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Uranium is a chemical element; it has chemical symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 and 92 , of which 6 are . Uranium radioactively decays, usually by emitting an alpha particle. The half-life of this decay varies between 159,200 and 4.5 billion years for different isotopes, making them useful for dating the age of the Earth. The most common isotopes in natural uranium are uranium-238 (which has 146 and accounts for over 99% of uranium on Earth) and uranium-235 (which has 143 neutrons). Uranium has the highest atomic weight of the primordially occurring elements. Its density is about 70% higher than that of lead and slightly lower than that of gold or tungsten. It occurs naturally in low concentrations of a few parts per million in soil, rock and water, and is commercially uranium mining from uranium-bearing such as uraninite.
Many contemporary uses of uranium exploit its unique atomic nucleus properties. Uranium is used in nuclear power plants and because it is the only naturally occurring element with a fissile 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 undergo enrichment so that enough uranium-235 is present. Uranium-238 is fissionable by fast neutrons and is fertile material, meaning it can be transmuted to fissile plutonium-239 in a nuclear reactor. Another fissile isotope, uranium-233, can be produced from natural thorium and is studied for future industrial use in nuclear technology. Uranium-238 has a small probability for spontaneous 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 sustained nuclear chain reaction. This generates the heat in nuclear reactor and produces the fissile material for nuclear weapons. The primary civilian use for uranium harnesses the heat energy to produce electricity. Depleted uranium (U) is used in kinetic energy penetrators and vehicle armour.
The 1789 discovery of uranium in the mineral uraninite is credited to Martin Heinrich Klaproth, who named the new element after the recently discovered planet Uranus. Eugène-Melchior Péligot was the first person to isolate the metal, and its radioactive properties were discovered in 1896 by Henri Becquerel. Research by Otto Hahn, Lise Meitner, Enrico Fermi and others, such as J. Robert Oppenheimer 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 uranium metal and uranium-derived plutonium-239. Dismantling of these weapons and related nuclear facilities is carried out within various nuclear disarmament programs and costs billions of dollars. Weapon-grade uranium obtained from nuclear weapons is diluted with uranium-238 and reused as fuel for nuclear reactors. Spent nuclear fuel forms radioactive waste, which mostly consists of uranium-238 and poses a significant health threat and environmental impact.
Uranium metal reacts with almost all non-metallic elements (except ) and their compounds, with reactivity increasing with temperature. Hydrochloric and dissolve uranium, but non-oxidizing acids other than hydrochloric acid attack the element very slowly. When finely divided, it can react with cold water; in air, uranium metal becomes coated with a dark layer of uranium dioxide. Uranium in ores is extracted chemically and converted into uranium dioxide or other chemical forms usable in industry.
Uranium-235 was the first isotope that was found to be fissile. Other naturally occurring isotopes are fissionable, but not fissile. On bombardment with slow neutrons, uranium-235 most of the time splits into two smaller atomic nucleus, releasing nuclear binding energy and more neutrons. If too many of these neutrons are absorbed by other uranium-235 nuclei, a nuclear 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 a neutron poison, absorbing some of the free neutrons. Such neutron absorbent materials are often part of reactor (see nuclear reactor physics for a description of this process of reactor control).
As little as of uranium-235 can be used to make an atomic bomb. The nuclear weapon detonated over Hiroshima, called Little Boy, relied on uranium fission. However, the first nuclear bomb (the Gadget used at Trinity) and the bomb that was detonated over Nagasaki (Fat Man) were both plutonium bombs.
Uranium metal has three allotropy forms:
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. Other uses of depleted uranium include counterweights for aircraft control surfaces, as ballast for missile re-entry vehicles and as a shielding material. Due to its high density, this material is found in inertial guidance systems and in gyroscope . 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. The main risk of exposure to depleted uranium is chemical poisoning by uranium oxide rather than radioactivity (uranium being only a weak alpha decay).
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 from the nuclear fusion process.
Commercial nuclear power plants use fuel that is typically enriched to around 3% uranium-235. The CANDU reactor 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-239 through the following reaction:
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. "Statement regarding the Good Morning America broadcast," The Homer Laughlin China Co. , 16 March 2011, accessed 25 March 2012.
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. 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.
Uranium was also used in photography chemicals (especially uranium nitrate as a toner), in lamp filaments for stage lighting bulbs, to improve the appearance of dentures, and in the leather and wood industries for stains and dyes. Uranium salts are 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 , isolated and .
The discovery of the radioactivity of uranium ushered in additional scientific and practical uses of the element. The long half-life of uranium-238 (4.47 years) makes it well-suited for use in estimating the age of the earliest 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.
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.
