Many contemporary uses of uranium exploit its unique nuclear properties. Uranium-235 is the only naturally occurring fissileisotope, which makes it widely used in nuclear power plants and nuclear weapons. However, because of the low abundance of uranium-235 in natural uranium (which is, overwhelmingly, mostly uranium-238), uranium needs to undergo enrichment so that enough uranium-235 is present. Uranium-238 is fissionable by fast neutrons and is fertile, 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 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 in kinetic energy penetrators and armor plating.[10]
Uranium metal reacts with almost all non-metallic elements (except noble gases) and their compounds, with reactivity increasing with temperature.[17]Hydrochloric and nitric 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 of uranium oxide.[12] 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 nuclei, 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 control rods (see nuclear reactor physics for a description of this process of reactor control).
As little as 15lb (6.8kg) of uranium-235 can be used to make an atomic bomb.[18] 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.
β (tetragonal) stable from 668 to 775°C (1,234 to 1,427°F). Tetragonal, space group P42/mnm, P42nm, or P4n2, lattice parameters a= 565.6pm, b= c= 1075.9pm.[20]
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 parameter a=352.4pm.[20]
Applications
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.[21] 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).[18]
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.[11] Other uses of depleted uranium include counterweights for aircraft control surfaces, as ballast for missile re-entry vehicles and as a shielding material.[12] Due to its high density, this material is found in inertial guidance systems and in gyroscopiccompasses.[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.[22] 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.[23]
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 20terajoules of energy (2×1013joules), assuming complete fission; as much energy as 1.5million kilograms (1,500 tonnes) of coal.[10]
Commercial nuclear power plants use fuel that is typically enriched to around 3% uranium-235.[10] 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-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 as uranium glass and in Fiestaware.[24]
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.[25][26] 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
The discovery of the radioactivity of uranium ushered in additional scientific and practical uses of the element. The long half-life of uranium-238 (4.47×109 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.[12]
History
Pre-discovery use
The use of pitchblende, uranium in its natural oxide form, dates back to at least the year 79 AD, when it was used in the Roman Empire to add a yellow color to ceramic glazes.[12] 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.[29] Starting in the late Middle Ages, pitchblende was extracted from the Habsburg silver mines in Joachimsthal, Bohemia (now Jáchymov in the Czech Republic) in the Ore Mountains, and was used as a coloring agent in the local glassmaking industry.[30] In the early 19th century, the world's only known sources of uranium ore were these mines.
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.[30] 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).[30][31] 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.[32]
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 unexposed photographic plate in a drawer and noting that the plate had become "fogged".[34] 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,[35][36] until the Manhattan Project and the Cold 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 by Enrico Fermi in 1934 found that bombarding uranium with neutrons produces beta rays (electrons or positrons from the elements produced; see beta particle).[37] The fission products were at first mistaken for new elements with atomic numbers 93 and 94, which the Dean of the Sapienza University of Rome, Orso Mario Corbino, named ausenium and hesperium, respectively.[38][39][40][41] 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[37] in Hahn's laboratory in Berlin. Lise Meitner and her nephew, physicist Otto Robert Frisch, published the physical explanation in February 1939 and named the process "nuclear fission".[42] 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.[37] 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.[43] 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.[44]
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.[45] 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.[37][46]
Two types of atomic bomb 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. Little Boy became the first nuclear weapon used in war when it was detonated over Hiroshima, Japan, on 6 August 1945. Exploding with a yield equivalent to 12,500 tonnes of TNT, the blast and thermal wave of the bomb destroyed nearly 50,000 buildings and killed about 75,000 people (see Atomic bombings of Hiroshima and Nagasaki).[34] 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.[47]
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.[48] 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).[49][50] 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,[51] 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.[37][52]
In 1972, 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, Africa, collectively known as the Oklo Fossil Reactors. The ore deposit is 1.7billion years old; then, uranium-235 constituted about 3% of uranium on Earth.[53] 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-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.[53]
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 Franceinto the 1970s and 1980s[22] spread a significant amount of fallout from uranium daughter isotopes around the world.[54] Additional fallout and pollution occurred from several nuclear accidents.[55]
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.[56] 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.[57]
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 600short tons (540metric 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.[18] 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.[18] 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.[18]
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,[58] 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.[59]
Occurrence
Uranium is a naturally 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 the 48th most abundant element in the Earth’s crust.[60] The decay of uranium, thorium, and potassium-40 in Earth's mantle is thought to be the main source of heat[61][62] that keeps the Earth's outer core in the liquid state and drives mantle convection, which in turn drives plate tectonics.
