Uranium is a chemical element with symbol U and atomic number 92.
It is a silvery-white metal in the actinide series of the periodic
table. A uranium atom has 92 protons and 92 electrons, of which 6 are
Uranium is weakly radioactive because all isotopes
of uranium are unstable, with half-lives varying between 159,200 years
and 4.5 billion years. The most common isotopes in natural uranium are
uranium-238 (which has 146 neutrons and accounts for over 99%) and
uranium-235 (which has 143 neutrons).
Uranium has the highest atomic
weight of the primordially occurring elements. Its density is about
70% higher than that of lead, and slightly lower than that of gold or
tungsten. It occurs naturally in low concentrations of a few parts per
million in soil, rock and water, and is commercially extracted from
uranium-bearing minerals such as uraninite.
In nature, uranium is found as uranium-238 (99.2739–99.2752%),
uranium-235 (0.7198–0.7202%), and a very small amount of uranium-234
Uranium decays slowly by emitting an alpha
particle. The half-life of uranium-238 is about 4.47 billion years and
that of uranium-235 is 704 million years, making them useful in
dating the age of the Earth.
Many contemporary uses of uranium exploit its unique nuclear
Uranium-235 is the only naturally occurring fissile
isotope, which makes it widely used in nuclear power plants and
nuclear weapons. However, because of the tiny amounts found in nature,
uranium needs to undergo enrichment so that enough uranium-235 is
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 also important 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.
Depleted uranium (238U) is used in kinetic energy penetrators and
Uranium is used as a colorant in uranium glass,
producing lemon yellow to green colors.
Uranium glass fluoresces green
in ultraviolet light. It was also used for tinting and shading in
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
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
Soviet Union produced tens of thousands of nuclear weapons that
used uranium metal and uranium-derived plutonium-239. The security of
those weapons and their fissile material following the breakup of the
Soviet Union in 1991 is an ongoing concern for public health and
safety. See Nuclear proliferation.
3.1 Pre-discovery use
3.3 Fission research
3.4 Nuclear weaponry
3.6 Prehistoric naturally occurring fission
3.7 Contamination and the
Cold War legacy
4.2 Biotic and abiotic
4.3 Production and mining
4.4 Resources and reserves
5.1 Oxidation states and oxides
5.1.2 Aqueous chemistry
5.1.4 Effects of pH
5.2 Hydrides, carbides and nitrides
6.1 Natural concentrations
7 Human exposure
7.1 Effects and precautions
8 See also
11 External links
A neutron-induced nuclear fission event involving uranium-235
When refined, uranium is a silvery white, weakly radioactive metal. It
has a Mohs hardness of 6, sufficient to scratch glass and
approximately equal to that of titanium, rhodium, manganese and
niobium. It is malleable, ductile, slightly paramagnetic, strongly
electropositive and a poor electrical conductor.
has a very high density of 19.1 g/cm3, denser than lead
(11.3 g/cm3), but slightly less dense than tungsten and gold
Uranium metal reacts with almost all non-metal elements (with the
exception of the noble gases) and their compounds, with reactivity
increasing with temperature. Hydrochloric and nitric acids
dissolve uranium, but non-oxidizing acids other than hydrochloric acid
attack the element very slowly. When finely divided, it can react
with cold water; in air, uranium metal becomes coated with a dark
layer of uranium oxide.
Uranium in ores is extracted chemically and
converted into uranium dioxide or other chemical forms usable in
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, its uranium-235 isotope will most of
the time divide 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 special 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
As little as 15 lb (7 kg) of uranium-235 can be used to make
an atomic bomb. The first nuclear bomb used in war, Little Boy,
relied on uranium fission, but the very first nuclear explosive (the
Gadget used at Trinity) and the bomb that destroyed Nagasaki (Fat Man)
were both plutonium bombs.
Uranium metal has three allotropic forms:
α (orthorhombic) stable up to 668 °C. Orthorhombic, space group
No. 63, Cmcm, lattice parameters a = 285.4 pm, b = 587 pm, c
= 495.5 pm.