Henri Becquerel discovered radioactivity by using uranium in 1896. Becquerel made the discovery in Paris by leaving a sample of a uranium salt, KUO(SO) (potassium uranyl sulfate), on top of an unexposed photographic plate in a drawer and noting that the plate had become "fogged". 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, until the Manhattan Project and the Cold War placed a large demand on uranium for fission research and weapon development.
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. 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, most of which was supplied by Westinghouse Lamp Plant in a makeshift production process.
Uranium miners have a higher incidence of cancer. An excess risk of lung cancer among Navajo people uranium miners, for example, has been documented and linked to their occupation. 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.
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 Russia and several other former Soviet states. Police in Asia, Europe, and South America on at least 16 occasions from 1993 to 2005 have intercepted shipments of smuggled bomb-grade uranium or plutonium, most of which was from ex-Soviet sources. From 1993 to 2005 the Material Protection, Control, and Accounting Program, operated by the federal 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.
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 Sievert, which is equivalent to a single full-body CT scan, (3000 examinations from 18 hospitals) 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 USD). 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. Russia's Nuclear Fuel Cycle. World Nuclear Association. Updated December 2021.
Uranium's concentration in the Earth's crust is (depending on the reference) 2 to 4 parts per million, or about 40 times as abundant as silver. The Earth's crust from the surface to 25 km (15 mi) down is calculated to contain 10 kg (2 lb) of uranium while the may contain 10 kg (2 lb). The concentration of uranium in soil ranges from 0.7 to 11 parts per million (up to 15 parts per million in farmland soil due to use of phosphate containing uranium impurities),Schnug, E., Sun, Y., Zhang, L., Windmann, H., Lottermoser, B.G., Ulrich, A. E., Bol, R., Makeawa, M., and Haneklaus, S.H. (2023) "Elemental loads with phosphate fertilizers – a constraint for soil productivity?" In: Bolan, N.S. and Kirkham, M.B. (eds.) Managing Soil Constraints for Sustaining Productivity. CRC Press. and its concentration in sea water is 3 parts per billion.
Uranium is more plentiful than antimony, tin, cadmium, mercury, or silver, and it is about as abundant as arsenic or molybdenum. Uranium is found in hundreds of minerals, including uraninite (the most common uranium ore), carnotite, autunite, uranophane, torbernite, and coffinite. Significant concentrations of uranium occur in some substances such as phosphate rock deposits, and minerals such as lignite, and monazite sands in uranium-rich ores (it is recovered commercially from sources with as little as 0.1% uranium).
Other organisms, such as the lichen Trapelia involuta or such as the bacterium Citrobacter, can absorb concentrations of uranium that are up to 300 times the level of their environment. 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. The proteobacterium Geobacter has also been shown to bioremediate uranium in ground water. The mycorrhizal fungus Glomus intraradices increases uranium content in the roots of its symbiotic plant.
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.
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. Dry weight concentrations of uranium in food plants are typically lower with one to two micrograms per day ingested through the food people eat.
Uranium ore is mined in several ways: open-pit mining, underground, , and borehole mining. 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. High-grade ores found in Athabasca Basin deposits in Saskatchewan, Canada can contain up to 23% uranium oxides on average. 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 UO. Yellowcake is then calcined to remove impurities from the milling process before refining and conversion.
Commercial-grade uranium can be produced through the redox of uranium with alkali metal or alkaline earth metals. Uranium metal can also be prepared through electrolysis of or , dissolved in molten calcium chloride () and sodium chloride (sodium) solution. Very pure uranium is produced through the thermal decomposition of uranium on a hot filament.
Australia has 28% of the world's known uranium ore reserves and the world's largest single uranium deposit is located at the Olympic Dam Mine in South Australia. There is a significant reserve of uranium in Bakouma, a sub-prefecture in the prefecture of Mbomou in the Central African Republic.
Some uranium also originates from dismantled nuclear weapons. For example, in 1993–2013 Russia supplied the United States with 15,000 tonnes of low-enriched uranium within the Megatons to Megawatts Program.
An additional 4.6 billion tonnes of uranium are estimated to be dissolved in sea water ( scientists in the 1980s showed that extraction of uranium from sea water using was technically feasible). There have been experiments to extract uranium from sea water, 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.
In the late 1960s, UN geologists 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.
The ultimate available supply is believed to be sufficient for at least the next 85 years, though some studies indicate underinvestment in the late twentieth century may produce supply problems in the 21st century. 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. In other words, there is little high grade ore and proportionately much more low grade ore available.
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 are, respectively, uranium dioxide () and uranium trioxide (). Other such as uranium monoxide (UO), diuranium pentoxide (), and uranium peroxide () also exist.