Uranium's concentration in the Earth's crust is (depending on the reference) 2 to 4 parts per million,[11][22] or about 40 times as abundant as silver.[17] The Earth's crust from the surface to 25km (15mi) down is calculated to contain 1017kg (2×1017lb) of uranium while the oceans may contain 1013kg (2×1013lb).[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 phosphate fertilizers),[63] and its concentration in sea water is 3 parts per billion.[22]
Uranium is more plentiful than antimony, tin, cadmium, mercury, or silver, and it is about as abundant as arsenic or molybdenum.[12][22] Uranium is found in hundreds of minerals, including uraninite (the most common uranium ore), carnotite, autunite, uranophane, torbernite, and coffinite.[12] 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[12] (it is recovered commercially from sources with as little as 0.1% uranium[17]).
Origin
Like all elements with 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.[64] 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.[65][66] 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. Further uranium-236 was produced by the decay of 244Pu, accounting for the observed higher-than-expected abundance of thorium and lower-than-expected abundance of uranium.[67] While the natural abundance of uranium has been supplemented by the decay of extinct 242Pu (half-life 375,000years) and 247Cm (half-life 16million 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.[68]
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 U in red and from U in green
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).[69][70] Recent research suggests that this pathway includes reduction of the soluble U(VI) via an intermediate U(V) pentavalent state.[71][72] Other organisms, such as the lichenTrapelia involuta or microorganisms such as the bacteriumCitrobacter, can absorb concentrations of uranium that are up to 300 times the level of their environment.[73]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.[30][74] The proteobacterium Geobacter has also been shown to bioremediate uranium in ground water.[75] The mycorrhizal fungus Glomus intraradices increases uranium content in the roots of its symbiotic plant.[76]
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.[77]
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.[30] Dry weight concentrations of uranium in food plants are typically lower with one to two micrograms per day ingested through the food people eat.[30]
Worldwide production of uranium in 2021 was 48,332 tonnes, of which 21,819 t (45%) was mined in Kazakhstan. Other important uranium mining countries are Namibia (5,753t), Canada (4,693t), Australia (4,192t), Uzbekistan (3,500t), and Russia (2,635t).[78]
Uranium ore is mined in several ways: open pit, underground, in-situ leaching, and borehole 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.[79] High-grade ores found in Athabasca Basin deposits in Saskatchewan, Canada can contain up to 23% uranium oxides on average.[80] 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.[81]
It is estimated that 6.1million tonnes of uranium exists in ores that are economically viable at US$130 per kg of uranium,[83] while 35million tonnes are classed as mineral resources (reasonable prospects for eventual economic extraction).[84]
Some uranium also originates from dismantled nuclear weapons.[87] For example, in 1993–2013 Russia supplied the United States with 15,000 tonnes of low-enriched uranium within the Megatons to Megawatts Program.[88]
An additional 4.6billion tonnes of uranium are estimated to be dissolved in sea water (Japanese scientists in the 1980s showed that extraction of uranium from sea water using ion exchangers was technically feasible).[89][90] There have been experiments to extract uranium from sea water,[91] but the yield has been low due to the carbonate present in the water. In 2012, ORNL researchers announced the successful development of a new absorbent material dubbed HiCap which performs surface retention of solid or gas molecules, atoms or ions and also effectively removes toxic metals from water, according to results verified by researchers at Pacific Northwest National Laboratory.[92][93]
Monthly uranium spot price in US$ per pound. The 2007 price peak is clearly visible.