β (tetragonal) stable from 668 °C to 775 °C. Tetragonal,
space group P42/mnm, P42nm, or P4n2, lattice parameters a =
565.6 pm, b = c = 1075.9 pm.
γ (body-centered cubic) from 775 °C to melting point—this is
the most malleable and ductile state. Body-centered cubic, lattice
parameter a = 352.4 pm.
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. 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).
Depleted uranium is also used as a shielding material in some
containers used to store and transport radioactive materials. While
the metal itself is radioactive, its high density makes it more
effective than lead in halting radiation from strong sources such as
radium. Other uses of depleted uranium include counterweights for
aircraft control surfaces, as ballast for missile re-entry vehicles
and as a shielding material. Due to its high density, this material
is found in inertial guidance systems and in gyroscopic compasses.
Depleted uranium is preferred over similarly dense metals due to its
ability to be easily machined and cast as well as its relatively low
cost. The main risk of exposure to depleted uranium is chemical
poisoning by uranium oxide rather than radioactivity (uranium being
only a weak alpha 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.
The most visible civilian use of uranium is as the thermal power
source used in nuclear power plants.
The main use of uranium in the civilian sector is to fuel nuclear
power plants. One kilogram of uranium-235 can theoretically produce
about 20 terajoules of energy (2×1013 joules), assuming
complete fission; as much energy as 1500 tonnes of coal.
Commercial nuclear power plants use fuel that is typically enriched to
around 3% uranium-235. The CANDU and
Magnox designs are the only
commercial reactors capable of using unenriched uranium fuel. Fuel
United States Navy reactors is typically highly enriched in
uranium-235 (the exact values are classified). In a breeder reactor,
uranium-238 can also be converted into plutonium through the following
92U + n → 239
92U + γ β−→ 239
93Np β−→ 239
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.
The discovery and isolation of radium in uranium ore (pitchblende) by
Marie Curie sparked the development of uranium mining to extract the
radium, which was used to make glow-in-the-dark paints for clock and
aircraft dials. This left a prodigious quantity of uranium as a
waste product, since it takes three tonnes of uranium to extract one
gram of radium. This waste product was diverted to the glazing
industry, making uranium glazes very inexpensive and abundant. Besides
the pottery glazes, uranium tile glazes accounted for the bulk of the
use, including common bathroom and kitchen tiles which can be produced
in green, yellow, mauve, black, blue, red and other colors.
Uranium glass used as lead-in seals in a vacuum capacitor
Uranium was also used in photographic chemicals (especially uranium
nitrate as a toner), in lamp filaments for stage lighting
bulbs, to improve the appearance of dentures, and in the
leather and wood industries for stains and dyes.
Uranium salts are
mordants of silk or wool.
Uranyl acetate and uranyl formate are used
as electron-dense "stains" in transmission electron microscopy, to
increase the contrast of biological specimens in ultrathin sections
and in negative staining of viruses, isolated cell organelles and
The discovery of the radioactivity of uranium ushered in additional
scientific and practical uses of the element. The long half-life of
the isotope uranium-238 (4.51×109 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
X-ray targets in the making of high-energy X-rays.
The planet Uranus, which uranium is named after
The use of uranium in its natural oxide form dates back to at least
the year 79 CE, when it was used to add a yellow color to ceramic
glazes. 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
University of Oxford
University of Oxford in 1912. Starting in the late Middle
Ages, pitchblende was extracted from the
Habsburg silver mines in
Jáchymov in the Czech Republic), and was
used as a coloring agent in the local glassmaking industry. In the
early 19th century, the world's only known sources of uranium ore were
Henri Becquerel discovered the phenomenon of radioactivity by
exposing a photographic plate to uranium in 1896.
The discovery of the element is credited to the German chemist Martin
Heinrich Klaproth. While he was working in his experimental laboratory
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. 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). He named the newly discovered element after the
Uranus (named after the primordial Greek god of the sky), which
had been discovered eight years earlier by William Herschel.
