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Uranium

2007 Schools Wikipedia Selection. Related subjects: Chemical elements


              92             protactinium ← uranium → neptunium
              Nd
             ↑
             U
             ↓
             (Uqb)

                                  Periodic Table - Extended Periodic Table

                                                                   General
                                       Name, Symbol, Number uranium, U, 92
                                                 Chemical series actinides
                                            Group, Period, Block n/a, 7, f
                                         Appearance silvery gray metallic;
                                                    corrodes to a spalling
                                                   black oxide coat in air
                                                                   Uranium
                                           Atomic mass 238.02891 (3) g/mol
                                Electron configuration [Rn] 5f^3 6d^1 7s^2
                                Electrons per shell 2, 8, 18, 32, 21, 9, 2
                                                       Physical properties
                                                               Phase solid
                                       Density (near r.t.) 19.1 g·cm^−3
                                    Liquid density at m.p. 17.3 g·cm^−3
                                                   Melting point 1405.3  K
                                                  (1132.2 ° C, 2070 ° F)
                                                      Boiling point 4404 K
                                                    (4131 ° C, 7468 ° F)
                                          Heat of fusion 9.14 kJ·mol^−1
                                   Heat of vaporization 417.1 kJ·mol^−1
                         Heat capacity (25 °C) 27.665 J·mol^−1·K^−1

   CAPTION: Vapor pressure

                                      P/Pa   1    10  100  1 k  10 k 100 k
                                     at T/K 2325 2564 2859 3234 3727 4402

                                                         Atomic properties
                                            Crystal structure orthorhombic
                                              Oxidation states 3+,4+,5+,6+
                                                      (weakly basic oxide)
                                    Electronegativity 1.38 (Pauling scale)
                                     Ionization energies 1st: 597.6 kJ/mol
                                                          2nd: 1420 kJ/mol
                                                      Atomic radius 175 pm
                                               Van der Waals radius 186 pm
                                                             Miscellaneous
                                            Magnetic ordering paramagnetic
                              Electrical resistivity (0 °C) 0.280 µΩ·m
                       Thermal conductivity (300 K) 27.5 W·m^−1·K^−1
                       Thermal expansion (25 °C) 13.9 µm·m^−1·K^−1
                               Speed of sound (thin rod) (20 °C) 3155 m/s
                                                   Young's modulus 208 GPa
                                                     Shear modulus 111 GPa
                                                      Bulk modulus 100 GPa
                                                        Poisson ratio 0.23
                                             CAS registry number 7440-61-1
                                                         Selected isotopes

                 CAPTION: Main article: Isotopes of uranium

                         iso    NA     half-life     DM   DE ( MeV)   DP
                        ^232U syn     68.9 y       α & SF 5.414     ^228Th
                        ^233U syn     159,200 y    SF & α 4.909     ^229Th
                        ^234U 0.0058% 245,500 y    SF & α 4.859     ^230Th
                        ^235U 0.72%   7.038×10^8 y SF & α 4.679     ^231Th
                        ^236U syn     2.342×10^7 y SF & α 4.572     ^232Th
                        ^238U 99.275% 4.468×10^9 y SF & α 4.270     ^234Th

                                                                References

   Uranium ( IPA: /jəˈreɪniəm/) is a chemical element in the periodic
   table that has the symbol U and atomic number 92. Heavy, silvery-white,
   metallic, naturally radioactive, uranium belongs to the actinide
   series. Its isotopes ^235U and to a lesser degree ^233U are used as the
   fuel for nuclear reactors and the explosive material for nuclear
   weapons. Depleted uranium ( ^238U) is used in kinetic energy
   penetrators and armor plating.

Notable characteristics

   When refined, uranium is a silvery white, weakly radioactive metal,
   which is slightly softer than steel. It is malleable, ductile, and
   slightly paramagnetic. Uranium metal has very high density, 65% more
   dense than lead, but slightly less dense than gold. When finely
   divided, it can react with cold water; in air, uranium metal becomes
   coated with a layer of uranium oxide. Uranium in ores is extracted
   chemically and converted into uranium dioxide or other chemical forms
   usable in industry.

   Uranium metal has three allotropic forms:
     * alpha (orthorhombic) stable up to 667.7 °C
     * beta (tetragonal) stable from 667.7 °C to 774.8 °C
     * gamma (body-centred cubic) from 774.8 °C to melting point - this is
       the most malleable and ductile state.

