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= Uranium =
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Introduction
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Uranium is a chemical element; it has symbol U and atomic number 92.
It is a silvery-grey metal in the actinide series of the periodic
table. A uranium atom has 92 protons and 92 electrons, of which 6 are
valence electrons. Uranium radioactively decays, usually by emitting
an alpha particle. The half-life of this decay varies between 159,200
and 4.5 billion years for different isotopes, making them useful for
dating the age of the Earth. The most common isotopes in natural
uranium are uranium-238 (which has 146 neutrons and accounts for over
99% of uranium on Earth) and uranium-235 (which has 143 neutrons).
Uranium has the highest atomic weight of the primordially occurring
elements. Its density is about 70% higher than that of lead and
slightly lower than that of gold or tungsten. It occurs naturally in
low concentrations of a few parts per million in soil, rock and water,
and is commercially extracted from uranium-bearing minerals such as
uraninite.
Many contemporary uses of uranium exploit its unique nuclear
properties. Uranium is used in nuclear power plants and nuclear
weapons because it is the only naturally occurring element with a
fissile isotope - uranium-235 - present in non-trace amounts. However,
because of the low abundance of uranium-235 in natural uranium (which
is overwhelmingly uranium-238), uranium needs to undergo enrichment so
that enough uranium-235 is present. Uranium-238 is fissionable by fast
neutrons and is fertile, 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 ((238)U) is used in kinetic energy
penetrators and armor plating.
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
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Uranium is a silvery white, weakly radioactive metal. It has a Mohs
hardness of 6, sufficient to scratch glass and roughly equal to that
of titanium, rhodium, manganese and niobium. It is malleable, ductile,
slightly paramagnetic, strongly electropositive and a poor electrical
conductor. Uranium metal has a very high density of 19.1 g/cm(3),
denser than lead (11.3 g/cm(3)), but slightly less dense than tungsten
and gold (19.3 g/cm(3)).
Uranium metal reacts with almost all non-metallic elements (except
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
dioxide. Uranium in ores is extracted chemically and converted into
uranium dioxide or other chemical forms usable in industry.
Uranium-235 was the first isotope that was found to be fissile. Other
naturally occurring isotopes are fissionable, but not fissile. On
bombardment with slow neutrons, uranium-235 most of the time splits
into two smaller 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 of uranium-235 can be used to make an atomic bomb. The
nuclear weapon detonated over Hiroshima, called Little Boy, relied on
uranium fission. However, the first nuclear bomb (the 'Gadget' used at
Trinity) and the bomb that was detonated over Nagasaki (Fat Man) were
both plutonium bombs.
Uranium metal has three 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 to. Tetragonal, space group
'P'42/'mnm', 'P'42'nm', or 'P'4'n'2, 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.
Military
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The major application of uranium in the military sector is in
high-density penetrating projectiles. 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 health 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.
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 20 terajoules of energy (2 joules), assuming complete fission;
as much energy as 1.5 million kilograms (1,500 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
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:
: + n + γ
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 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 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 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.
Pre-discovery use
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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. Yellow glass with 1% uranium
oxide was found in a Roman villa on Cape Posillipo in the Gulf of
Naples, Italy, by R. T. Gunther of the University of Oxford in 1912.
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. In the early 19th century, the
world's only known sources of uranium ore were these mines.
Discovery
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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. 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 planet
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 at
the Conservatoire National des Arts et Métiers (Central School of Arts
and Manufactures) in Paris, isolated the first sample of uranium metal
by heating uranium tetrachloride with potassium.
Henri Becquerel discovered radioactivity by using uranium in 1896.
Becquerel made the discovery in Paris by leaving a sample of a uranium
salt, KUO(SO) (potassium uranyl sulfate), on top of an unexposed
photographic plate in a drawer and noting that the plate had become
"fogged". He determined that a form of invisible light or rays emitted
by uranium had exposed the plate.
During World War I when the Central Powers suffered a shortage of
molybdenum to make artillery gun barrels and high speed tool steels,
they routinely used ferrouranium alloy as a substitute, as it presents
many of the same physical characteristics as molybdenum. When this
practice became known in 1916 the US government requested several
prominent universities to research the use of uranium in manufacturing
and metalwork. Tools made with these formulas remained in use for
several decades, until the Manhattan Project and the Cold War placed a
large demand on uranium for fission research and weapon development.