In 1841, Eugène-Melchior Péligot, Professor of Analytical Chemistry
Conservatoire National des Arts et Métiers
Conservatoire National des Arts et Métiers (Central School of
Arts and Manufactures) in Paris, isolated the first sample of uranium
metal by heating uranium tetrachloride with potassium.
Henri Becquerel discovered radioactivity by using uranium in 1896.
Becquerel made the discovery in
Paris by leaving a sample of a uranium
salt, K2UO2(SO4)2 (potassium uranyl sulfate), on top of an unexposed
photographic plate in a drawer and noting that the plate had become
"fogged". He determined that a form of invisible light or rays
emitted by uranium had exposed the plate.
Cubes and cuboids of uranium produced during the Manhattan project
A team led by
Enrico Fermi in 1934 observed that bombarding uranium
with neutrons produces the emission of beta rays (electrons or
positrons from the elements produced; see beta particle). The
fission products were at first mistaken for new elements with atomic
numbers 93 and 94, which the Dean of the Faculty of Rome, Orso Mario
Corbino, christened ausonium and hesperium,
respectively. 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 in Hahn's laboratory in Berlin.
Lise Meitner and her
nephew, the physicist Otto Robert Frisch, published the physical
explanation in February 1939 and named the process "nuclear
fission". Soon after, Fermi hypothesized that the fission of
uranium might release enough neutrons to sustain a fission reaction.
Confirmation of this hypothesis came in 1939, and later work found
that on average about 2.5 neutrons are released by each fission of the
rare uranium isotope uranium-235. Fermi urged Alfred O.C. Nier to
separate uranium isotopes for determination of the fissile component,
and on February 29, 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. After mailed to Columbia University's
cyclotron, John Dunning confirmed the sample to be the isolated
fissile material on March 1. 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.
On 2 December 1942, as part of the Manhattan Project, another team led
Enrico Fermi was able to initiate the first artificial
self-sustained nuclear chain reaction, Chicago Pile-1. An initial plan
using enriched uranium-235 was abandoned as it was as yet unavailable
in sufficient quantities. Working in a lab below the stands of
Stagg Field at the University of Chicago, the team created the
conditions needed for such a reaction by piling together
400 short tons (360 metric tons) of graphite, 58 short
tons (53 metric tons) of uranium oxide, and six short tons (5.5
metric tons) of uranium metal, a majority of which was supplied by
Westinghouse Lamp Plant
Westinghouse Lamp Plant in a makeshift production process.
The mushroom cloud over
Hiroshima after the dropping of the
uranium-based atomic bomb nicknamed 'Little Boy'
Two major types of atomic bombs were developed by the United States
during World War II: a uranium-based device (codenamed "Little Boy")
whose fissile material was highly enriched uranium, and a
plutonium-based device (see
Trinity test and "Fat Man") whose
plutonium was derived from uranium-238. The uranium-based Little Boy
device became the first nuclear weapon used in war when it was
detonated over the Japanese city of
Hiroshima on 6 August 1945.
Exploding with a yield equivalent to 12,500 tonnes of TNT, the
blast and thermal wave of the bomb destroyed nearly 50,000 buildings
and killed approximately 75,000 people (see Atomic bombings of
Hiroshima and Nagasaki). 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
Four light bulbs lit with electricity generated from the first
artificial electricity-producing nuclear reactor, EBR-I (1951)
Graphite Reactor at
Oak Ridge National Laboratory
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. 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). 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, 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.
Prehistoric naturally occurring fission
Main article: Natural nuclear fission reactor
In 1972, the French physicist
Francis Perrin discovered fifteen
ancient and no longer active natural nuclear fission reactors in three
separate ore deposits at the
Oklo mine in Gabon, West Africa,
collectively known as the
Oklo Fossil Reactors. The ore deposit is 1.7
billion years old; then, uranium-235 constituted about 3% of the total
uranium on Earth. This is high enough to permit a sustained
nuclear fission chain reaction to occur, provided other supporting
conditions exist. The capacity of the surrounding sediment to contain
the 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.
