{{Element |
Naam = Uraan / Uranium |
Symbool = U |
Atoomnummer = 92 |
Groep = Scandiumgroep |
Periode = Periode 7 |
Blok = F blok |
Reeks = Actiniden |
Kleur = Grijs metalliek |
Atoommassa = 238,03 |
Elektronenconfiguratie = <nowiki>[</nowiki>Rn<nowiki>]</nowiki>5f<sup>3</sup> 6d<sup>1</sup> 7s<sup>2</sup> |
Oxidatietoestanden = +3, +4, +5, +6 |
Elektronegativiteit = 1,38 |
Atoomstraal = 139 |
Ionisatiepotentiaal1 = 597,64 |
Ionisatiepotentiaal2 =  |
Ionisatiepotentiaal3 =  |
Dichtheid = 18950 |
Hardheid = 6,0 |
Smeltpunt = 1408 |
Kookpunt = 4404 |
Aggregatietoestand = Vast |
Smeltwarmte = 15,5 |
Verdampingswarmte = 420 |
vdWstraal =  |
Kristalstructuur = Ortho |
Molair volume = 11,7 |
Dampdruk =  |
Geluidssnelheid =  |
Specifieke warmte = 120 |
Weerstand = 20 |
Warmtegeleiding = 27,6 |
}}
'''Uranium''' of '''uraan''' is een scheikundig element met symbool U en atoomnummer 92. Het is een metalliek grijs actinide.

== Ontdekking ==
Uranium werd in Uranus, die acht jaar eerder was ontdekt.

De zoektocht naar en ontginning van radioactieve ertsen begon in de Colorado uranium gewonnen voor het Manhattanproject. Maar uiteindelijk werd het meeste uranium voor het Manhattan project en vooral voor de atoombom Little Boy geleverd door de Belgische regering in ballingschap, vanuit U-234.
Rond 1960 nam de behoefte aan militair uranium in de Verenigde Staten af door de nucleaire ontwapening. Tegelijkertijd kwam er meer behoefte aan uranium voor gebruik in kernreactoren.

==Toepassingen==
Naast het genoemde gebruik van Noordvaarder sinds 1993 niet meer toegestaan.

==Opmerkelijke eigenschappen==
Na raffinage is uranium een zilverwit licht radioactief metaal dat iets zachter is dan staal. Het is buigzaam, vervormbaar, een beetje paramagnetisch en heeft een zeer hoge dichtheid; 65% dichter dan lood. Als fijn verdeeld poeder reageert uranium met koud water en bij aanwezigheid van zuurstof wordt het langzaamaan bedekt met een laagje uraniumoxide.

Uraniumhexafluoride (UF<sub>6</sub>) is een witte, vaste stof, die al een damp vormt bij temperaturen boven 56 °C. Het wordt gebruikt bij het verrijkingsproces van uranium. Verrijkt uranium bevat meer dan de 0,711 procent <sup>235</sup>U die in natuurlijk uranium aanwezig is.

== Verschijning ==
erts dat van nature het meest op aarde voorkomt. Het bestaat voornamelijk uit uraniumdioxide (UO<sub>2</sub>). Voor verdere verwerking wordt dit verwerkt tot "Yellowcake" (ammoniumdiuranaat), dit bevat 70 tot 80 gewichtsprocent uraniumoxide (U<sub>3</sub>O<sub>8</sub>).
Om <sup>235</sup>U te winnen moeten grote hoeveelheden erts gedolven worden, want in 1000 kilo erts zit slechts 1 kilo van dit uraniumisotoop. Bij de mijn blijven in veel gevallen grote hoeveelheden radioactief afval en verzuurde modder achter.
thumb|right|250px|Uraniumerts
thumb|right|250px|Verrijkt uranium

Landen waar uranium wordt gevonden zijn Australië, Niger, Canada, de VS en Zuid-Afrika. Ook in Sudetenland in Tsjechië komt uranium voor. In februari 2003 werd in Iran uranium aangetroffen, op 200 kilometer afstand van de stad Yazd. Er werd meteen een fabriek gebouwd om het uranium te winnen.

Daarnaast zijn er volgens het boek ''The origins of the Korean War'' van Stalin had in de jaren 1945-1948 een deal gemaakt met Kim Il Sung, dat de Sovjet-Unie Noord-Korea wapens zou leveren in ruil voor uranium, monaziet en andere radioactieve grondstoffen voor het kernwapenprogramma van de Sovjet-Unie, hoewel in de jaren 40 op kleine schaal, op bevel van Stalin in het diepste geheim, ook uranium werd gedolven door dwangarbeiders in de omgeving van Oest-Omtsjoeg (het kampsysteem Boetoegytsjag, bekend als de "Vallei des doods"), in de oblast Magadan, Oost-Siberië, zoals onthuld in de documentaire ''Goud'' van Theo Uittenbogaard.