Fission research
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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). 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. 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, physicist Otto Robert Frisch, published the physical
explanation in February 1939 and named the process "nuclear fission".
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. 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. 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.
On 2 December 1942, as part of the Manhattan Project, another team led
by Enrico Fermi was able to initiate the first artificial
self-sustained nuclear chain reaction, Chicago Pile-1. An initial plan
using enriched uranium-235 was abandoned as it was as yet unavailable
in sufficient quantities. Working in a lab below the stands of Stagg
Field at the University of Chicago, the team created the conditions
needed for such a reaction by piling together 360 tonnes of graphite,
53 tonnes of uranium oxide, and 5.5 tonnes of uranium metal, most of
which was supplied by Westinghouse Lamp Plant in a makeshift
production process.
Nuclear weaponry
==================
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). 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.
Reactors
==========
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. 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
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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 1.7 billion
years old; then, uranium-235 constituted about 3% of uranium on Earth.
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.
Contamination and the Cold War legacy
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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 Radiation Exposure
Compensation Act, a 1990 law in the US, required $100,000 in
"compassion payments" to uranium miners diagnosed with cancer or other
respiratory ailments.
During the Cold War between the Soviet Union and the United States,
huge stockpiles of uranium were amassed and tens of thousands of
nuclear weapons were created using enriched uranium and plutonium made
from uranium. After the break-up of the Soviet Union in 1991, an
estimated 600 short tons (540 metric tons) of highly enriched weapons
grade uranium (enough to make 40,000 nuclear warheads) had been stored
in often inadequately guarded facilities in the 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 about US$550 million to help safeguard
uranium and plutonium stockpiles in Russia. This money was used for
improvements and security enhancements at research and storage
facilities.
Safety of nuclear facilities in Russia has been significantly improved
since the stabilization of political and economical turmoil of the
early 1990s. For example, in 1993 there were 29 incidents ranking
above level 1 on the International Nuclear Event Scale, and this
number dropped under four per year in 1995-2003. The number of
employees receiving annual radiation doses above 20 mSv, which is
equivalent to a single full-body CT scan, 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.
Occurrence
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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. Uranium is the 48th most abundant
element in the Earth's crust. The decay of uranium, thorium, and
potassium-40 in Earth's mantle is thought to be the main source of
heat 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, or about 40 times as abundant as
silver. The Earth's crust from the surface to 25 km (15 mi) down is
calculated to contain 10(17) kg (2 lb) of uranium while the oceans may
contain 10(13) kg (2 lb). The concentration of uranium in soil ranges
from 0.7 to 11 parts per million (up to 15 parts per million in
farmland soil due to use of phosphate fertilizers containing uranium
impurities), 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).
Origin
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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. 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. Besides the two extant primordial uranium
isotopes, (235)U and (238)U, the r-process also produced significant
quantities of (236)U, which has a shorter half-life and so is an
extinct radionuclide, having long since decayed completely to (232)Th.
Further uranium-236 was produced by the decay of (244)Pu, 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 (242)Pu
(half-life 375,000 years) and (247)Cm (half-life 16 million years),
producing (238)U and (235)U respectively, this occurred to an almost
negligible extent due to the shorter half-lives of these parents and
their lower production than (236)U and (244)Pu, the parents of
thorium: the (247)Cm/(235)U ratio at the formation of the Solar System
was .
Biotic and abiotic
====================
Some bacteria, such as 'Shewanella putrefaciens', 'Geobacter
metallireducens' and some strains of 'Burkholderia fungorum', can use
uranium for their growth and convert U(VI) to U(IV). Recent research
suggests that this pathway includes reduction of the soluble U(VI) via
an intermediate U(V) pentavalent state.
Other 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 symbiotic plant.
In nature, uranium(VI) forms highly soluble carbonate complexes at
alkaline pH. This leads to an increase in mobility and availability of
uranium to groundwater and soil from nuclear wastes which leads to
health hazards. However, it is difficult to precipitate uranium as
phosphate in the presence of excess carbonate at alkaline pH. A
'Sphingomonas' sp. strain BSAR-1 has been found to express a high
activity alkaline phosphatase (PhoK) that has been applied for
bioprecipitation of uranium as uranyl phosphate species from alkaline
solutions. The precipitation ability was enhanced by overexpressing
PhoK protein in 'E. coli'.
