Uranium is a silvery-white metallic chemical element in the
actinide series of the periodic table, with symbol U and atomic number 92. A
uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons.
Uranium is weakly radioactive because all its isotopes are unstable (with
half-lives of the 6 naturally known isotopes, uranium-233 to uranium-238,
varying between 69 years and 4½ billion years). The most common isotopes of
uranium are uranium-238 (which has 146 neutrons and accounts for almost 99.3%
of the uranium found in nature) and uranium-235 (which has 143 neutrons,
accounting for 0.7% of the element found naturally). Uranium has the second
highest atomic weight of the primordially occurring elements, lighter only than
plutonium. Its density is about 70% higher than that of lead, but not as dense
as 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 (0.0050–0.0059%).[4] 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
properties. Uranium-235 has the distinction of being the only naturally occurring
fissile isotope. 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. While 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 armor plating.
Uranium is used as a colorant in uranium glass, producing
orange-red to lemon yellow hues. It was also used for tinting and shading in
early photography. The 1789 discovery of uranium in the mineral pitchblende is
credited to Martin Heinrich Klaproth, who named the new element after the
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 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. 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.
Characteristics
A diagram showing a chain transformation of uranium-235 to
uranium-236 to barium-141 and krypton-92
A neutron-induced nuclear fission event involving
uranium-235
When refined, uranium is a silvery white, weakly radioactive
metal, which is harder than most elements. It is malleable, ductile, slightly
paramagnetic, strongly electropositive and is a poor electrical conductor.
Uranium metal has very high density, being approximately 70% denser than lead,
but slightly less dense than gold.
Uranium metal reacts with almost all non-metal elements(with
an exception of the group 18 elements) 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.[8] 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 industry.
Uranium-235 was the first isotope that was found to be
fissile. Other naturally occurring isotopes are fissionable, but not fissile.
Upon 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 control).
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, while the very first nuclear explosive (The gadget) and the
bomb that destroyed Nagasaki (Fat Man) were plutonium bombs.
Uranium metal has three allotropic forms:
α (orthorhombic)
stable up to 660 °C
β (tetragonal)
stable from 660 °C to 760 °C
γ (body-centered
cubic) from 760 °C to melting point—this is the most malleable and ductile
state.
Applications
Military
Shiny metallic cylinder with a sharpened tip. The overall
length is 9 cm and diameter about 2 cm.
Depleted uranium is used by various militaries 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. At high impact speed, the density, hardness,
and pyrophoricity of the projectile enable destruction of heavily armored
targets. Tank armor and other removable vehicle armor are also hardened with
depleted uranium plates. The use of depleted uranium became politically and
environmentally contentious after the use of depleted uranium munitions by the
US, UK and other countries during wars in the Persian Gulf and the Balkans
raised questions of 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
Photograph featuring sunflowers in front and a plant on the
back. The plant has a wide smoking chimney with diameter comparable to its
height.
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 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 through the following reaction: 238U (n, gamma) → 239U
-(beta) → 239Np -(beta) → 239Pu.
A glass place on a glass stand. The plate is glowing green
while the stand is colorless.
Uranium glass glowing under UV light
Before (and, occasionally, after)[citation needed] 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.
A glass cylinder capped on both ends with metal electrodes.
Inside the glass bulb there is a metal cylinder connected to the electrodes.
Uranium glass used as lead-in seals in a vacuum capacitor
Uranium was also used in photographic chemicals (especially
uranium nitrate as a toner), in lamp filaments,[citation needed] 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
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 used for X-ray targets in the making
of high-energy X-rays.
History
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.
Pre-discovery use
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 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), and was used as a coloring agent in the local glassmaking
industry.[19] In the early 19th century, the world's only known sources of
uranium ore were these mines.
Discovery
Two fuzzy black features on a fuzzy white paper-like
background. There is a handwriting at the top of the picture.
Antoine 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 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. Uranium was not seen as being
particularly dangerous during much of the 19th century, leading to the
development of various uses for the element. One such use for the oxide was the
aforementioned but no longer secret coloring of pottery and glass.
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.
Fission research
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 of 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.
Further work found that the far more common uranium-238 isotope can be
transmuted into plutonium, which, like uranium-235, is also fissionable 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 by Enrico Fermi was able to initiate the first artificial self-sustained
nuclear chain reaction, Chicago Pile-1. 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 in a makeshift production process.
Nuclear weaponry
White fragmentred mushroom-like smoke cloud evolving from
the ground.
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).[23] 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
An industrial room with four large illuminated light bulbs
hanging down from a bar.
Four light bulbs lit with electricity generated from the
first artificial electricity-producing nuclear reactor, EBR-I (1951)
The 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.
Contamination and the Cold War legacy
A graph showing evolution of number of nuclear weapons in
the US and USSR and in the period 1945–2005. US dominates early and USSR later
years with and crossover around 1978.
U.S. and USSR/Russian nuclear weapons stockpiles, 1945–2005
Above-ground nuclear 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 USA, 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. 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.