Contamination and the
Cold War legacy
U.S. and USSR/Russian nuclear weapons stockpiles, 1945–2005
Above-ground nuclear tests by the
Soviet Union and the United States
in the 1950s and early 1960s and by
France into the 1970s and
1980s spread a significant amount of fallout from uranium daughter
isotopes around the world. Additional fallout and pollution
occurred from several nuclear accidents.
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. The
Compensation Act, a 1990 law in the USA, required $100,000 in
"compassion payments" to uranium miners diagnosed with cancer or other
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. Since the break-up of the
Soviet Union in 1991, an
estimated 600 short tons (540 metric tons) of highly
enriched weapons grade uranium (enough to make 40,000 nuclear
warheads) have been stored in often inadequately guarded facilities in
the Russian Federation and several other former Soviet states.
Police in Asia, Europe, and
South America on at least 16 occasions
from 1993 to 2005 have intercepted shipments of smuggled bomb-grade
uranium or plutonium, most of which was from ex-Soviet sources.
From 1993 to 2005 the Material Protection, Control, and Accounting
Program, operated by the federal government of the United States,
spent approximately US $550 million to help safeguard uranium and
plutonium stockpiles in Russia. This money was used for
improvements and security enhancements at research and storage
facilities. Scientific American reported in February 2006 that in some
of the facilities security consisted of chain link fences which were
in severe states of disrepair. According to an interview from the
article, one facility had been storing samples of enriched (weapons
grade) uranium in a broom closet before the improvement project;
another had been keeping track of its stock of nuclear warheads using
index cards kept in a shoe box.
Along with all elements having atomic weights higher than that of
iron, uranium is only naturally formed in supernovae. Primordial
thorium and uranium are only produced in the r-process (rapid neutron
capture), because the s-process (slow neutron capture) is too slow and
cannot pass the gap of instability after bismuth. 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 has long since decayed completely to 232Th, which was
itself enriched by the decay of 244Pu, accounting for the observed
higher-than-expected abundance of thorium and lower-than-expected
abundance of uranium. While the natural abundance of uranium has
been supplemented by the decay of extinct 242Pu (half-life
0.375 million years) and 247Cm (half-life
16 million years), producing 238U and 235U respectively,
this occurred to an almost negligible extent due to the shorter
half-lives of these parents and their lower production than 236U and
244Pu, the parents of thorium: the 247Cm:235U ratio at the formation
of the Solar System was (7.0 ± 1.6) × 10−5.
Biotic and abiotic
Uranium in the environment
Uraninite, also known as pitchblende, is the most common ore mined to
The evolution of Earth's radiogenic heat flow over time: contribution
from 235U in red and from 238U in green
Uranium is a naturally occurring element that can be found in low
levels within all rock, soil, and water.
Uranium is the 51st element
in order of abundance in the Earth's crust.
Uranium is also the
highest-numbered element to be found naturally in significant
Earth and is almost always found combined with other
elements. The decay of uranium, thorium, and potassium-40 in the
Earth's mantle is thought to be the main source of heat that
keeps the outer core liquid and drives mantle convection, which in
turn drives plate tectonics.
Uranium's average concentration in the Earth's crust is (depending on
the reference) 2 to 4 parts per million, or about 40 times as
abundant as silver. The Earth's crust from the surface to
25 km (15 mi) down is calculated to contain 1017 kg
(2×1017 lb) of uranium while the oceans may contain 1013 kg
(2×1013 lb). The concentration of uranium in soil ranges from
0.7 to 11 parts per million (up to 15 parts per million in farmland
soil due to use of phosphate fertilizers), and its concentration in
sea water is 3 parts per billion.
Uranium is more plentiful than antimony, tin, cadmium, mercury, or
silver, and it is about as abundant as arsenic or molybdenum.