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%).[95] In 2008, Kazakhstan was forecast to increase production and become the world's largest supplier of uranium by 2009;[96][97] 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.[78] Most uranium was produced not by conventional undergroundmining of ores (29% of production), but by in situ leaching (66%).[78][98]
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.[99]
The ultimate available supply is believed to be sufficient for at least the next 85 years,[84] though some studies indicate underinvestment in the late twentieth century may produce supply problems in the 21st century.[100] 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.[101] 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 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.[102]
The most common forms of uranium oxide are triuranium octoxide (U 3O 8) and UO 2.[104] 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.[104] 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.[104]
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.[105] 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 organicchelating agents, the most commonly encountered of which is uranyl acetate.[105]
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.
Uranium in a non-complexing aqueous medium (e.g. perchloric acid/sodium hydroxide).[106]
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).[106]
Relative concentrations of the different chemical forms of uranium in an aqueous carbonate solution.[106]
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 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.[107] 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.[107]
Uranium carbides and uranium nitrides are both relatively inertsemimetallic compounds that are minimally soluble in acids, react with water, and can ignite in air to form U 3O 8.[107] 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.[108] 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).[108]
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.[107] 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.[107]
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:[107]
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.[107]
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.[107] 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.[107] 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.[107]
Uranium, like all elements with an atomic number greater than 82, has no stable isotopes. All isotopes of uranium are radioactive because the strong nuclear force does not prevail over electromagnetic repulsion in nuclides containing more than 82 protons.[109] Nevertheless, the two most stable isotopes, 238U and 235U, have half-lives long enough to occur in nature as primordial radionuclides, with measurable quantities having survived since the formation of the Earth.[110] These two nuclides, along with thorium-232, are the only confirmed primordial nuclides heavier than nearly-stable bismuth-209.[7][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 of plutonium-244;[111] uranium-239, which is formed when 238U undergoes spontaneous fission, releasing neutrons that are captured by another 238U atom; uranium-237, which is formed when 238U 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 235U and as a decay product of plutonium-244;[111] 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.[114]
Uranium-238 is the most stable isotope of uranium, with a half-life of about 4.463×109 years,[7] 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.[17] 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 238U nucleus can absorb one neutron to produce the radioactive isotope uranium-239. 239U decays by beta emission to neptunium-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 first atomic bomb detonated in the "Trinity test" on 16 July 1945 in New Mexico.[37]
Uranium-235 has a half-life of about 7.04×108 years; it is the next most stable uranium isotope after 238U and is also predominantly an alpha emitter, decaying to thorium-231.[7] Uranium-235 is important for both nuclear reactors and nuclear 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 of 235U, which is called the actinium 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 to daughter elements useful in radiometric dating.
Uranium-236 has a half-life of 2.342×107 years[7] 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.[67] Uranium-236 occurs in spent nuclear fuel when neutron capture on 235U 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 239Pu, and is not itself fissile; as such, it is considered long-lived radioactive waste.[115]
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[7] 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 235U, is fissile.[12] 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 233U and the greater natural abundance of thorium (three times that of uranium),[116]233U has been investigated for use as nuclear fuel as a possible alternative to 235U and 239Pu,[117] though is not in widespread use as of 2022[update].[116] The decay chain of uranium-233 forms part of the neptunium series and ends at nearly-stable bismuth-209 (half-life 2.01×1019years)[7] and stable thallium-205.