   Natural uranium metal contains about 0.71% U-235, 99.28% U-238, and
   about 0.0054% U-234. In order to produce enriched uranium, the process
   of isotope separation removes a substantial portion of the U-235 for
   use in nuclear power, weapons, or other uses. The remainder, depleted
   uranium, contains only 0.2% to 0.4% U-235. Because natural uranium
   begins with such a low percentage of U-235, the enrichment process
   produces large quantities of depleted uranium. For example, producing 1
   kg of 5% enriched uranium requires 11.8 kg of natural uranium, and
   leaves about 10.8 kg of depleted uranium with only 0.3% U-235
   remaining.

   Its two principal isotopes are ^235U and ^238U. Naturally-occurring
   uranium also contains a small amount of the ^234U isotope, which is a
   decay product of ^238U. The isotope ^235U or enriched uranium is
   important for both nuclear reactors and nuclear weapons because it is
   the only isotope existing in nature to any appreciable extent that is
   fissile, that is, fissionable by thermal neutrons. The isotope ^238U is
   also important because it absorbs neutrons to produce a radioactive
   isotope that subsequently decays to the isotope ^239Pu (plutonium),
   which also is fissile.

   The artificial ^233U isotope is also fissile and is made from
   thorium-232 by neutron bombardment.

   Uranium was the first element that was found to be fissile. Upon
   bombardment with slow neutrons, its ^235U isotope becomes the very
   short lived ^236U which immediately divides into two smaller nuclei,
   releasing nuclear binding energy and more neutrons. If these neutrons
   are absorbed by other ^235U nuclei, a nuclear chain reaction occurs
   and, if there is nothing to absorb some neutrons and slow the reaction,
   the reaction is explosive. The first atomic bomb worked by this
   principle (nuclear fission). A more accurate name for both this and the
   hydrogen bomb ( nuclear fusion) would be "nuclear bomb" or "nuclear
   weapon", because only the nuclei participate.

Applications

   Before radiation was discovered, uranium was primarily used in small
   amounts for yellow glass and pottery dyes (such as uranium glass and in
   Fiestaware.) There was also some use in photographic chemicals (esp.
   uranium nitrate.) It was used in filaments for lamps and in the leather
   and wood industries for stains and dyes. Uranium salts are mordants of
   silk or wool. Uranium was also used to improve the appearance of
   dentures. After the discovery of uranium radiation, additional
   scientific and practical values of uranium were pursued.

   After the discovery in 1939 that it could undergo nuclear fission,
   uranium gained importance with the development of practical uses of
   nuclear energy. The first atomic bomb used in warfare, " Little Boy",
   was a uranium bomb. This bomb contained enough of the uranium-235
   isotope to start a runaway chain reaction which in a fraction of a
   second caused a large number of the uranium atoms to undergo fission,
   thereby releasing a fireball of energy.

   The main use of uranium in the civilian sector is to fuel commercial
   nuclear power plants. Generally this is in the form of enriched
   uranium, which has been processed to have higher-than-natural levels of
   ^235U and can be used for a variety of purposes relating to nuclear
   fission. Commercial nuclear power plants use fuel typically enriched to
   2–3% ^235U, though some reactor designs (such as the Candu reactors)
   can use natural uranium (unenriched, less than 1% ^235U) fuel. Fuel
   used for United States Navy submarine reactors is typically highly
   enriched in ^235U (the exact values are classified information). When
   uranium is enriched over 85% it is known as "weapons grade". In a
   breeder reactor, ^238U can also be converted into plutonium.

   Currently the major application of uranium in the U.S. military sector
   is in high-density penetrators. This ammunition consists of depleted
   uranium alloyed with 1–2% other elements. The applications of these
   armor-piercing rounds range from the 20 mm Phalanx gun of the U.S. Navy
   for piercing attacking missiles, through the 30 mm gun in A-10
   aircraft, to 105mm and larger tank barrels. At high impact speed, the
   density, hardness, and flammability of the projectile enable
   destruction of heavily armored targets. Tank armour and the removable
   armour on combat vehicles are also hardened with depleted uranium (DU)
   plates. The use of DU became a contentious political-environmental
   issue after US, UK and other countries' use of DU munitions in wars in
   the Persian Gulf and the Balkans raised questions of uranium compounds
   left in the soil.