The most common forms of uranium oxide are triuranium octoxide () and . 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. At ambient temperatures, will gradually convert to . Because of their stability, uranium oxides are generally considered the preferred chemical form for storage or disposal.
Unlike the uranyl salts of uranium and polyatomic ion uranium-oxide cationic forms, the , salts containing a polyatomic uranium-oxide anion, are generally not water-soluble.
| +Puigdomenech, Ignasi (2004) Hydra/Medusa Chemical Equilibrium Database and Plotting Software. KTH Royal Institute of Technology | |
| Uranium in a non-complexing aqueous medium (e.g. perchloric acid/sodium hydroxide). | 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). | Relative concentrations of the different chemical forms of uranium in an aqueous carbonate solution. |
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.
and are both relatively Chemically inert compounds that are minimally soluble in , react with water, and can ignite in air to form . Carbides of uranium include uranium monocarbide (Ucarbon), uranium dicarbide (), and diuranium tricarbide (). Both UC and are formed by adding carbon to molten uranium or by exposing the metal to carbon monoxide at high temperatures. Stable below 1800 °C, is prepared by subjecting a heated mixture of UC and to mechanical stress. Uranium nitrides obtained by direct exposure of the metal to nitrogen include uranium mononitride (UN), uranium dinitride (), and diuranium trinitride ().
At room temperatures, 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:
The resulting , 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.
One method of preparing uranium tetrachloride () is to directly combine chlorine with either uranium metal or uranium hydride. The reduction of by hydrogen produces uranium trichloride () while the higher chlorides of uranium are prepared by reaction with additional chlorine. All uranium chlorides react with water and air.
and of uranium are formed by direct reaction of, respectively, bromine and iodine with uranium or by adding to those element's acids. Known examples include: , , , and . has never been prepared. Uranium oxyhalides are water-soluble and include Uranyl fluoride, , Uranyl chloride, and Uranyl bromide. Stability of the oxyhalides decrease as the atomic weight of the component halide increases.
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 of plutonium-244; uranium-239, which is formed when U undergoes spontaneous fission, releasing neutrons that are captured by another U atom; uranium-237, which is formed when U captures a neutron but emits two more, which then decays to neptunium-237; uranium-236, which occurs in trace quantities due to neutron capture on U and as a decay product of plutonium-244; and finally, uranium-233, which is formed in the decay chain of neptunium-237. Additionally, uranium-232 would be produced by the double beta decay of natural thorium-232, though this energetically possible process has never been observed.
Uranium-238 is the most stable isotope of uranium, with a half-life of about years, roughly the age of the Earth. Uranium-238 is predominantly an alpha emitter, decaying to thorium-234. It ultimately decays through the uranium series, which has 18 members, into lead-206. Uranium-238 is not fissile, but is a fertile isotope, because after neutron activation it can be converted to plutonium-239, another fissile isotope. Indeed, the U nucleus can absorb one neutron to produce the radioactive isotope uranium-239. U decays by beta emission to neptunium-239, also a beta-emitter, that decays in its turn, within a few days into plutonium-239. Pu was used as fissile material in the first atomic bomb detonated in the "Trinity test" on 16 July 1945 in New Mexico.
Uranium-235 has a half-life of about years; it is the next most stable uranium isotope after U and is also predominantly an alpha emitter, decaying to thorium-231. Uranium-235 is important for both and , 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. The decay chain of U, which is called the actinium series, has 15 members and eventually decays into lead-207. The constant rates of decay in these decay series makes the comparison of the ratios of parent to decay product useful in radiometric dating.
Uranium-236 has a half-life of years 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 an extinct progenitor of its alpha decay daughter, thorium-232. Uranium-236 occurs in spent nuclear fuel when neutron capture on U does not induce fission, or as a decay product of plutonium-240. Uranium-236 is not fertile, as three more neutron captures are required to produce fissile Pu, and is not itself fissile; as such, it is considered long-lived radioactive waste.
Uranium-234 is a member of the uranium series and occurs in equilibrium with its progenitor, U; it undergoes alpha decay with a half-life of 245,500 years 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, like U, is fissile. It can be bred from thorium-232 via neutron bombardment, usually in a nuclear reactor; this process is known as the thorium fuel cycle. Owing to the fissility of U and the greater natural abundance of thorium (three times that of uranium), U has been investigated for use as nuclear fuel as a possible alternative to U and Pu, though is not in widespread use . The decay chain of uranium-233 forms part of the neptunium series and ends at nearly-stable bismuth-209 (half-life ) and stable thallium-205.
Uranium-232 is an alpha emitter with a half-life of 68.9 years. This isotope is produced as a byproduct in production of U and is considered a nuisance, as it is not fissile and decays through short-lived alpha and gamma radiation such as Tl. It is also expected that thorium-232 should be able to undergo double beta decay, which would produce uranium-232, but this has not yet been observed experimentally.