Occurrence
Biotic and abiotic
A shiny gray 5-centimeter piece of matter with a rough
surface.
Uraninite, also known as pitchblende, is the most common ore
mined to extract uranium.
The evolution of Earth's radiogenic heat flow over time:
contribution from 235U in pink and from 238U in light blue.
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 quantities on Earth and is almost
always found combined with other elements. Along with all elements having
atomic weights higher than that of iron, it is only naturally formed in
supernovae. 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.
Five cylinder-like bodies on a flat surface: four in a group
and one separate.
Citrobacter species can have concentrations of uranium in
their bodies 300 times higher than in the surrounding environment.
Some bacteria such as S. putrefaciens and G. metallireducens
have been shown to reduce 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 higher than in 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
World uranium production (mines) and demand
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.
The worldwide production of uranium in 2010 amounted to
53,663 tonnes, of which 17,803 t (33.2%) was mined in Kazakhstan. Other
important uranium mining countries are Canada (9,783 t), Australia (5,900 t),
Namibia (4,496 t), Niger (4,198 t) and Russia (3,562 t).
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 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 U3O8. 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 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
filament.
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 world wide in 2005, a 54% increase on the previous
year. This trend continued through 2006, when expenditure on exploration rocketed
to over $774 million, an increase of over 250% compared to 2004. The OECD
Nuclear Energy Agency said exploration figures for 2007 would likely match
those for 2006.
Australia has 31% of the world's known uranium ore reserves
and the world's largest single uranium deposit, located at the Olympic Dam Mine
in South Australia.There is a significant reserve of uranium in Bakouma a
sub-prefecture in the prefecture of Mbomou in Central African Republic.
Some nuclear fuel comes from nuclear weapons being
dismantled, such as from the Megatons to Megawatts Program.
An additional 4.6 billion tonnes of uranium are estimated to
be in sea water (Japanese scientists in the 1980s showed that extraction of
uranium from sea water using ion exchangers was technically feasible). There
have been experiments to extract uranium from sea water, but the yield has been
low due to the carbonate present in the water. In 2012, ORNL researchers
announced the successful development of a new absorbent material dubbed HiCap
which performs surface retention of solid or gas molecules, atoms or ions and
also effectively removes toxic metals from water, according to results verified
by researchers at Pacific Northwest National Laboratory.
Supplies
Monthly uranium spot price in US$ per pound. The 2007 price
peak is clearly visible.[66]
In 2005, seventeen countries produced concentrated uranium
oxides, with Canada (27.9% of world production) and Australia (22.8%) being the
largest producers and Kazakhstan (10.5%), Russia (8.0%), Namibia (7.5%), Niger
(7.4%), Uzbekistan (5.5%), the United States (2.5%), Argentina (2.1%), Ukraine
(1.9%) and China (1.7%) also producing significant amounts. 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 Canada with 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 uranium 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.
Compounds
Oxidation states and oxides
Oxides
Ball and stick model of layered crystal structure containing
two types of atoms.
Ball and stick model of cubic-like crystal structure
containing two types of atoms.
Triuranium octoxide (left) and uranium dioxide (right) are
the two most common uranium oxides.
Calcined uranium yellowcake as produced in many large mills
contains a distribution of uranium oxidation species in various forms ranging
from most oxidized to least oxidized. Particles with short residence times in a
calciner will generally be less oxidized than those with long retention times
or particles recovered in the stack scrubber. Uranium content is usually
referenced to U
3O
8, which dates to the days of the Manhattan project when U
3O
8 was used as an analytical chemistry reporting standard.
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 (UO
2) and uranium trioxide (UO
3).[73] Other uranium oxides such as uranium monoxide (UO),
diuranium pentoxide (U
2O
5), and uranium peroxide (UO
4·2H
2O) also exist.
The most common forms of uranium oxide are triuranium
octoxide (U
3O
8) and UO
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, UO
2 will gradually convert to U
3O
8. Because of their stability, uranium oxides are generally
considered the preferred chemical form for storage or disposal.
Aqueous chemistry
Uranium in its oxidation states III, IV, V, VI
Salts of many oxidation states of uranium are water-soluble
and may be studied in aqueous solutions. The most common ionic forms are U3+
(brown-red), U4+
(green), UO+
2 (unstable), and UO2+
2 (yellow), for U(III), U(IV), U(V), and U(VI),
respectively. 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.
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
A graph of potential vs. pH showing stability regions of
various uranium compounds
A graph of potential vs. pH showing stability regions of
various uranium compounds
Uranium in a non-complexing aqueous medium (e.g. perchloric
acid/sodium hydroxide). Uranium
in carbonate solution
A graph of potential vs. pH showing stability regions of
various uranium compounds
A graph of potential vs. pH showing stability regions of
various uranium compounds
Relative concentrations of the different chemical forms of
uranium in a non-complexing aqueous medium (e.g. perchloric acid/sodium
hydroxide). 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 which 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
3O
Carbides of uranium
include uranium monocarbide (UC), uranium dicarbide (UC
2), and diuranium tricarbide (U
2C
3). Both UC and UC
2 are formed by adding carbon to molten uranium or by
exposing the metal to carbon monoxide at high temperatures. Stable below 1800
°C, U
2C
3 is prepared by subjecting a heated mixture of UC and UC
2 to mechanical stress. Uranium nitrides obtained by direct
exposure of the metal to nitrogen include uranium mononitride (UN), uranium
dinitride (UN
2), and diuranium trinitride (U
2N
3).