   Other uses include:
     * The long half-life of the isotope ^238U (4.51 × 10^9 years) make 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 and uranium-lead dating).
     * Uranyl acetate, UO[2](CH[3]COO)[2] is used in analytical chemistry.
       It forms an insoluble salt with sodium.
     * Uranium metal is used for X-ray targets in the making of
       high-energy X-rays.
     * Its high atomic mass makes ^238U suitable for radiation shielding.
     * It is alloyed with iron to make “ferrouranium” that imparts special
       properties to steels by increasing elastic limit and tensile
       strength and as a cathode in photoelectric tubes responsive to
       ultraviolet radiation.
     * Distinctive 234U/238U activity ratios (ARs) are a useful
       environmental tracer of sources of ground water to discharge
       springs.
     * It is a more powerful deoxidiser than vanadium and will
       denitrogenise steel.
     * It is used in high-speed steels as an alloying agent to improve
       strength and toughness.
     * Depleted uranium (uranium with the percentage of ^235U lowered to
       0.2%) has found use as counterweights for aircraft control
       surfaces, as ballast for missile re-entry vehicles and as a
       shielding material. Due to its high density, this material has also
       found use in inertial guidance devices and in gyroscopic compasses.

History

   The use of uranium, in its natural oxide form, dates back to at least
   CE 79, when it was used to add a yellow colour to ceramic glazes
   (yellow glass with 1% uranium oxide was found near Naples, Italy). When
   this was rediscovered, in the earlier part of the 19th century, the
   world’s only known source of uranium 'earths' were the old Habsburg
   silver mines in Joachimsthal, Bohemia, and the local glassmaking
   industry kept a tight lid on the secret ingredient and its supply as
   long as it could.

   The discovery of the element is credited to the German chemist Martin
   Heinrich Klaproth, who in 1789 found uranium in a mineral called
   pitchblende. It was named after Uranus the planet, which had been
   discovered eight years earlier by William Herschel. It was first
   isolated as a metal in 1841 by Eugene-Melchior Peligot. In 1850 the
   first commercial use of Uranium in glass was developed by Lloyd &
   Summerfield of Birmingham, England. Uranium was found to be radioactive
   by French physicist Henri Becquerel in 1896, who first discovered the
   process of radioactivity with uranium minerals.

   During the Manhattan Project, the wartime Allied program to develop the
   first atomic bombs during World War II, the United States government
   bought up many reserves of uranium around the world, although the
   process of enriching it to applicable levels required gargantuan
   facilities (see Oak Ridge National Laboratory). Eventually enough
   uranium, mainly from the Democratic Republic of the Congo (Belgian
   Congo), was enriched for one atomic bomb nicknamed " Little Boy", which
   was dropped on Hiroshima, Japan on August 6th, 1945. The other nuclear
   weapons developed during the war used plutonium as their fissionable
   material, which itself requires uranium to produce. Initially it was
   believed that uranium was relatively rare, and that nuclear
   proliferation could be avoided by simply buying up all known uranium
   stocks, though within a decade large deposits of it were discovered in
   many places around the world.

   During the Manhattan Project, the names tuballoy and oralloy were used
   to refer to natural uranium and enriched uranium respectively,
   originally for purposes of secrecy. These names are still used
   occasionally to refer to natural or enriched uranium. Less commonly, 25
   was used to refer to Uranium-235 by scientists at the Project. The
   names Q-metal, depletalloy, and D-38, once applied to depleted uranium,
   have fallen into disuse.

   70% of the world's known uranium is located in Australia. The
   Australian government is currently advocating an expansion of uranium
   mining, although issues with state governments and indigenous interests
   complicate the issue.

Bacterial biochemistry

   It has been shown in some recent work at Manchester that bacteria can
   reduce and fix uranium in soils.

Occurrence

   Uranium ore
   Enlarge
   Uranium ore

   Uranium is a naturally occurring element found in low levels within all
   rock, soil, and water. This is the highest-numbered element to be found
   naturally in significant quantities on earth.

   It is considered to be more plentiful than antimony, beryllium,
   cadmium, gold, mercury, silver, or tungsten and is about as abundant as
   arsenic or molybdenum. It is found in many minerals including uraninite
   (also called pitchblende, most common uranium ore), 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 these sources).

   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 ore is rock containing uranium mineralisation in concentrations
   that can be mined economically, typically 1 to 4 pounds of uranium
   oxide per ton or 0.05 to 0.20 percent uranium oxide.