Uranium-232 is an alpha emitter with a half-life of 68.9 years.[7] This isotope is produced as a byproduct in production of 233U and is considered a nuisance, as it is not fissile and decays through short-lived alpha and gamma emitters such as 208Tl.[117] 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.[7]
All isotopes from 232U to 236U inclusive have minor cluster decay branches (less than 10−10%), and all these bar 233U, in addition to 238U, have minor spontaneous fission branches;[7] the greatest branching ratio for spontaneous fission is about 5×10−5% for 238U, or about one in every two million decays.[118] The shorter-lived trace isotopes 237U and 239U exclusively undergo beta decay, with respective half-lives of 6.752 days and 23.45 minutes.[7]
In total, 28 isotopes of uranium have been identified, ranging in mass number from 214[119] to 242, with the exception of 220.[7][120] Among the uranium isotopes not found in natural samples or nuclear fuel, the longest-lived is 230U, an alpha emitter with a half-life of 20.23 days.[7] This isotope has been considered for use in targeted alpha-particle therapy (TAT).[121] All other isotopes have half-lives shorter than one hour, except for 231U (half-life 4.2 days) and 240U (half-life 14.1 hours).[7] The shortest-lived known isotope is 221U, with a half-life of 660 nanoseconds, and it is expected that the hitherto unknown 220U has an even shorter half-life.[122] The proton-rich isotopes lighter than 232U primarily undergo alpha decay, except for 229U and 231U, which decay to protactinium isotopes via positron emission and electron capture, respectively; the neutron-rich 240U, 241U, and 242U undergo beta decay to form neptunium isotopes.[7][120]
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 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'.
To be considered 'enriched', the uranium-235 fraction should be between 3% and 5%.[123] 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 235U concentration should be no more than 0.3%.[124] 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.[124]
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.[34] 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).[34] 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.[10] An alternative laser method of enrichment is known as atomic vapor laser isotope separation (AVLIS) and employs visible tunable lasers such as dye lasers.[125] Another method used is liquid thermal diffusion.[11]
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.
Human exposure
A person can be exposed to uranium (or its radioactive daughters, such as radon) by inhaling dust in air or by ingesting contaminated water and food. The amount of uranium in air is usually very small; however, people who work in factories that process phosphatefertilizers, 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.[126][127] 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. The Occupational Safety and Health Administration (OSHA) has set the permissible exposure limit for uranium exposure in the workplace as 0.25mg/m3 over an 8-hour workday. The National Institute for Occupational Safety and Health (NIOSH) has set a recommended exposure limit (REL) of 0.2mg/m3 over an 8-hour workday and a short-term limit of 0.6mg/m3. At 10mg/m3, uranium is immediately dangerous to life and health.[128]
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%.[30] However, soluble uranium compounds tend to quickly pass through the body, whereas insoluble uranium compounds, especially when inhaled by way of dust into the lungs, pose a more serious exposure hazard. After entering the bloodstream, the absorbed uranium tends to bioaccumulate and stay for many years in bone tissue because of uranium's affinity for phosphates.[30] Incorporated uranium becomes uranyl ions, which accumulate in bone, liver, kidney, and reproductive tissues.[129]
Radiological and chemical toxicity of uranium combine by the fact that elements of high atomic number Z 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.[130]
Uranium is not absorbed through the skin, and alpha particles released by uranium cannot penetrate the skin.[27]
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.[30][134][135] Uranium is also a reproductive toxicant.[136][137] 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.[138] While the CDC has published one study that no human cancer has been seen as a result of exposure to natural or depleted uranium,[139] exposure to uranium and its decay products, especially radon, is a significant health threat.[140] 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.[141]
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.[142] Finely divided uranium metal presents a fire hazard because uranium is pyrophoric; small grains will ignite spontaneously in air at room temperature.[12]
Uranium metal is commonly handled with gloves as a sufficient precaution.[143] Uranium concentrate is handled and contained so as to ensure that people do not inhale or ingest it.[143]
↑ The thermal expansion is anisotropic: the coefficients for each crystal axis (at 20°C) are αa=25.27×10−6/K, αb=0.76×10−6/K, αc=20.35×10−6/K, and αaverage =αvolume/3 =15.46×10−6/K.
Related Research Articles
The actinide or actinoid series encompasses at least the 14 metallic chemical elements in the 5f series, with atomic numbers from 89 to 102, actinium through nobelium. The actinide series derives its name from the first element in the series, actinium. The informal chemical symbol An is used in general discussions of actinide chemistry to refer to any actinide.
Nuclear fission is a reaction in which the nucleus of an atom splits into two or more smaller nuclei. The fission process often produces gamma photons, and releases a very large amount of energy even by the energetic standards of radioactive decay.