Uranium is found in hundreds of minerals, including uraninite (the
most common uranium ore), carnotite, autunite, uranophane, torbernite,
and coffinite. Significant concentrations of uranium occur in some
substances such as phosphate rock deposits, and minerals such as
lignite, and monazite sands in uranium-rich ores (it is recovered
commercially from sources with as little as 0.1% uranium).
Citrobacter species can have concentrations of uranium in their bodies
300 times the level of the surrounding environment.
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).
Some organisms, such as the lichen Trapelia involuta or microorganisms
such as the bacterium Citrobacter, can absorb concentrations of
uranium that are up to 300 times the level of their environment.
Citrobacter species absorb uranyl ions when given glycerol phosphate
(or other similar organic phosphates). After one day, one gram of
bacteria can encrust themselves with nine grams of uranyl phosphate
crystals; this creates the possibility that these organisms could be
used in bioremediation to decontaminate uranium-polluted
water. The proteobacterium
Geobacter has also been shown to
bioremediate uranium in ground water. The mycorrhizal fungus
Glomus intraradices increases uranium content in the roots of its
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.
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. Dry
weight concentrations of uranium in food plants are typically lower
with one to two micrograms per day ingested through the food people
Production and mining
World uranium production (mines) and demand
Yellowcake is a concentrated mixture of uranium oxides that is further
refined to extract pure uranium.
Worldwide production of U3O8 (yellowcake) in 2013 amounted to 70,015
tonnes, of which 22,451 t (32%) was mined in Kazakhstan. Other
important uranium mining countries are
Canada (9,331 t), Australia
Niger (4,518 t),
Namibia (4,323 t) and
Uranium ore is mined in several ways: by open pit, underground,
in-situ leaching, and borehole mining (see uranium mining).
Low-grade uranium ore mined typically contains 0.01 to 0.25% uranium
oxides. Extensive measures must be employed to extract the metal from
its ore. High-grade ores found in
Athabasca Basin deposits in
Canada can contain up to 23% uranium oxides on
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.
then calcined to remove impurities from the milling process before
refining and conversion.
Commercial-grade uranium can be produced through the reduction of
uranium halides with alkali or alkaline earth metals.
can also be prepared through electrolysis of KUF
5 or UF
4, dissolved in molten calcium chloride (CaCl
2) and sodium chloride (NaCl) solution. Very pure uranium is
produced through the thermal decomposition of uranium halides on a hot
Resources and reserves
It is estimated that 5.5 million tonnes of uranium exists in ore
reserves that are economically viable at US$59 per lb of uranium,
while 35 million tonnes are classed as mineral resources (reasonable
prospects for eventual economic extraction). Prices went from
about $10/lb in May 2003 to $138/lb in July 2007. This has caused a
big increase in spending on exploration, with US$200 million being
spent worldwide in 2005, a 54% increase on the previous year. This
trend continued through 2006, when expenditure on exploration rocketed
to over $774 million, an increase of over 250% compared to 2004. The
Energy Agency said exploration figures for 2007 would
likely match those for 2006.
Australia has 31% of the world's known uranium ore reserves and
the world's largest single uranium deposit, located at the Olympic Dam
Mine in South Australia. There is a significant reserve of uranium
Bakouma a sub-prefecture in the prefecture of
Mbomou in Central
Some nuclear fuel comes from nuclear weapons being dismantled,
such as from the Megatons to Megawatts Program.
An additional 4.6 billion tonnes of uranium are estimated to be in sea
water (Japanese scientists in the 1980s showed that extraction of
uranium from sea water using ion exchangers was technically
feasible). There have been experiments to extract uranium from
sea water, but the yield has been low due to the carbonate present
in the water. In 2012,
ORNL researchers announced the successful
development of a new absorbent material dubbed HiCap which performs
surface retention of solid or gas molecules, atoms or ions and also
effectively removes toxic metals from water, according to results
verified by researchers at Pacific Northwest National
This section needs to be updated. Please update this article to
reflect recent events or newly available information. (September 2016)
Monthly uranium spot price in US$ per pound. The 2007 price peak is
In 2005, seventeen countries produced concentrated uranium oxides:
Canada (27.9% of world production),
Australia (22.8%), Kazakhstan
Niger (7.4%), Uzbekistan
United States (2.5%),
Ukraine (1.9%) and
Kazakhstan continues to increase production and may
have become the world's largest producer of uranium by 2009 with an
expected production of 12,826 tonnes, compared to
11,100 t and
Australia with 9,430 t. In the late
1960s, UN geologists also discovered major uranium deposits and other
rare mineral reserves in Somalia. The find was the largest of its
kind, with industry experts estimating the deposits at over 25% of the
world's then known uranium reserves of 800,000 tons.