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
eat.
Production and 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).
Uranium ore is mined in several ways: open pit, underground, in-situ
leaching, and borehole mining. Low-grade uranium ore mined typically
contains 0.01 to 0.25% uranium oxides. Extensive measures must be
employed to extract the metal from its ore. High-grade ores found in
Athabasca Basin deposits in Saskatchewan, Canada can contain up to 23%
uranium oxides on average. Uranium ore is crushed and rendered into a
fine powder and then leached with either an acid or alkali. The
leachate is subjected to one of several sequences of precipitation,
solvent extraction, and ion exchange. The resulting mixture, called
yellowcake, contains at least 75% uranium oxides UO. Yellowcake is
then calcined to remove impurities from the milling process before
refining and conversion.
Commercial-grade uranium can be produced through the reduction of
uranium halides with alkali or alkaline earth metals. Uranium metal
can also be prepared through electrolysis of or
Uranium tetrafluoride, dissolved in molten calcium chloride () and
sodium chloride (NaCl) solution. Very pure uranium is produced through
the thermal decomposition of uranium halides on a hot filament.
U production-demand.png|World uranium production (mines) and demand
Yellowcake.jpg|alt=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, OWID.svg|Uranium production 2015, in tonnes
Resources and reserves
========================
It is estimated that 6.1 million tonnes of uranium exists in ores that
are economically viable at US$130 per kg of uranium, while 35 million
tonnes are classed as mineral resources (reasonable prospects for
eventual economic extraction).
Australia has 28% of the world's known uranium ore reserves and the
world's largest single uranium deposit is located at the Olympic Dam
Mine in South Australia. There is a significant reserve of uranium in
Bakouma, a sub-prefecture in the prefecture of Mbomou in the Central
African Republic.
Some uranium also originates from dismantled nuclear weapons. For
example, in 1993-2013 Russia supplied the United States with 15,000
tonnes of low-enriched uranium within the Megatons to Megawatts
Program.
An additional 4.6 billion tonnes of uranium are estimated to be
dissolved in sea water (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
Laboratory.
Supplies
==========
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%). In 2008, Kazakhstan was forecast to increase
production and become the world's largest supplier of uranium by 2009;
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. Most uranium was
produced not by conventional underground mining of ores (29% of
production), but by in situ leaching (66%).
In the late 1960s, UN geologists discovered major uranium deposits and
other rare mineral reserves in Somalia. The find was the largest of
its kind, with industry experts estimating the deposits at over 25% of
the world's then known uranium reserves of 800,000 tons.
The ultimate available supply is believed to be sufficient for at
least the next 85 years, though some studies indicate underinvestment
in the late twentieth century may produce supply problems in the 21st
century.
Uranium deposits seem to be log-normal distributed. There is a
300-fold increase in the amount of uranium recoverable for each
tenfold decrease in ore grade.
In other words, there is little high grade ore and proportionately
much more low grade ore available.
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 , which dates to the days of
the Manhattan Project when 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
() and uranium trioxide (). Other uranium oxides such as uranium
monoxide (UO), diuranium pentoxide (), and uranium peroxide () also
exist.
The most common forms of uranium oxide are triuranium octoxide () and
. Both oxide forms are solids that have low solubility in water and
are relatively stable over a wide range of environmental conditions.
Triuranium octoxide is (depending on conditions) the most stable
compound of uranium and is the form most commonly found in nature.
Uranium dioxide is the form in which uranium is most commonly used as
a nuclear reactor fuel. At ambient temperatures, will gradually
convert to . Because of their stability, uranium oxides are generally
considered the preferred chemical form for storage or disposal.
Aqueous chemistry
===================
Salts of many oxidation states of uranium are water-soluble and may be
studied in aqueous solutions. The most common ionic forms are
(brown-red), (green), (unstable), and uranyl (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 liberate hydrogen from water and are therefore
considered to be highly unstable. The ion represents the uranium(VI)
state and is known to form compounds such as uranyl carbonate, uranyl
chloride and uranyl sulfate. 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.