In nuclear physics, a nuclear chain reaction occurs when one single nuclear reaction causes an average of one or more subsequent nuclear reactions, thus leading to the possibility of a self-propagating series or "positive feedback loop" of these reactions. The specific nuclear reaction may be the fission of heavy isotopes. A nuclear chain reaction releases several million times more energy per reaction than any chemical reaction.
Enriched uranium is a type of uranium in which the percent composition of uranium-235 has been increased through the process of isotope separation. Naturally occurring uranium is composed of three major isotopes: uranium-238, uranium-235, and uranium-234. 235U is the only nuclide existing in nature that is fissile with thermal neutrons.
In nuclear engineering, fissile material is material that can undergo nuclear fission when struck by a neutron of low energy. A self-sustaining thermal chain reaction can only be achieved with fissile material. The predominant neutron energy in a system may be typified by either slow neutrons or fast neutrons. Fissile material can be used to fuel thermal-neutron reactors, fast-neutron reactors and nuclear explosives.
The nuclear fuel cycle, also called nuclear fuel chain, is the progression of nuclear fuel through a series of differing stages. It consists of steps in the front end, which are the preparation of the fuel, steps in the service period in which the fuel is used during reactor operation, and steps in the back end, which are necessary to safely manage, contain, and either reprocess or dispose of spent nuclear fuel. If spent fuel is not reprocessed, the fuel cycle is referred to as an open fuel cycle ; if the spent fuel is reprocessed, it is referred to as a closed fuel cycle.
Mixed oxide fuel, commonly referred to as MOX fuel, is nuclear fuel that contains more than one oxide of fissile material, usually consisting of plutonium blended with natural uranium, reprocessed uranium, or depleted uranium. MOX fuel is an alternative to the low-enriched uranium fuel used in the light-water reactors that predominate nuclear power generation.
A breeder reactor is a nuclear reactor that generates more fissile material than it consumes. These reactors can be fueled with more-commonly available isotopes of uranium and thorium, such as uranium-238 and thorium-232, as opposed to the rare uranium-235 which is used in conventional reactors. These materials are called fertile materials since they can be bred into fuel by these breeder reactors.
A natural nuclear fission reactor is a uranium deposit where self-sustaining nuclear chain reactions occur. The idea of a nuclear reactor existing in situ within an ore body moderated by groundwater was briefly explored by Paul Kuroda in 1956. The existence of an extinct or fossil nuclear fission reactor, where self-sustaining nuclear reactions have occurred in the past, are established by analysis of isotope ratios of uranium and of the fission products. The first such fossil reactor was first discovered in 1972 in Oklo, Gabon by researchers from the French Alternative Energies and Atomic Energy Commission (CEA) when chemists performing quality control for the French nuclear industry noticed sharp depletions of fissionable 235 U in gaseous uranium made from Gabonese ore.
Uranium-238 is the most common isotope of uranium found in nature, with a relative abundance of 99%. Unlike uranium-235, it is non-fissile, which means it cannot sustain a chain reaction in a thermal-neutron reactor. However, it is fissionable by fast neutrons, and is fertile, meaning it can be transmuted to fissile plutonium-239. 238U cannot support a chain reaction because inelastic scattering reduces neutron energy below the range where fast fission of one or more next-generation nuclei is probable. Doppler broadening of 238U's neutron absorption resonances, increasing absorption as fuel temperature increases, is also an essential negative feedback mechanism for reactor control.
Uranium-234 is an isotope of uranium. In natural uranium and in uranium ore, 234U occurs as an indirect decay product of uranium-238, but it makes up only 0.0055% of the raw uranium because its half-life of just 245,500 years is only about 1/18,000 as long as that of 238U. Thus the ratio of 234 U to 238 U in a natural sample is equivalent to the ratio of their half-lives. The primary path of production of 234U via nuclear decay is as follows: uranium-238 nuclei emit an alpha particle to become thorium-234. Next, with a short half-life, 234Th nuclei emit a beta particle to become protactinium-234 (234Pa), or more likely a nuclear isomer denoted 234mPa. Finally, 234Pa or 234mPa nuclei emit another beta particle to become 234U nuclei.