The ultimate available supply is believed to be sufficient for at
least the next 85 years, although some studies indicate
underinvestment in the late twentieth century may produce supply
problems in the 21st century.
Uranium deposits seem to be
log-normal distributed. There is a 300-fold increase in the amount of
uranium recoverable for each tenfold decrease in ore grade. In
other words, there is little high grade ore and proportionately much
more low grade ore available.
Reactions of uranium metal
Oxidation states and oxides
Triuranium octoxide (left) and uranium dioxide (right) are the two
most common uranium oxides.
Calcined uranium yellowcake, as produced in many large mills, contains
a distribution of uranium oxidation species in various forms ranging
from most oxidized to least oxidized. Particles with short residence
times in a calciner will generally be less oxidized than those with
long retention times or particles recovered in the stack scrubber.
Uranium content is usually referenced to U
8, which dates to the days of the
Manhattan Project when U
8 was used as an analytical chemistry reporting standard.
Phase relationships in the uranium-oxygen system are complex. The most
important oxidation states of uranium are uranium(IV) and uranium(VI),
and their two corresponding oxides are, respectively, uranium dioxide
2) and uranium trioxide (UO
3). Other uranium oxides such as uranium monoxide (UO), diuranium
5), and uranium peroxide (UO
2O) also exist.
The most common forms of uranium oxide are triuranium octoxide (U
8) and UO
2. Both oxide forms are solids that have low solubility in water
and are relatively stable over a wide range of environmental
Triuranium octoxide is (depending on conditions) the most
stable compound of uranium and is the form most commonly found in
Uranium dioxide is the form in which uranium is most commonly
used as a nuclear reactor fuel. At ambient temperatures, UO
2 will gradually convert to U
8. Because of their stability, uranium oxides are generally considered
the preferred chemical form for storage or disposal.
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. A
few solid and semi-metallic compounds such as UO and US exist for the
formal oxidation state uranium(II), but no simple ions are known to
exist in solution for that state. Ions of U3+ liberate hydrogen from
water and are therefore considered to be highly unstable. The UO2+
2 ion represents the uranium(VI) state and is known to form compounds
such as uranyl carbonate, uranyl chloride and uranyl sulfate. UO2+
2 also forms complexes with various organic chelating agents, the most
commonly encountered of which is uranyl acetate.
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.
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
Uranium in carbonate solution
Relative concentrations of the different chemical forms of uranium in
a non-complexing aqueous medium (e.g. perchloric acid/sodium
Relative concentrations of the different chemical forms of uranium in
an aqueous carbonate solution.
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. 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.
Uranium carbides and uranium nitrides are both relatively inert
semimetallic compounds that are minimally soluble in acids, react with
water, and can ignite in air to form U
8. Carbides of uranium include uranium monocarbide (UC), uranium
2), and diuranium tricarbide (U
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
3 is prepared by subjecting a heated mixture of UC and UC
2 to mechanical stress.
Uranium nitrides obtained by direct
exposure of the metal to nitrogen include uranium mononitride (UN),
uranium dinitride (UN
2), and diuranium trinitride (U
Uranium hexafluoride is the feedstock used to separate uranium-235
from natural uranium.
All uranium fluorides are created using uranium tetrafluoride (UF
4 itself is prepared by hydrofluorination of uranium dioxide.
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
17, and UF
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:
2 + 4 HF → UF
4 + 2 H
2O (500 °C, endothermic)
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.