Production and distribution

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

   Owners and operators of U.S. civilian nuclear power reactors purchased
   from U.S. and foreign suppliers a total of 21,300 tons of uranium
   deliveries during 2001. The average price paid was $26.39 per kilogram
   of uranium, a decrease of 16 percent compared with the 1998 price. In
   2001, the U.S. produced 1,018 tons of uranium from seven mining
   operations, all of which are west of the Mississippi River.

   The ultimate supply of uranium is very large. It is estimated that for
   a ten times increase in price, the supply of uranium that can be
   economically mined is increased 300 times.

Uranium exploration and mining

   Uranium concentration in US soils
   Enlarge
   Uranium concentration in US soils

   Uranium is distributed worldwide. The world's largest single uranium
   deposit is located at the Olympic Dam Mine in South Australia.

   Australia has the world's largest uranium reserves — 40 percent of the
   planet's known supply. Almost all the uranium is exported, but under
   strict International Atomic Energy Agency safeguards to satisfy the
   Australian people and government that none of the uranium is used in
   nuclear weapons. Australian uranium is used strictly for electricity
   production.

   In spite of Australia's huge reserves, Canada remains the largest
   exporter of uranium ore, with mines located in the Athabasca Basin in
   northern Saskatchewan. Cameco, the world’s largest, low-cost uranium
   producer accounting for 18% of the world’s uranium production, operates
   three mines in the area.

   There are also significant ore finds in Sweden but it is currently not
   legal to exploit them.

   U.S mining has been in a slump due to the presence of former weapons
   material available for reprocessing into fuel; the stockpiles of former
   Soviet uranium and the CES countries' need for dollars; and the start
   of production at huge high-grade uranium mines in Canada are depressing
   the market price.

Compounds

   Uranium tetrafluoride (UF[4]) is known as "green salt" and is an
   intermediate product in the production of uranium hexafluoride. It has
   the appearance of an emerald-green solid.

   Uranium hexafluoride (UF[6]) is a colorless crystalline solid which
   forms a vapor at temperatures above 56.4 °C. UF[6] is the compound of
   uranium used for the two most common enrichment processes, gaseous
   diffusion enrichment, and gas centrifuge enrichment. It is simply
   called "hex" in the industry. It is corrosive to many metals and reacts
   violently to water and oils.
   Powdered yellowcake in a drum.
   Enlarge
   Powdered yellowcake in a drum.

   Yellowcake is uranium concentrate. It takes its name from the colour
   and texture of the concentrates produced by early mining operations,
   despite the fact that modern mills using higher calcining temperatures
   produce "yellowcake" that is dull yellow to almost black. Initially,
   the compounds formed in yellowcakes were not identified; in 1970, the
   U.S. Bureau of Mines still referred to yellowcakes as the final
   precipitate formed in the milling process and considered it to be
   ammonium diuranate or sodium diuranate. The compositions were variable
   and depended upon precipitating conditions. Among the compounds
   identified in yellowcakes include: uranyl hydroxide, uranyl sulfate,
   sodium para-uranate, and uranyl peroxide, along with various uranium
   oxides. Modern yellowcake typically contains 70 to 90 percent uranium
   oxide (U[3]O[8]) by weight. (Other uranium oxides, such as UO[2] and
   UO[3], exist; the most stable oxide, U[3]O[8], is actually considered
   to be a 1:2 molar mixture of these.)

   Uranium dioxide a dark brown, crystalline powder, once used in the late
   1800s to mid-1900s in ceramic glazes is now used mainly as nuclear
   fuel, specifically in the form of fuel rods.

   Uranyl nitrate (UO[2](NO[3])[2]) is an extraordinarily toxic, soluble
   uranium salt. It appears as a yellow crystalline solid.

   Uranium rhodium germanium (URhGe) is the first discovered alloy that
   becomes superconducting in the presence of an extremely strong
   electromagnetic field.

   Uranium carbonate (UO[2](CO[3])) is found in both the mineral and
   organic fractions of coal and its fly ash and is the main component of
   uranium in mine tailing seepage water.

   Uranium trihydride (UH[3]) appears as a black powder, is highly
   reactive, and pyrophoric.

Isotopes

   Naturally occurring uranium is composed of three major isotopes, ^238U,
   ^235U, and ^234U, with ^238U being the most abundant (99.3% natural
   abundance). All three isotopes are radioactive, creating radioisotopes,
   with the most abundant and stable being ^238U with a half-life of 4.5 ×
   10^9 years, ^235U with a half-life of 7 × 10^8 years, and ^234U with a
   half-life of 2.5 × 10^5 years. ^238U is an α emitter, decaying through
   the uranium natural decay series into ^206Pb.