== Isotopen ==
{| rules="all" cellpadding=1 style="border: 1px solid #999; background-color: #eeeeee; font-size: 90%;"
! colspan=6 bgcolor="#cedaf2" | Stabielste isotopen
|-----
! Iso
! RA (%)
! Halveringstijd
! VV
! VE (MeV)
! VP
|----
| <sup>233</sup>U
| syn
| 1,592×10<sup>3</sup> j
| α
| 4,909
| <sup>229</sup>Th
|----
| <sup>234</sup>U
| 0,0055
| 2,455×10<sup>5</sup> j
| α
| 4,859
| <sup>230</sup>Th
|----
| <sup>235</sup>U
| 0,720
| 7,038×10<sup>8</sup> j
| α
| 4,679
| <sup>231</sup>Th
|----
| <sup>236</sup>U
| syn
| 2,342×10<sup>7</sup> j
| α
| 4,572
| <sup>232</sup>Th
|----
| <sup>238</sup>U
| 99,2745
| 4,468×10<sup>9</sup> j
| α
| 4,270
| <sup>234</sup>Th
|}
De twee voornaamste uraniumisotopen die op aarde voorkomen zijn <sup>235</sup>U en <sup>238</sup>U. <sup>235</sup>U is onmisbaar voor zowel kernreactoren als kernwapens, doordat het de enige isotoop is die op aarde in voldoende mate wordt gevonden. Bovendien is het splijtbaar door thermische neutronen. De isotoop <sup>238</sup>U is ook belangrijk doordat deze neutronen absorbeert, waarbij hij vervalt naar <sup>239</sup>Pu (plutonium), dat ook splijtbaar is door thermische neutronen. Ook de kunstmatige isotoop <sup>233</sup>U is splijtbaar; deze wordt gevormd uit <sup>232</sup>Th (thorium) door middel van een neutronenbombardement.

Uranium was het eerste element dat splijtbaar bleek te zijn. Door een bombardement met langzame neutronen verandert de isotoop <sup>235</sup>U in het zeer kort levende <sup>236</sup>U, dat zich onmiddellijk deelt in twee kleinere atoomkernen, waarbij energie vrijkomt en bovendien nog meer neutronen. Als deze neutronen vervolgens ook geabsorbeerd worden door andere <sup>235</sup>U-atomen ontstaat een kettingreactie. Als er geen materiaal in de buurt is om het overschot aan neutronen te absorberen, ontstaat een explosieve situatie. De eerste atoombom werkte met dit principe van kernsplijting. Zo'n twee miljard jaar geleden was de relatieve aanwezigheid van <sup>235</sup>U dusdanig groot dat onder bijzondere omstandigheden natuurlijke kernsplijting kon voorkomen. De enige plaatsen waarvan bekend is dat het daar heeft plaatsgevonden liggen in de omgeving van Oklo in Gabon.

Een ander type atoombom is de waterstofbom, die werkt op basis van kernfusie.

==Toxicologie en veiligheid==
Uranium en uraniumverbindingen zijn giftig. Bij blootstelling aan minder dan de lethale dosis bestaat er kans op schade aan de lever. Bij hogere doses kunnen ook andere organen onherstelbaar beschadigd raken. Daarnaast zijn alle uraniumisotopen radioactief en kunnen daardoor bij opeenhoping in het lichaam genetische schade aanrichten, die uiteindelijk kanker kan veroorzaken. Omdat uranium alleen α-straling uitzendt, is dit gevaar bij uranium dat zich buiten het lichaam bevindt, niet aanwezig, omdat deze deeltjes de huid niet kunnen doordringen.

In poedervorm is uranium brandbaar. Verarmd uranium vindt toepassing in anti-tankwapens. Het projectiel uit verarmd uranium doorboort het pantser. Het uranium brandt dan en verbruikt daarbij alle zuurstof binnen in de tank.

== Externe links ==
*

<!-- moved otheruses below to align infobox; eases flipping through atomic numbers -->

'''Uranium''' () is a silver-gray symbol '''U''' and dense than radioactive. It occurs naturally in low concentrations (a few parts per million) in soil, rock and water, and is commercially extracted from uranium-bearing minerals such as uraninite (see uranium mining).

In nature, uranium atoms exist as billion years and that of uranium-235 is 704 nuclear properties.  Uranium-235 has the distinction of being the only naturally occurring fertile (capable of being transmuted to fissile fission spontaneously or when bombarded with fast neutrons, the much higher probability of uranium-235 and to a lesser degree uranium-233 to fission when bombarded with slow neutrons generates the heat in armor plating.<ref name="BuildingBlocks479">Emsley, ''Nature's Building Blocks'' (2001), page 479</ref>

Uranium is used as a colorant in discovery of uranium in the mineral pitchblende is credited to first nuclear weapon used in war. An ensuing breakup of the Soviet Union in 1991 is a concern for public health and safety.