All isotopes from U to U inclusive have minor cluster decay branches (less than %), and all these bar U, in addition to U, have minor spontaneous fission branches; the greatest branching ratio for spontaneous fission is about % for U, or about one in every two million decays. The shorter-lived trace isotopes U and U exclusively undergo beta decay, with respective half-lives of 6.752 days and 23.45 minutes.
In total, 28 isotopes of uranium have been identified, ranging in mass number from 214 to 242, with the exception of 220. Among the uranium isotopes not found in natural samples or nuclear fuel, the longest-lived is U, an alpha emitter with a half-life of 20.23 days. This isotope has been considered for use in targeted alpha-particle therapy (TAT). All other isotopes have half-lives shorter than one hour, except for U (half-life 4.2 days) and U (half-life 14.1 hours). The shortest-lived known isotope is U, with a half-life of 660 nanoseconds, and it is expected that the hitherto unknown U has an even shorter half-life. The proton-rich isotopes lighter than U primarily undergo alpha decay, except for U and U, which decay to protactinium isotopes via positron emission and electron capture, respectively; the neutron-rich U, U, and U undergo beta decay to form neptunium isotopes.
To be considered 'enriched', the uranium-235 fraction should be between 3% and 5%. 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 U concentration should be no more than 0.3%. 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.
The gas centrifuge process, where gaseous uranium hexafluoride () is separated by the difference in molecular weight between UF and UF using high-speed , is the cheapest and leading enrichment process. 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 diffusion 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). 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. An alternative laser method of enrichment is known as atomic vapor laser isotope separation (AVLIS) and employs visible such as . Another method used is liquid thermal diffusion.
The only significant deviation from the U to U ratio in any known natural samples occurs in Oklo, Gabon, where natural nuclear fission reactors consumed some of the U some two billion years ago when the ratio of U to U 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 U 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 U) confirmed the theory by finding isotope ratios of common (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.
The health impacts of natural and of deleted uranium are chemical rather than due to radiation.
The Occupational Safety and Health Administration (OSHA) has set the permissible exposure limit for uranium exposure in the workplace as 0.25 mg/m over an 8-hour workday. The National Institute for Occupational Safety and Health (NIOSH) has set a recommended exposure limit (REL) of 0.2 mg/m over an 8-hour workday and a short-term limit of 0.6 mg/m. At 10 mg/m, uranium is IDLH.
Most ingested uranium is excreted during digestion. Only 0.5% is absorbed when insoluble forms of uranium, such as its oxide, are ingested, whereas absorption of the more soluble uranyl ion can be up to 5%. However, soluble uranium compounds tend to quickly pass through the body, whereas insoluble uranium compounds, especially when inhaled by way of dust into the , pose a more serious exposure hazard. After entering the bloodstream, the absorbed uranium tends to bioaccumulation and stay for many years in bone tissue because of uranium's affinity for phosphates. Incorporated uranium becomes uranyl ions, which accumulate in bone, liver, kidney, and reproductive tissues.
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.Busby, C. and Schnug, E. (2008). "Advanced biochemical and biophysical aspects of uranium contamination". In: De Kok, L.J. and Schnug, E. (Eds) Loads and Fate of Fertilizer Derived Uranium. Backhuys Publishers, Leiden, The Netherlands.
Uranium is not absorbed through the skin, and released by uranium cannot penetrate the skin.
Uranium can be decontaminated from steel surfaces and .
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. Finely divided uranium metal presents a fire hazard because uranium is pyrophoricity; small grains will ignite spontaneously in air at room temperature.
Uranium metal is commonly handled with gloves as a sufficient precaution. Uranium concentrate is handled and contained so as to ensure that people do not inhale or ingest it.
Enrichment
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 pressurized 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'.
Human exposure
A person can be exposed to uranium (or its decay product, 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 containing uraium impurities, 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. 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.
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 Metal toxicity. Uranium is also a reproductive toxicant. Radiological effects are generally local because alpha radiation, the primary form of U 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. The Centers for Disease Control have published one study stating that neither natural nor depleted uranium have been classified with respect to carcinogenicity. Exposure to its decay products, especially radon, is a significant health threat, and uranium processing produces wastes contaminated with radium which in turn produces radon gas.
target="_blank" rel="nofollow"> Radon Exposures to Workers at the Fernald Feed Materials Production Center. Page reviewed: April 8, 2020. U.S. National Institute for Occupational Safety and Health (NIOSH) Because of its long half-life, purified uranium will not produce significant amounts of daughter nuclides for millions of years. 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.Chart of the Nuclides, US Atomic Energy Commission 1968
See also
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