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
|width=50% |alt=A graph of potential vs. pH showing stability regions
of various uranium compounds |width=50% |alt=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). |Uranium in carbonate solution
|alt=A graph of potential vs. pH showing stability regions of various
uranium compounds |alt=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). |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 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 . Carbides of uranium include
uranium monocarbide (UC), uranium dicarbide (), and diuranium
tricarbide (). Both UC and are formed by adding carbon to molten
uranium or by exposing the metal to carbon monoxide at high
temperatures. Stable below 1800 °C, is prepared by subjecting a
heated mixture of UC and to mechanical stress. Uranium nitrides
obtained by direct exposure of the metal to nitrogen include uranium
mononitride (UN), uranium dinitride (), and diuranium trinitride ().
Halides
=========
All uranium fluorides are created using uranium tetrafluoride ();
itself is prepared by hydrofluorination of uranium dioxide. Reduction
of with hydrogen at 1000 °C produces uranium trifluoride (). Under
the right conditions of temperature and pressure, the reaction of
solid with gaseous uranium hexafluoride () can form the intermediate
fluorides of , , and Uranium pentafluoride.
At room temperatures, has a high vapor pressure, making it useful in
the gaseous diffusion process to separate the rare uranium-235 from
the common uranium-238 isotope. This compound can be prepared from
uranium dioxide and uranium hydride by the following process:
: + 4 HF → + 2 (500 °C, endothermic)
: + → (350 °C, endothermic)
The resulting , a white solid, is highly reactive (by fluorination),
easily sublimes (emitting a vapor that behaves as a nearly ideal gas),
and is the most volatile compound of uranium known to exist.
One method of preparing uranium tetrachloride () is to directly
combine chlorine with either uranium metal or uranium hydride. The
reduction of by hydrogen produces uranium trichloride () while the
higher chlorides of uranium are prepared by reaction with additional
chlorine. All uranium chlorides react with water and air.
Bromides and iodides of uranium are formed by direct reaction of,
respectively, bromine and iodine with uranium or by adding to those
element's acids. Known examples include: Uranium(III) bromide,
Uranium(IV) bromide, Uranium(III) iodide, and Uranium(IV) iodide. has
never been prepared. Uranium oxyhalides are water-soluble and include
Uranyl fluoride, , Uranyl chloride, and Uranyl bromide. Stability of
the oxyhalides decrease as the atomic weight of the component halide
increases.
Isotopes
======================================================================
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. Nevertheless,
the two most stable isotopes, (238)U and (235)U, have half-lives long
enough to occur in nature as primordial radionuclides, with measurable
quantities having survived since the formation of the Earth. These two
nuclides, along with thorium-232, are the only confirmed primordial
nuclides heavier than nearly-stable bismuth-209.
Natural uranium consists of three major isotopes: uranium-238 (99.28%
natural abundance), uranium-235 (0.71%), and uranium-234 (0.0054%).
There are also five other trace isotopes: uranium-240, a decay product
of plutonium-244; uranium-239, which is formed when (238)U undergoes
spontaneous fission, releasing neutrons that are captured by another
(238)U atom; uranium-237, which is formed when (238)U captures a
neutron but emits two more, which then decays to neptunium-237;
uranium-236, which occurs in trace quantities due to neutron capture
on (235)U and as a decay product of
plutonium-244;{{refn|name=pu244|The occurrence of plutonium-244 as a
primordial nuclide is disputed, though some reports of its detection
have also been attributed to infall from the interstellar medium.}}
and finally, uranium-233, which is formed in the decay chain of
neptunium-237. Additionally, uranium-232 would be produced by the
double beta decay of natural thorium-232, though this energetically
possible process has never been observed.
Uranium-238 is the most stable isotope of uranium, with a half-life of
about years, roughly the age of the Earth. Uranium-238 is
predominantly an alpha emitter, decaying to thorium-234. It ultimately
decays through the uranium series, which has 18 members, into
lead-206. Uranium-238 is not fissile, but is a fertile isotope,
because after neutron activation it can be converted to plutonium-239,
another fissile isotope. Indeed, the (238)U nucleus can absorb one
neutron to produce the radioactive isotope uranium-239. (239)U decays
by beta emission to neptunium-239, also a beta-emitter, that decays in
its turn, within a few days into plutonium-239. (239)Pu was used as
fissile material in the first atomic bomb detonated in the "Trinity
test" on 16 July 1945 in New Mexico.