   Uranium isotopes can be separated to increase the concentration of one
   isotope relative to another. This process is called "enrichment" (see
   enriched uranium). To be considered "enriched" the ^235U fraction has
   to be increased to significantly greater than 0.711% (by weight)
   (typically to levels from 3% to 7%). ^235U is typically the main
   fissile material for nuclear power reactors. Either ^235U or ^239Pu are
   used for making nuclear weapons. The process produces huge quantities
   of uranium that is depleted of ^235U and with a correspondingly
   increased fraction of ^238U, called depleted uranium or "DU". To be
   considered "depleted", the ^235U isotope concentration has to have been
   decreased to significantly less than 0.711% (by weight). Typically the
   amount of ^235U left in depleted uranium is 0.2% to 0.3%. This
   represents anywhere from 28% to 42% of the original fraction of ^235U.

   Another way to look at this is as follows: Pressurised Heavy Water
   Reactors (PHWR) use natural uranium (0.71% fissile material). From
   Pressurised water reactors (PWRs) of typical design (most USA reactors
   are PWR) we note the fuel goes in with about 4% ^235U and 96% ^238U and
   comes out with about 1% ^235U, 1% ^239Pu and 95% ^238U. If the ^239Pu
   were removed (fuel reprocessing is not allowed in the USA) and this
   were added to the depleted uranium then we would have 1.2% fissile
   material in the reprocessed depleted uranium and at the same time have
   1% fissile material in the left over spent fuel. Both of these would be
   considered "enriched" fuels for a PHWR style reactor.

   ^233U, an artificial isotope, is used as a reactor fuel in India. It
   has also been tested in nuclear weapons, but the results were
   unpromising.

Hazards

   All isotopes and compounds of uranium are toxic, teratogenic, and
   radioactive. It has been shown that some compounds of uranium could
   cause renal damage, but no conclusive evidence has yet been produced.

   No deaths are causally associated with prolonged occupational exposure
   to inhaled uranium compounds . Although accidental inhalation exposure
   to a high concentration of uranium hexafluoride has resulted in human
   fatalities, those deaths were not associated with uranium . On the
   basis of the available data, exposure to environmental uranium or to
   uranium at levels found at hazardous waste sites will not be lethal to
   humans.

   Radiological effects are generally local because this is the nature of
   alpha radiation, the primary form from U-238 decay. Uranium compounds
   in general are poorly absorbed by the lining in the lungs and may
   remain a radiological hazard indefinitely. Uranyl (UO[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.

   Finely-divided uranium metal presents a fire hazard because uranium is
   pyrophoric, so small grains will ignite spontaneously in air at room
   temperature.

   A person can be exposed to uranium (or its radioactive daughters) by
   inhaling dust in air or from smoking tobacco products which have been
   grown using certain phosphate fertilizers, or ingesting water and food.
   The general population is exposed to uranium primarily through food and
   water; the average daily intake of uranium from food ranges from 0.07
   to 1.1 micrograms per day. The amount of uranium in air is usually very
   small; however, people who live near government facilities that made or
   tested nuclear weapons, or facilities that mine or process uranium ore
   or enrich uranium for reactor fuel, may have increased exposure to
   uranium. Houses or structures which are over uranium deposits (either
   natural or man-made slag deposits) may have an increased incidence of
   exposure to radon gas, a radioactive carcinogen.

   Uranium can enter the body when it is inhaled or swallowed, or under
   rare circumstances it may enter through cuts in the skin. Uranium does
   not absorb through the skin, and alpha particles released by uranium
   cannot penetrate the skin, so uranium that is outside the body is much
   less harmful than it would be if it were inhaled or swallowed. When
   uranium enters the body it can lead to kidney damage. Uranium itself is
   not a chemical carcinogen.

   Uranium mining carries the danger of airborne radioactive dust and the
   release of radioactive radon gas and its daughter products (an added
   danger to the already dangerous activity of all hard rock mining). As a
   result, without proper ventilation, uranium miners have a dramatically
   increased risk of later development of lung cancer and other pulmonary
   diseases. There is also the possible danger of groundwater
   contamination with the toxic chemicals used in the separation of the
   uranium ore.

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