Uranium

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This article is about the chemical element. For other uses, see Uranium (disambiguation).

Uranium, 92U
spalling black oxide coat in air
Standard atomic weight Ar°(U)
Uranium in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson
Nd
↑
U
↓
—
protactinium ← uranium → neptunium
kJ/mol
Heat of vaporization417.1 kJ/mol
Molar heat capacity27.665 J/(mol·K)
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K) 2325 2564 2859 3234 3727 4402
Atomic properties
Oxidation statescommon: +6
−1,
Discovery
Martin Heinrich Klaproth (1789)
First isolationEugène-Melchior Péligot (1841)
Isotopes of uranium
Main isotopes[7] Decay
abun­dance half-life (t1/2) mode pro­duct
232U synth 68.9 y α
228Th
SF –
233U trace 1.592×105 y[8] α
229Th
SF –
234U 0.005% 2.455×105 y α
230Th
SF –
235U 0.720% 7.038×108 y α
231Th
SF –
236U trace 2.342×107 y α 232Th
SF –
238U 99.3% 4.468×109 y α
234Th
SF –
β−β−
 		238Pu
 Category: Uranium
| references

Uranium is a

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 extracted from uranium-bearing minerals such as uraninite.[9]

Many contemporary uses of uranium exploit its unique

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]

The 1789

discovery of uranium in the mineral pitchblende 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
.

Characteristics

A diagram showing a chain transformation of uranium-235 to uranium-236 to barium-141 and krypton-92
A neutron-induced nuclear fission event involving uranium-235

Uranium is a silvery white, weakly radioactive

electrical conductor.[11][12] Uranium metal has a very high density of 19.1 g/cm3,[13] denser than lead (11.3 g/cm3),[14] but slightly less dense than tungsten and gold (19.3 g/cm3).[15][16]

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.[citation needed] 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 15 lb (6.8 kg) 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.

Uranium metal has three allotropic forms:[19]

  • α (
    lattice parameters a = 285.4 pm, b = 587 pm, c = 495.5 pm.[20]
  • 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 = 565.6 pm, b = c = 1075.9 pm.[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 = 352.4 pm.[20]

Applications

Military

Shiny metallic cylinder with a sharpened tip. The overall length is 9 cm and diameter about 2 cm.
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

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

inertial guidance systems and in gyroscopic compasses.[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

fast neutrons from the nuclear fusion process.[23]

Civilian

The main use of uranium in the civilian sector is to fuel

terajoules of energy (2×1013 joules), assuming complete fission; as much energy as 1.5 million 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]

239
93Np
 β−→  239
94Pu
A glass place on a glass stand. The plate is glowing green while the stand is colorless.
Uranium glass glowing under UV light

Before (and, occasionally, after) the discovery of radioactivity, uranium was primarily used in small amounts for yellow glass and pottery glazes, such as uranium glass and in Fiestaware.[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.
A glass cylinder capped on both ends with metal electrodes. Inside the glass bulb there is a metal cylinder connected to the electrodes.
Uranium glass used as lead-in seals in a vacuum capacitor

Uranium was also used in

cell organelles and macromolecules
.

The discovery of the radioactivity of uranium ushered in additional scientific and practical uses of the element. The long half-life of 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

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.

Discovery

The planet Uranus, which uranium is named after

The

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]

In 1841,

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.[30][33]

radioactivity by exposing a photographic plate
to uranium in 1896.

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

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]

Nuclear weaponry

White fragmentred mushroom-like smoke cloud evolving from the ground.
Mushroom cloud over Hiroshima after the dropping of the uranium-fired 'Little Boy'

Two types of atomic bomb were developed by the United States during

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

An industrial room with four large illuminated light bulbs hanging down from a bar.
Four light bulbs lit with electricity generated from the first artificial electricity-producing nuclear reactor, EBR-I (1951)

The

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]

Prehistoric naturally occurring fission

Main article: Natural nuclear fission reactor

In 1972, French physicist

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

A graph showing evolution of number of nuclear weapons in the US and USSR and in the period 1945–2005. US dominates early and USSR later years with and crossover around 1978.
U.S. and USSR/Russian nuclear weapons stockpiles, 1945–2005

Above-ground

nuclear accidents.[55]

