U nucleus can absorb one neutron to produce the radioactive isotopeuranium-239.239U decays bybeta emission toneptunium-239, also a beta-emitter, that decays in its turn, within a few days into plutonium-239.239Pu was used as fissile material in the firstatomic bomb detonated in the "Trinity test" on 16 July 1945 inNew Mexico.[40]
Uranium-235 has a half-life of about7.04×108 years; it is the next most stable uranium isotope after238U and is also predominantly an alpha emitter, decaying to thorium-231.[8] Uranium-235 is important for bothnuclear reactors andnuclear weapons, because it is the only uranium isotope existing in nature on Earth in significant amounts that is fissile. This means that it can be split into two or three fragments (fission products) by thermal neutrons.[17] The decay chain of235U, which is called theactinium series, has 15 members and eventually decays into lead-207.[17] The constant rates of decay in these decay series makes the comparison of the ratios of parent todaughter elements useful in radiometric dating.
Uranium-236 has a half-life of2.342×107 years[8] and is not found in significant quantities in nature. The half-life of uranium-236 is too short for it to be primordial, though it has been identified as anextinct progenitor of its alpha decay daughter, thorium-232.[71] Uranium-236 occurs inspent nuclear fuel when neutron capture on235U does not induce fission, or as a decay product ofplutonium-240. Uranium-236 is not fertile, as three more neutron captures are required to produce fissile239Pu, and is not itself fissile; as such, it is considered long-lived radioactive waste.[119]
Uranium-234 is a member of the uranium series and occurs in equilibrium with its progenitor,238U; it undergoes alpha decay with a half-life of 245,500 years[8] and decays to lead-206 through a series of relatively short-lived isotopes.
Uranium-233 undergoes alpha decay with a half-life of 160,000 years and, like235U, is fissile.[12] It can be bred fromthorium-232 via neutron bombardment, usually in a nuclear reactor; this process is known as thethorium fuel cycle. Owing to the fissility of233U and the greater natural abundance of thorium (three times that of uranium),[120]233U has been investigated for use as nuclear fuel as a possible alternative to235U and239Pu,[121] though is not in widespread use as of 2022[update].[120] The decay chain of uranium-233 forms part of theneptunium series and ends at nearly-stable bismuth-209 (half-life2.01×1019 years)[8] and stablethallium-205.
Uranium-232 is an alpha emitter with a half-life of 68.9 years.[8] This isotope is produced as a byproduct in production of233U and is considered a nuisance, as it is not fissile and decays through short-lived alpha andgamma emitters such as208Tl.[121] It is also expected that thorium-232 should be able to undergodouble beta decay, which would produce uranium-232, but this has not yet been observed experimentally.[8]
All isotopes from232U to236U inclusive have minorcluster decay branches (less than10−10%), and all these bar233U, in addition to238U, have minorspontaneous fission branches;[8] the greatestbranching ratio for spontaneous fission is about5×10−5% for238U, or about one in every two million decays.[122] The shorter-lived trace isotopes237U and239U exclusively undergobeta decay, with respective half-lives of 6.752 days and 23.45 minutes.[8]
In total, 28 isotopes of uranium have been identified, ranging inmass number from 214[123] to 242, with the exception of 220.[8][124] Among the uranium isotopes not found in natural samples or nuclear fuel, the longest-lived is230U, an alpha emitter with a half-life of 20.23 days.[8] This isotope has been considered for use intargeted alpha-particle therapy (TAT).[125] All other isotopes have half-lives shorter than one hour, except for231U (half-life 4.2 days) and240U (half-life 14.1 hours).[8] The shortest-lived known isotope is221U, with a half-life of 660 nanoseconds, and it is expected that the hitherto unknown220U has an even shorter half-life.[126] The proton-rich isotopes lighter than232U primarily undergo alpha decay, except for229U and231U, which decay toprotactinium isotopes viapositron emission andelectron capture, respectively; the neutron-rich240U,241U, and242U undergobeta decay to formneptunium isotopes.[8][124]
Cascades ofgas 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 forgas cooled reactors andpressurized heavy water reactors. Most neutrons released by a fissioning atom of uranium-235 must impact other uranium-235 atoms to sustain thenuclear 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%.[127] 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', the235U concentration should be no more than 0.3%.[128] 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 ofdepleted uranium hexafluoride above $130 per kilogram in July 2007 from $5 in 2001.[128]
