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Uranium



    protactiniumuraniumneptunium
    Nd
    U
    Click for description
    General
    Name Symbol Number Uranium U, 92
    Chemical series Actinides
    Period Block 7 , f
    Density Mohs scale|Hardness] 19050 kg/m3 ND
    Appearance silvery-white metal
    Atomic properties
    Atomic weight 2380289 amu
    Atomic radius (calc) 175 pm
    Covalent radius ND pm
    van der Waals radius 186 pm
    Electron configuration href="Radon.html" title="Radon" >Rn7s25f26d1
    e- 's per energy level 2818322192
    Oxidation states (Oxide) 5 (weak base)
    Crystal structure Orthohombic
    Physical properties
    State of matter Solid (paramagnetic)
    Melting point 1405 K (2912 °F)
    Boiling point 2070 K (7473 °F)
    Molar volume 1249 ×10-6 m3/mol
    Heat of vaporization 477 kJ/mol
    Heat of fusion 1548 kJ/mol
    Vapor pressure ND Pa at 2200 K
    Velocity of sound 3155 m/s at 29315 K
    Miscellaneous
    Electronegativity 138 (Pauling scale)
    Specific capacity] 120 J/
    Electrical conductivity 3.8 106/m ohm
    Thermal conductivity 276 W/
    1st ionization potential 5976 kJ/mol
    2nd ionization potential 1420 kJ/mol
    Most stable isotopes

    iso NA half-life DM DE MeV DP
    232U {syn} 689 y α & SF 5414 228Th
    233U {syn} 159200 y SF & α 4909 229Th
    234U 0006% 245500 y SF & α 4859 230Th
    235U 072% 7038 E8 y SF & α 4679 231Th
    236U {syn} 2342 E7 y SF & α 4572 232Th
    238U 99275% 4468 E9 y SF & α 4270 234Th
    SI units & standard and pressure|STP] are used except where noted
    Uranium is a chemical element in the periodic table that has the symbol U and atomic number 92. A heavy silvery-white toxic metallic and naturally-radioactive element uranium belongs to the actinide series and its isotope uranium-235 is used as the fuel for nuclear reactors and nuclear weapons Uranium is commonly found in very small amounts in rock soil water plants and animals (including humans)

    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 When finely divided it can react with cold water; in air uranium metal becomes coated with uranium oxide Uranium in ores can be extracted and chemically converted into uranium dioxide or other chemical forms usable in industry
    Uranium metal has three allotropic forms:
    • alpha (orthorhombic) stable up to 6677°C
    • beta (tetragonal) stable from 6677 C to 7748°C
    • gamma (body-centered cubic) from 7748°C to melting point - this is the most malleable and ductile state

    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 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 232thorium by neutron bombardment
    Uranium was the first element that was found to be fissile ie upon bombardment with slow neutrons its 235U isotope becomes the very short lived 236U that immediately divides into two smaller nuclei liberating 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 it 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 weapon" because only the nuclei participate