Uranium miners have a higher incidence of

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

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

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

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 25 km (15 mi) down is calculated to contain 1017 kg (2×1017 lb) of uranium while the oceans may contain 1013 kg (2×1013 lb).[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 375,000 years) and 247Cm (half-life 16 million years), producing 238U and 235U respectively, this occurred to an almost negligible extent due to the shorter half-lives of these parents and their lower production than 236U and 244Pu, the parents of thorium: the 247Cm/235U ratio at the formation of the Solar System was (7.0±1.6)×10−5.[68]

Biotic and abiotic

Main article: Uranium in the environment
A shiny gray 5-centimeter piece of matter with a rough surface.
Uraninite, also known as pitchblende, is the most common ore mined to extract uranium.
radiogenic heat
flow over time: contribution from 235U in red and from 238U in green

Some bacteria, such as

Burkholderia fungorum, can 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
bacterium Citrobacter, 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
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

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]

Production and mining

Main article: Uranium mining

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 (5,753 t), Canada (4,693 t), Australia (4,192 t), Uzbekistan (3,500 t), and Russia (2,635 t).[78]

Uranium ore is mined in several ways:

calcined to remove impurities from the milling process before refining and conversion.[81]

Commercial-grade uranium can be produced through the reduction of uranium halides with alkali or alkaline earth metals.[12] Uranium metal can also be prepared through electrolysis of KUF
5 or UF
4, dissolved in molten calcium chloride (CaCl
2) and sodium chloride (NaCl) solution.[12] Very pure uranium is produced through the thermal decomposition of uranium halides on a hot filament.[12]

  • World uranium production (mines) and demand[78]
    World uranium production (mines) and demand[78]
  • A yellow sand-like rhombic mass on black background.
    Yellowcake is a concentrated mixture of uranium oxides that is further refined to extract pure uranium.
  • Uranium production 2015, in tonnes[82]
    Uranium production 2015, in tonnes[82]

Resources and reserves

Uranium price 1990–2022.

It is estimated that 6.1 million tonnes of uranium exists in ores that are economically viable at US$130 per kg of uranium,[83] while 35 million tonnes are classed as mineral resources (reasonable prospects for eventual economic extraction).[84]

Australia has 28% of the world's known uranium ore reserves

sub-prefecture in the prefecture of Mbomou in the Central African Republic.[86]

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 4.6 billion tonnes of uranium are estimated to be dissolved in

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]

Supplies

Main article: Uranium market
See also: 2000s commodities boom
Monthly uranium spot price in US$ per pound. The 2007 price peak is clearly visible.[94]

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 underground mining 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.

Compounds

Main article: Uranium compounds
Reactions of uranium metal

Oxidation states and oxides

Oxides

See also: Uranium oxide
Ball and stick model of layered crystal structure containing two types of atoms.
Ball and stick model of cubic-like crystal structure containing two types of atoms.
Triuranium octoxide (left) and uranium dioxide (right) are the two most common uranium oxides.

Calcined uranium yellowcake, as produced in many large mills, contains a distribution of uranium oxidation species in various forms ranging from most oxidized to least oxidized. Particles with short residence times in a calciner will generally be less oxidized than those with long retention times or particles recovered in the stack scrubber. Uranium content is usually referenced to U
3O
8, which dates to the days of the Manhattan Project when U
3O
8 was used as an analytical chemistry reporting standard.[102]

uranium peroxide
(UO
4·2H
2O) also exist.

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

complexes with various organic chelating 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.

Pourbaix diagrams[106]
A graph of potential vs. pH showing stability regions of various uranium compounds
A graph of potential vs. pH showing stability regions of various uranium compounds
Uranium in a non-complexing aqueous medium
(e.g. perchloric acid/sodium hydroxide).[106] 		Uranium in carbonate solution
A graph of potential vs. pH showing stability regions of various uranium compounds
A graph of potential vs. pH showing stability regions of various uranium compounds
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]

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

Snow-like substance in a sealed glass ampoule.
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

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 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.

atomic weight of the component halide increases.[107]

Isotopes

Main article: Isotopes of uranium

Uranium, like all elements with an atomic number greater than 82, has no

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]

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,

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.

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.

neptunium series and ends at nearly-stable bismuth-209 (half-life 2.01×1019 years)[7] and stable thallium
-205.

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

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]

Enrichment

Main article: Enriched uranium
A photo of a large hall filled with arrays of long white standing cylinders.
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%).

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

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

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]

Uranium can be decontaminated from steel surfaces[131] and aquifers.[132][133]

Effects and precautions

Normal functioning of the

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]

See also

Notes

  1. ^ 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.

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Sources cited

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