Uranium-235 has a half-life of about years; it is the next most
stable uranium isotope after (238)U and is also predominantly an alpha
emitter, decaying to thorium-231. 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. The decay chain of (235)U,
which is called the actinium series, has 15 members and eventually
decays into lead-207. The constant rates of decay in these decay
series makes the comparison of the ratios of parent to daughter
elements useful in radiometric dating.
Uranium-236 has a half-life of years and is not found in significant
quantities in nature. The half-life of uranium-236 is too short for it
to be primordial, though it has been identified as an extinct
progenitor of its alpha decay daughter, thorium-232. Uranium-236
occurs in spent nuclear fuel when neutron capture on (235)U does not
induce fission, or as a decay product of plutonium-240. Uranium-236 is
not fertile, as three more neutron captures are required to produce
fissile (239)Pu, and is not itself fissile; as such, it is considered
long-lived radioactive waste.
Uranium-234 is a member of the uranium series and occurs in
equilibrium with its progenitor, (238)U; it undergoes alpha decay with
a half-life of 245,500 years and decays to lead-206 through a series
of relatively short-lived isotopes.
Uranium-233 undergoes alpha decay with a half-life of 160,000 years
and, like (235)U, is fissile. It can be bred from thorium-232 via
neutron bombardment, usually in a nuclear reactor; this process is
known as the thorium fuel cycle. Owing to the fissility of (233)U and
the greater natural abundance of thorium (three times that of
uranium), (233)U has been investigated for use as nuclear fuel as a
possible alternative to (235)U and (239)Pu, though is not in
widespread use . The decay chain of uranium-233 forms part of the
neptunium series and ends at nearly-stable bismuth-209 (half-life )
and stable thallium-205.
Uranium-232 is an alpha emitter with a half-life of 68.9 years. This
isotope is produced as a byproduct in production of (233)U and is
considered a nuisance, as it is not fissile and decays through
short-lived alpha and gamma emitters such as (208)Tl. It is also
expected that thorium-232 should be able to undergo double beta decay,
which would produce uranium-232, but this has not yet been observed
experimentally.
All isotopes from (232)U to (236)U inclusive have minor cluster decay
branches (less than %), and all these bar (233)U, in addition to
(238)U, have minor spontaneous fission branches; the greatest
branching ratio for spontaneous fission is about % for (238)U, or
about one in every two million decays. The shorter-lived trace
isotopes (237)U and (239)U exclusively undergo beta decay, with
respective half-lives of 6.752 days and 23.45 minutes.
In total, 28 isotopes of uranium have been identified, ranging in mass
number from 214 to 242, with the exception of 220. Among the uranium
isotopes not found in natural samples or nuclear fuel, the
longest-lived is (230)U, an alpha emitter with a half-life of 20.23
days. This isotope has been considered for use in targeted
alpha-particle therapy (TAT). All other isotopes have half-lives
shorter than one hour, except for (231)U (half-life 4.2 days) and
(240)U (half-life 14.1 hours). The shortest-lived known isotope is
(221)U, with a half-life of 660 nanoseconds, and it is expected that
the hitherto unknown (220)U has an even shorter half-life. The
proton-rich isotopes lighter than (232)U primarily undergo alpha
decay, except for (229)U and (231)U, which decay to protactinium
isotopes via positron emission and electron capture, respectively; the
neutron-rich (240)U, (241)U, and (242)U undergo beta decay to form
neptunium isotopes.
Enrichment
============
In nature, uranium is found as uranium-238 (99.2742%) and uranium-235
(0.7204%). Isotope separation concentrates (enriches) the fissile
uranium-235 for nuclear weapons and most nuclear power plants, except
for gas cooled reactors and pressurized heavy water reactors. Most
neutrons released by a fissioning atom of uranium-235 must impact
other uranium-235 atoms to sustain the nuclear chain reaction. The
concentration and amount of uranium-235 needed to achieve this is
called a 'critical mass'.
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 (235)U concentration should be no more than
0.3%. The price of uranium has risen since 2001, so enrichment
tailings containing more than 0.35% uranium-235 are being considered
for re-enrichment, driving the price of depleted uranium hexafluoride
above $130 per kilogram in July 2007 from $5 in 2001.
The gas centrifuge process, where gaseous uranium hexafluoride () is
separated by the difference in molecular weight between (235)UF and
(238)UF 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 thermal
diffusion.