Halides
Snow-like substance in a sealed glass ampoule.
Uranium hexafluoride is the feedstock used to separate
uranium-235 from natural uranium.
All uranium fluorides are created using uranium
tetrafluoride (UF
4); UF
4 itself is prepared by hydrofluorination of uranium
dioxide. Reduction of UF
4 with hydrogen at 1000 °C produces uranium trifluoride (UF
3). Under the right conditions of temperature and pressure,
the reaction of solid UF
4 with gaseous uranium hexafluoride (UF
6) can form the intermediate fluorides of U
2F
9, U
4F
17, and UF
5.[77]
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:
UO
2 + 4 HF → UF
4 + 2 H
2O (500 °C,
endothermic)
UF
4 + F
2 → UF
6 (350 °C,
endothermic)
The resulting UF
6, a white solid, is highly reactive (by fluorination),
easily sublimes (emitting a vapor that behaves as a nearly ideal gas), and is
the most volatile compound of uranium known to exist.
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
air.
Bromides and iodides of uranium are formed by direct
reaction of, respectively, bromine and iodine with uranium or by adding UH
3 to those element's acids. Known examples include: UBr
3, UBr
4, UI
3, and UI
4. Uranium oxyhalides are water-soluble and include UO
2F
2, UOCl
2, UO
2Cl
2, and UO
2Br
2. Stability of the oxyhalides decrease as the atomic weight
of the component halide increases.
Isotopes
Natural concentrations
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.
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. Uranium-235
has 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, all of which eventually decay into
lead-206, by a variety of different decay paths.
The decay series of 235U, which is called the actinium
series has 15 members, all of which eventually decay 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 is a member of the "Uranium Series",
and it decays to lead-206 through a series of relatively short-lived isotopes.
Uranium-233 is made from thorium-232 by neutron bombardment,
usually in a nuclear reactor, and 233U is also fissile. Its decay series ends
with 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 it can be split
into two or three fragments (fission products) by thermal neutrons.
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 New Mexico.
Enrichment
A photo of a large hall filled with arrays of long white
standing cylinders.
Cascades of gas centrifuges are used to enrich uranium ore
to concentrate its fissionable isotopes.
In nature, uranium is found as uranium-238 (99.2742%) and
uranium-235 (0.7204%). 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%.[81] 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
(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 thermal diffusion.
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, 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.
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.[19] Uranium
is not absorbed through the skin, and alpha particles released by uranium
cannot penetrate the skin.
Incorporated uranium becomes uranyl ions, which accumulate
in bone, liver, kidney, and reproductive tissues. Uranium can be decontaminated
from steel surfaces[85] 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.[89][90] Radiological effects are generally local because alpha
radiation, the primary form of 238U decay, has a very short range, and will not
penetrate skin. Uranyl (UO2+
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 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.
Compilation of 2004 review on uranium toxicity[87] Body
system Human studies Animal studies In
vitro
Renal Elevated
levels of protein excretion, urinary catalase and diuresis Damage to proximal convoluted
tubules, necrotic cells cast from tubular epithelium, glomerular changes (renal
failure) No studies
Brain/CNS Decreased
performance on neurocognitive tests Acute
cholinergic toxicity; Dose-dependent accumulation in cortex, midbrain, and
vermis; Electrophysiological changes in hippocampus No studies
DNA 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
Bone/muscle No
studies Inhibition of periodontal
bone formation; and alveolar wound healing No
studies
Reproductive Uranium
miners have more first born female children Moderate
to severe focal tubular atrophy; vacuolization of Leydig cells No studies
Lungs/respiratory No
adverse health effects reported Severe
nasal congestion and hemorrhage, lung lesions and fibrosis, edema and swelling,
lung cancer No studies
Gastrointestinal Vomiting,
diarrhea, albuminuria No
studies No studies
Liver No effects
seen at exposure dose Fatty
livers, focal necrosis No
studies
Skin No exposure
assessment data available Swollen
vacuolated epidermal cells, damage to hair follicles and sebaceous glands No studies
Tissues surrounding embedded DU fragments Elevated uranium urine concentrations Elevated uranium urine
concentrations, perturbations in biochemical and neuropsychological testing No studies
Immune system Chronic
fatigue, rash, ear and eye infections, hair and weight loss, cough. May be due
to combined chemical exposure rather than DU alone No studies No
studies
Eyes No studies Conjunctivitis, irritation
inflammation, edema, ulceration of conjunctival sacs No studies
Blood No studies Decrease in RBC count and hemoglobin
concentration No studies
Cardiovascular Myocarditis
resulting from the uranium ingestion, which ended 6 months after ingestion No effects No studies
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