    Applications

    Uranium metal is very dense and heavy Depleted uranium (almost pure U-238 with less than 02% U-235) is used by some militaries as shielding to protect tanks and also in parts of bullets and missiles as it is extremely dense The military also uses enriched uranium (more than natural levels of U-235) to power nuclear propelled navy ships and submarines and in nuclear weapons Fuel used for United States Navy reactors is typically highly enriched in U-235 (the exact values are classified information) In nuclear weapons uranium is also highly enriched usually over 90% (again the exact values are classified information)
    The main use of uranium in the civilian sector is to fuel commercial nuclear power plants where fuel is typically enriched in U-235 to 2-3% However the Canadian Candu reactors use natural unenriched uranium as fuel Depleted uranium is used in helicopters and airplanes as counterweights on certain wing parts Other uses include;
    • Ceramic glazes where small amounts of natural uranium (that is, not having gone through the enrichment process) may be added for color
    • Addition of uranium makes fluorescent yellow or green colored glass
    • The long half-life of the isotope uranium-238 (451 × 109) make it well-suited for use in estimating the age of the earliest igneous rocks
    • U-238 is converted into plutonium in breeder reactors Plutonium can be used in reactors or in nuclear weapons
    • Uranyl acetate UO2(CH3COO)2 is used in analytical chemistry It forms an insoluble salt with sodium
    • Some lighting fixtures utilize uranium as do some photographic chemicals (esp uranium nitrate)
    • Phosphate fertilizers often contain high amounts of natural uranium because the mineral material from which they are made is typically high in uranium
    • Uranium metal is used for X-ray targets in making of high-energy X-rays
    • The element has 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 79 AD, when it was used to add a yellow color to ceramic glazes (yellow glass with 1% uranium oxide was found near Naples Italy)
    The discovery of the element is credited to the German chemist Martin Heinrich Klaproth who in 1789 found uranium as part of the mineral called pitchblende It was named after the planet Uranus which had been discovered eight years earlier It was first isolated as a metal in 1841 by Eugene-Melchior Peligot 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 uranium gained new importance on the world political scene Before the discovery of plutonium only uranium was considered for the development of an atomic bomb though the process of enriching it to applicable levels required gargantuan facilities (see Oak National Laboratory]) Eventually enough uranium was enriched for one atomic bomb which was dropped on Hiroshima Japan in 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 though within a decade large deposits of it were discovered in many places around the world
    The exploration and mining of radioactive ores in the United States began around the turn of the 20th century Sources for radium (contained in uranium ore) were sought for use as luminous paint for watch dials and other instruments as well as for health-related applications (some of which in retrospect were incredibly unhealthy) Because of the need for the element during World War II, the Manhattan Project contracted with numerous vanadium mining companies in the American Southwest and also purchased uranium ore from the Belgian Congo American uranium ores mined in Colorado were primarily mixes of vanadium and uranium but because of wartime secrecy the Manhattan Project would only publicly admit to purchasing the vanadium and did not pay the uranium miners for the uranium ore (in a much later lawsuit many miners were able to reclaim lost profits from the US government) American uranium ores did not have nearly as high uranium concentrations as the ore from the Belgian Congo but they were pursued vigorously to ensure nuclear self-sufficiency Similar efforts were undertaken in the Soviet Union which did not have native stocks of uranium when it started developing its own weapons program
    In the beginning of the Cold War to ensure adequate supplies of uranium for national defense Congress passed the US Atomic Act of 1946] creating the Atomic Commission] (AEC) which had the power to withdraw prospective uranium mining land from public purchase and also to manipulate the price of uranium to meet national needs By setting a high price for uranium ore the AEC created a uranium "boom" in the early 1950s which attracted many prospectors to the four corners region of the country Moab Utah became the Uranium-capital of the world when geologist Charles Steen discovered such an ore in 1952 Military requirements declined in the 1960s and the government completed its uranium procurement program by the end of 1970 Simultaneously a new market emerged - commercial nuclear power plants
    Because uranium ores emit radon gas and their harmful and highly radioactive daughter products uranium mining is significantly more dangerous than other (already dangerous) hard rock mining requiring adequate ventilation systems if the mines are not open pit During the 1950s a significant amount of American uranium miners were Navajo Indians as many uranium deposits were discovered on Navajo reservations An unusually high number of these miners later developed lung cancer Some survivors and their descendants received compensation under the Radiation Exposure Compensation Act in 1990
    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

    Compounds

    Uranium tetrafluoride (UF4) is known as "green salt" and is an intermediate product in the production of uranium hexafluoride
    Uranium hexafluoride (UF6) is a white solid which forms a vapor at temperatures above 56 degrees Celsius UF6 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
    Yellowcake is uranium concentrate It takes its name from the color 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 green to almost black Yellowcake typically contains 70 to 90 percent uranium oxide (U3O8) by weight
    Ammonium diuranate is an intermediate product in the production of yellowcake and is bright yellow in colour It is sometimes confusingly called "yellowcake" as well but this is not a standard name