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. All uranium chlorides react with water and
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. Known examples include: UBr
3, and UI
Uranium oxyhalides are water-soluble and include UO
2, and UO
2. Stability of the oxyhalides decrease as the atomic weight of the
component halide increases.
Main article: Isotopes of uranium
Natural uranium consists of three major isotopes: uranium-238 (99.28%
natural abundance), uranium-235 (0.71%), and uranium-234 (0.0054%).
All three are radioactive, emitting alpha particles, with the
exception that all three of these isotopes have small probabilities of
undergoing spontaneous fission, rather than alpha emission. There are
also five other trace isotopes: 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; and
finally, uranium-233, which is formed in the decay chain of that
neptunium-237. 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.
Uranium-238 is the most stable isotope of uranium, with a half-life of
about 4.468×109 years, roughly the age of the Earth.
a half-life of about 7.13×108 years, and uranium-234 has a half-life
of about 2.48×105 years. For natural uranium, about 49% of its
alpha rays are emitted by each of 238U atom, and also 49% by 234U
(since the latter is formed from the former) and about 2.0% of them by
the 235U. When the
Earth was young, probably about one-fifth of its
uranium was uranium-235, but the percentage of 234U was probably much
lower than this.
Uranium-238 is usually an α emitter (occasionally, it undergoes
spontaneous fission), decaying through the "
Uranium Series" of nuclear
decay, which has 18 members, into lead-206, by a variety of different
The decay series of 235U, which is called the actinium series, has 15
members and eventually decays into lead-207. The constant rates of
decay in these decay series makes the comparison of the ratios of
parent to daughter elements useful in radiometric dating.
Uranium-234, which is a member of the uranium series (the decay chain
of uranium-238), decays to lead-206 through a series of relatively
Uranium-233 is made from thorium-232 by neutron bombardment, usually
in a nuclear reactor, and 233U is also fissile. Its decay series
ends at bismuth-209 and thallium-205.
Uranium-235 is important for both nuclear reactors and nuclear
weapons, because it is the only uranium isotope existing in nature on
Earth in any significant amount that is fissile. This means that it
can be split into two or three fragments (fission products) by thermal
Uranium-238 is not fissile, but is a fertile isotope, because after
neutron activation it can produce 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 15 July 1945 in
Main article: Enriched uranium
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
Isotope separation concentrates (enriches) the fissionable
uranium-235 for nuclear weapons and most nuclear power plants, except
for gas cooled reactors and pressurised heavy water reactors. Most
neutrons released by a fissioning atom of uranium-235 must impact
other uranium-235 atoms to sustain the nuclear chain reaction. The
concentration and amount of uranium-235 needed to achieve this is
called a 'critical mass'.
To be considered 'enriched', the uranium-235 fraction should be
between 3% and 5%. This process produces huge quantities of
uranium that is depleted of uranium-235 and with a correspondingly
increased fraction of uranium-238, called depleted uranium or 'DU'. To
be considered 'depleted', the uranium-235 isotope concentration should
be no more than 0.3%. 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
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. 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). The molecular laser isotope
separation method employs a laser beam of precise energy to sever the
bond between uranium-235 and fluorine. This leaves uranium-238 bonded
to fluorine and allows uranium-235 metal to precipitate from the
solution. An alternative laser method of enrichment is known as
atomic vapor laser isotope separation (AVLIS) and employs visible
tunable lasers such as dye lasers. Another method used is liquid
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 phosphate fertilizers, live
near government facilities that made or tested nuclear weapons, live
or work near a modern battlefield where depleted uranium weapons have
been used, or live or work near a coal-fired power plant, facilities
that mine or process uranium ore, or enrich uranium for reactor fuel,
may have increased exposure to uranium. Houses or structures
that are over uranium deposits (either natural or man-made slag
deposits) may have an increased incidence of exposure to radon gas.