Thegas centrifuge process, where gaseousuranium hexafluoride (UF 6) is separated by the difference in molecular weight between235UF6 and238UF6 using high-speedcentrifuges, is the cheapest and leading enrichment process.[37] Thegaseous diffusion process had been the leading method for enrichment and was used in theManhattan Project. In this process, uranium hexafluoride is repeatedlydiffused through asilver-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).[37] Themolecular laser isotope separation method employs alaser 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.[10] An alternative laser method of enrichment is known asatomic vapor laser isotope separation (AVLIS) and employs visibletunable lasers such asdye lasers.[129] Another method used is liquid thermal diffusion.[11]
The only significant deviation from the235U to238U ratio in any known natural samples occurs inOklo,Gabon, wherenatural nuclear fission reactors consumed some of the235U some two billion years ago when the ratio of235U to238U was more akin to that oflow enriched uranium allowing regular ("light") water to act as aneutron moderator akin to the process in humanmadelight water reactors. The existence of such natural fission reactors which had been theoretically predicted beforehand was proven as the slight deviation of235U concentration from the expected values were discovered duringuranium enrichment in France. Subsequent investigations to rule out any nefarious human action (such as stealing of235U) confirmed the theory by finding isotope ratios of commonfission 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.
Human exposure
A person can be exposed to uranium (or itsradioactive daughters, such asradon) 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 processphosphatefertilizers containing uranium impurities, live near government facilities that made or tested nuclear weapons, live or work near a modern battlefield where depleted uraniumweapons have been used, or live or work near acoal-fired power plant, facilities that mine or process uranium ore, or enrich uranium for reactor fuel, may have increased exposure to uranium.[130][131] 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.
Most ingested uranium is excreted duringdigestion. Only 0.5% is absorbed when insoluble forms of uranium, such as its oxide, are ingested, whereas absorption of the more solubleuranyl ion can be up to 5%.[32] However, soluble uranium compounds tend to quickly pass through the body, whereas insoluble uranium compounds, especially when inhaled by way of dust into thelungs, pose a more serious exposure hazard. After entering the bloodstream, the absorbed uranium tends tobioaccumulate and stay for many years inbone tissue because of uranium's affinity for phosphates.[32] Incorporated uranium becomesuranyl ions, which accumulate in bone, liver, kidney, and reproductive tissues.[133]
Elements of high atomic number like uranium exhibit phantom or secondary radiotoxicity through absorption of natural background gamma and X-rays and re-emission of photoelectrons, which in combination with the high affinity of uranium to the phosphate moiety of DNA cause increased single and double strand DNA breaks.[134]
Uranium is not absorbed through the skin, andalpha particles released by uranium cannot penetrate the skin.[29]
Normal functioning of thekidney,brain,liver,heart, and other systems can be affected by uranium exposure, because, besides being weakly radioactive, uranium is atoxic metal.[32][138][139] Uranium is also areproductive toxicant.[140][141] Radiological effects are generally local because alpha radiation, the primary form of238U 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.[142] TheCenters for Disease Control have published one study stating that neither natural nor depleted uranium have been classified with respect to carcinogenicity.[143] Exposure to its decay products, especiallyradon, is a significant health threat, and uranium processing produces wastes contaminated with radium which in turn produces radon gas.[144] Because of its long half-life, purified uranium will not produce significant amounts of daughter nuclides for millions of years. Exposure tostrontium-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.[145]
Although accidental inhalation exposure to a high concentration ofuranium hexafluoride has resulted in human fatalities, those deaths were associated with the generation of highly toxic hydrofluoric acid anduranyl fluoride rather than with uranium itself.[146] Finely divided uranium metal presents a fire hazard because uranium ispyrophoric; small grains will ignite spontaneously in air at room temperature.[12]
Uranium metal is commonly handled with gloves as a sufficient precaution.[147] Uranium concentrate is handled and contained so as to ensure that people do not inhale or ingest it.[147]
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