    Occurrence

    Uranium is a naturally-occurring element found at low levels in virtually all rock soil and water 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 pitchblende uraninite (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) Because uranium has such a long radioactive half-life (447x109 years for U-238) the total amount of it on Earth stays almost the same
    The decay of uranium and its nuclear reactions with thorium in the Earth's core is thought to be the source for much of the heat that keeps the outer core liquid which in turn drives plate tectonics
    Uranium ore is rock containing uranium mineralization in concentrations that can be mined economically typically 1 to 4 pounds of uranium oxide per ton or 005 to 020 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 KF5 or UF4 dissolved in a molten CaCl2 and NaCl Very pure uranium can be produced through the thermal decomposition of uranium halides on a hot filament
    Owners and operators of US civilian nuclear power reactors purchased from US and foreign suppliers a total of 21300 tons of uranium deliveries during 2001 The average price paid was $2639 per kilogram of uranium a decrease of 16 percent compared with the 1998 price In year 2001 the US produced 1018 tons of uranium from 7 mining operations all of which are west of the Mississippi River
    Uranium is distributed worldwide especially by the French Generally large countries produce more uranium than smaller ones because the worldwide distribution of uranium is very roughly uniform Canada is the world's largest producer of uranium with the world's richest deposits in Saskatchewan Saskatchewan through three large mines produces over a quarter of the world's uranium Because of this production extra capacity and the close government control of the industry the provincial government plays a central role in setting international uranium prices Australia also has extensive uranium deposits making up approximately 30% of the world's known uranium reserves The world's largest single uranium deposit is located at the Olympic Dam mine in South Australia [1] [2]

    Isotopes

    Naturally occurring uranium is composed of 3 major isotopes U-238 U-235 and U-234 with U-238 being the most abundant (993% natural abundance) These 3 isotopes are radioactive creating radioisotopes with the most abundant and stable being U-238 with a half-life of 4.5 × 109 years U-235 with a half-life of 7 × 108 years and U-234 with a half-life of 2.5 × 105 years
    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 to be 'enriched' the U-235 fraction has to be increased to significantly greater than the 0711% (by weight) (eg typically to levels from 3% to 7%) Uranium-235 is typically the main fissile material for nuclear power reactors Either U235 or Pu239 are used for making nuclear weapons The process produces huge quantities of uranium that is depleted of U-235 and with a correspondingly increased fraction of U-238 called depleted uranium or "DU" To be considered to be 'depleted' the U-235 isotope concentration has to have been decreased to significantly less than 0711% (by weight) Typically the amount of U235 left in depleted uranium is 02% to 03% This represents anywhere from 28% to 42% of the original fraction of U235
    Given that the half life of U235 is considerably shorter than U238 the "depleted" uranium is still significantly radioactive as is the natural uranium after refining
    Another way to look at this is as follows: Candu style reactors use natural uranium (071% fissile material) From a PWR reactors of typical design (most USA reactors are PWR) we note the fuel goes in with about 4% U235 and 96% U238 and comes out with about 1% U235 1% PU239 and 95%U238 If the PU239 were removed (fuel reprocessing is not allowed in the USA) and this were added to the "depleted uranium" then we would have 12% 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 CANDU style reactor

    Precautions

    All isotopes and compounds of uranium are toxic and radioactive Toxicity can be lethal In less than lethal doses toxicity is limited primarily to recoverable kidney damage Radiological effects are systemic Uranium compounds in general are poorly absorbed by the lining in the lungs and may remain a radiological hazard indefinitely Finely-divided uranium metal presents a fire hazard
    A person can be exposed to uranium by inhaling dust in air 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 007 to 1.1 micrograms per day The amount of uranium in air is usually very small 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
    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 cancer or kidney damage
    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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