Occupational Safety and Health Administration
Occupational Safety and Health Administration (OSHA) has set the
permissible exposure limit for uranium exposure in the workplace as
0.25 mg/m3 over an 8-hour workday. The National Institute for
Occupational Safety and Health (NIOSH) has set a recommended exposure
limit (REL) of 0.2 mg/m3 over an 8-hour workday and a short-term
limit of 0.6 mg/m3. At levels of 10 mg/m3, uranium is
immediately dangerous to life and health.
Most ingested uranium is excreted during digestion. Only 0.5% is
absorbed when insoluble forms of uranium, such as its oxide, are
ingested, whereas absorption of the more soluble uranyl ion can be up
to 5%. However, soluble uranium compounds tend to quickly pass
through the body, whereas insoluble uranium compounds, especially when
inhaled by way of dust into the 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.
Uranium is not absorbed through
the skin, and alpha particles released by uranium cannot penetrate the
Incorporated uranium becomes uranyl ions, which accumulate in bone,
liver, kidney, and reproductive tissues.
Uranium can be decontaminated
from steel surfaces and aquifers.
Effects and precautions
Normal functioning of the kidney, brain, liver, heart, and other
systems can be affected by uranium exposure, because, besides being
weakly radioactive, uranium is a toxic metal.
also a reproductive toxicant. 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.
2) ions, such as from uranium trioxide or uranyl nitrate and other
hexavalent uranium compounds, have been shown to cause birth defects
and immune system damage in laboratory animals. While the CDC has
published one study that no human cancer has been seen as a result of
exposure to natural or depleted uranium, exposure to uranium and
its decay products, especially radon, are widely known and significant
health threats. 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. Although accidental inhalation exposure to a
high concentration of uranium hexafluoride has resulted in human
fatalities, those deaths were associated with the generation of highly
toxic hydrofluoric acid and uranyl fluoride rather than with uranium
itself. Finely divided uranium metal presents a fire hazard
because uranium is pyrophoric; small grains will ignite spontaneously
in air at room temperature.
Uranium metal is commonly handled with gloves as a sufficient
Uranium concentrate is handled and contained so as to
ensure that people do not inhale or ingest it.
Compilation of 2004 review on uranium toxicity
Elevated levels of protein excretion, urinary catalase and diuresis
Damage to proximal convoluted tubules, necrotic cells cast from
tubular epithelium, glomerular changes (renal failure)
Decreased performance on neurocognitive tests
Acute cholinergic toxicity; Dose-dependent accumulation in cortex,
midbrain, and vermis; Electrophysiological changes in hippocampus
Increased reports of cancers
Increased mutagenicity (in mice) and induction of tumors
Binucleated cells with micronuclei, Inhibition of cell cycle kinetics
and proliferation; Sister chromatid induction, tumorigenic phenotype
Inhibition of periodontal bone formation; and alveolar wound healing
Uranium miners have more first-born female children
Moderate to severe focal tubular atrophy; vacuolization of Leydig
No adverse health effects reported
Severe nasal congestion and hemorrhage, lung lesions and fibrosis,
edema and swelling, lung cancer
Vomiting, diarrhea, albuminuria
No effects seen at exposure dose
Fatty livers, focal necrosis
No exposure assessment data available
Swollen vacuolated epidermal cells, damage to hair follicles and
Tissues surrounding embedded DU fragments
Elevated uranium urine concentrations
Elevated uranium urine concentrations, perturbations in biochemical
and neuropsychological testing
Chronic fatigue, rash, ear and eye infections, hair and weight loss,
cough. May be due to combined chemical exposure rather than DU alone
Conjunctivitis, irritation inflammation, edema, ulceration of
Decrease in RBC count and hemoglobin concentration
Myocarditis resulting from the uranium ingestion, which ended six
months after ingestion
List of countries by uranium production
List of countries by uranium reserves
List of uranium projects
Lists of nuclear disasters and radioactive incidents
Nuclear and radiation accidents
Nuclear and radiation accidents and incidents
Nuclear fuel cycle
Thorium fuel cycle
Uranium bubble of 2007
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