The only significant deviation from the (235)U to (238)U ratio in any
known natural samples occurs in Oklo, Gabon, where natural nuclear
fission reactors consumed some of the (235)U some two billion years
ago when the ratio of (235)U to (238)U was more akin to that of low
enriched uranium allowing regular ("light") water to act as a neutron
moderator akin to the process in humanmade light water reactors. The
existence of such natural fission reactors which had been
theoretically predicted beforehand was proven as the slight deviation
of (235)U concentration from the expected values were discovered
during uranium enrichment in France. Subsequent investigations to rule
out any nefarious human action (such as stealing of (235)U) 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 phosphate fertilizers
containing uraium impurities, live near government facilities that
made or tested nuclear weapons, live or work near a modern battlefield
where depleted uranium weapons have been used, or live or work near a
coal-fired power plant, facilities that mine or process uranium ore,
or enrich uranium for reactor fuel, may have increased exposure to
uranium. Houses or structures that are over uranium deposits (either
natural or man-made slag deposits) may have an increased incidence of
exposure to radon gas.
The health impacts of natural and of deleted uranium are chemical
rather than due to radiation.
The Occupational Safety and Health Administration (OSHA) has set the
permissible exposure limit for uranium exposure in the workplace as
0.25 mg/m(3) over an 8-hour workday. The National Institute for
Occupational Safety and Health (NIOSH) has set a recommended exposure
limit (REL) of 0.2 mg/m(3) over an 8-hour workday and a short-term
limit of 0.6 mg/m(3). At 10 mg/m(3), 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. Incorporated uranium becomes uranyl
ions, which accumulate in bone, liver, kidney, and reproductive
tissues.
Elements of high atomic number like uranium exhibit phantom or
secondary radiotoxicity through absorption of natural background gamma
and X-rays and re-emission of photoelectrons, which in combination
with the high affinity of uranium to the phosphate moiety of DNA cause
increased single and double strand DNA breaks.
Uranium is not absorbed through the skin, and alpha particles released
by uranium cannot penetrate the skin.
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. Uranium is also a
reproductive toxicant. Radiological effects are generally local
because alpha radiation, the primary form of (238)U decay, has a very
short range, and will not penetrate skin. Alpha radiation from inhaled
uranium has been demonstrated to cause lung cancer in exposed nuclear
workers. The Centers for Disease Control have published one study
stating that neither natural nor depleted uranium have been classified
with respect to carcinogenicity. Exposure to its decay products,
especially radon, is a significant health threat, and uranium
processing produces wastes contaminated with radium which in turn
produces radon gas. Because of its long half-life, purified uranium
will not produce significant amounts of daughter nuclides for millions
of years. Exposure to strontium-90, iodine-131, and other fission
products is unrelated to uranium exposure, but may result from medical
procedures or exposure to spent reactor fuel or fallout from nuclear
weapons.
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
precaution. Uranium concentrate is handled and contained so as to
ensure that people do not inhale or ingest it.
See also
======================================================================
* K-65 residues
* 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 and incidents
* Nuclear engineering
* Nuclear fuel cycle
* Nuclear physics
* Quintuple bond (earlier thought to be a phi bond), in the molecule U
* Thorium fuel cycle
* World Uranium Hearing
External links
======================================================================
* [
http://www.eia.gov/nuclear/ Nuclear fuel data and analysis] from
the U.S. Energy Information Administration
* [
http://www.wise-uranium.org/umaps.html World Uranium deposit maps]
*
*
[
https://web.archive.org/web/20051214080409/http://alsos.wlu.edu/qsearch.aspx?browse=science%2FUranium
Annotated bibliography for uranium from the Alsos Digital Library]
* [
https://www.nlm.nih.gov/toxnet/index.html NLM Hazardous Substances
Databank - Uranium, Radioactive]
* [
https://www.cdc.gov/niosh/npg/npgd0650.html CDC - NIOSH Pocket
Guide to Chemical Hazards]
* [
https://www.atsdr.cdc.gov/csem/csem.html ATSDR Case Studies in
Environmental Medicine: Uranium Toxicity] U.S. Department of Health
and Human Services
* [
http://www.periodicvideos.com/videos/092.htm Uranium] at 'The
Periodic Table of Videos' (University of Nottingham)
License
=========
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http://creativecommons.org/licenses/by-sa/3.0/
Original Article:
http://en.wikipedia.org/wiki/Uranium