Nuclear fuel is material used in nuclear power stations to produce heat to power turbines. Heat is created when nuclear fuel undergoes nuclear fission. Nuclear fuel has the highest energy density of all practical fuel sources. The processes involved in mining, refining, purifying, using, and disposing of nuclear fuel are collectively known as the nuclear fuel cycle.
Plutonium-239 is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 is also used for that purpose. Plutonium-239 is also one of the three main isotopes demonstrated usable as fuel in thermal spectrum nuclear reactors, along with uranium-235 and uranium-233. Plutonium-239 has a half-life of 24,110 years.
Uranium (92U) is a naturally occurring radioactive element (radioelement) with no stable isotopes. It has two primordial isotopes, uranium-238 and uranium-235, that have long half-lives and are found in appreciable quantity in Earth's crust. The decay product uranium-234 is also found. Other isotopes such as uranium-233 have been produced in breeder reactors. In addition to isotopes found in nature or nuclear reactors, many isotopes with far shorter half-lives have been produced, ranging from 214U to 242U. The standard atomic weight of natural uranium is 238.02891(3).
The thorium fuel cycle is a nuclear fuel cycle that uses an isotope of thorium, 232 Th , as the fertile material. In the reactor, 232 Th is transmuted into the fissile artificial uranium isotope 233 U which is the nuclear fuel. Unlike natural uranium, natural thorium contains only trace amounts of fissile material, which are insufficient to initiate a nuclear chain reaction. Additional fissile material or another neutron source is necessary to initiate the fuel cycle. In a thorium-fuelled reactor, 232 Th absorbs neutrons to produce 233 U . This parallels the process in uranium breeder reactors whereby fertile 238 U absorbs neutrons to form fissile 239 Pu . Depending on the design of the reactor and fuel cycle, the generated 233 U either fissions in situ or is chemically separated from the used nuclear fuel and formed into new nuclear fuel.
Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor. It is no longer useful in sustaining a nuclear reaction in an ordinary thermal reactor and, depending on its point along the nuclear fuel cycle, it will have different isotopic constituents than when it started.
Uranium-236 (236U) is an isotope of uranium that is neither fissile with thermal neutrons, nor very good fertile material, but is generally considered a nuisance and long-lived radioactive waste. It is found in spent nuclear fuel and in the reprocessed uranium made from spent nuclear fuel.
In nuclear power technology, burnup is a measure of how much energy is extracted from a primary nuclear fuel source. It is measured as the fraction of fuel atoms that underwent fission in %FIMA or %FIFA as well as, preferably, the actual energy released per mass of initial fuel in gigawatt-days/metric ton of heavy metal (GWd/tHM), or similar units.
Reprocessed uranium (RepU) is the uranium recovered from nuclear reprocessing, as done commercially in France, the UK and Japan and by nuclear weapons states' military plutonium production programs. This uranium makes up the bulk of the material separated during reprocessing.
A pressurized heavy-water reactor (PHWR) is a nuclear reactor that uses heavy water (deuterium oxide D2O) as its coolant and neutron moderator. PHWRs frequently use natural uranium as fuel, but sometimes also use very low enriched uranium. The heavy water coolant is kept under pressure to avoid boiling, allowing it to reach higher temperature (mostly) without forming steam bubbles, exactly as for a pressurized water reactor (PWR). While heavy water is very expensive to isolate from ordinary water (often referred to as light water in contrast to heavy water), its low absorption of neutrons greatly increases the neutron economy of the reactor, avoiding the need for enriched fuel. The high cost of the heavy water is offset by the lowered cost of using natural uranium and/or alternative fuel cycles. As of the beginning of 2001, 31 PHWRs were in operation, having a total capacity of 16.5 GW(e), representing roughly 7.76% by number and 4.7% by generating capacity of all current operating reactors.
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