==Characteristics==
thumb|left|150px|An induced nuclear fission event involving uranium-235
When refined, uranium is a silvery white, weakly radioactive electropositive and a poor electrical conductor.<ref name="SciTechEncy"/> It is malleable, ductile, and slightly paramagnetic.<ref name="LANL"/> Uranium metal has very high density, being approximately 70% more dense than lead, but slightly less dense than gold.

Uranium metal reacts with almost all nonmetallic elements and their compounds, with reactivity increasing with temperature.<ref name="ColumbiaEncy"></ref> Hydrochloric and nitric acids dissolve uranium, but nonoxidizing acids attack the element very slowly.<ref name="SciTechEncy"/> When finely divided, it can react with cold water; in air, uranium metal becomes coated with a dark layer of uranium oxide.<ref name="LANL"/> Uranium in ores is extracted chemically and converted into uranium dioxide or other chemical forms usable in industry.

Uranium was the first element that was found to be fissile. Upon bombardment with slow nuclei, releasing nuclear binding energy and more neutrons. If these neutrons are absorbed by other uranium-235 nuclei, a nuclear chain reaction occurs and, if there is nothing to absorb some neutrons and slow the reaction, the reaction is explosive. As little as 15&nbsp;lb (7&nbsp;kg) of uranium-235 can be used to make an atomic bomb.<ref name="EncyIntel"></ref> The first atomic bomb worked by this principle (nuclear fission).
<!-- NEEDS CITE
Uranium metal has three allotropic forms:
*alpha (orthorhombic) stable up to 667.7&nbsp;°C
*beta (tetragonal) stable from 667.7&nbsp;°C to 774.8&nbsp;°C
*gamma (body-centered cubic) from 774.8&nbsp;°C to melting point — this is the most malleable and ductile state.
/NEEDS CITE -->

==Applications==
===Military===
thumb|left|[[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 flammability of the projectile enable destruction of heavily armored targets. Tank armor and the removable armor on combat vehicles are also hardened with depleted uranium (DU) plates. The use of DU became a contentious political-environmental issue after the use of DU 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).<ref name="EncyIntel"/>

Depleted uranium is also used as a shielding material in some containers used to store and transport radioactive materials.<ref name="SciTechEncy"/> Other uses of DU include counterweights for aircraft control surfaces, as ballast for missile re-entry vehicles and as a shielding material.<ref name="LANL"/> Due to its high density, this material is found in inertial guidance devices and in gyroscopic alpha emitter).

During the later stages of World War II, the entire Cold War, and to a lesser extent afterwards, uranium has been used as the fissile explosive material to produce nuclear weapons. Two major types of fission bombs were built: a relatively simple device that uses uranium-235 and a more complicated mechanism that uses uranium-238-derived plutonium-239. Later, a much more complicated and far more powerful fusion bomb that uses a plutonium-based device in a uranium casing to cause a mixture of tritium and deuterium to undergo nuclear fusion was built.<ref></ref>

===Civilian===
thumb|left|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 commercial trillion joules of energy (20&nbsp;joules); as much electricity as 1500 tonnes of coal.<ref name="BuildingBlocks479"/>

Commercial classified). In a breeder reactor, uranium-238 can also be converted into plutonium through the following reaction:<ref name="LANL"/> <sup>238</sup>U (n, gamma) → <sup>239</sup>U -(beta) → <sup>239</sup>Np -(beta) → <sup>239</sup>Pu.

thumb|[[Uranium glass glowing under UV light]]
thumb|left|[[Uranium glass used as lead-in seals in a vacuum capacitor]]
Prior to the discovery of Fiestaware).

After Marie Curie discovered radium in uranium ore, a huge industry developed to mine uranium so as to extract the radium, which was used to make glow-in-the-dark paints for clock and aircraft dials[http://www.newscientist.com/article/mg15520902.900-dial-r-for-radioactive.html].  This left a prodigious quantity of uranium as a 'waste product', since it takes three metric tons of uranium to extract one gram of radium.  This 'waste product' was diverted to the glazing industry, making uranium glazes very inexpensive and abundant.  In addition to the pottery glazes, uranium tile glazes accounted for the bulk of the use, including common bathroom and kitchen tiles which can be colored green, yellow, mauve, black, blue, red and other colors with uranium.

Uranium was also used in photographic chemicals (esp. uranium nitrate as a toner),<ref name="LANL"/> in lamp filaments, 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 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 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 and uranium-lead dating). Uranium metal is used for X-ray targets in the making of high-energy X-rays.<ref name="LANL"/>

==History==
===Pre-discovery use===
The use of uranium in its natural Roman villa on Cape Bay of Naples, pitchblende was extracted from the Joachimsthal, Bohemia (now Jáchymov in the Czech Republic) and was used as a coloring agent in the local glassmaking industry.<ref name="BuildingBlocks477"/> In the early 19th century, the world's only known source of uranium ores were these old mines.

===Discovery===
thumb|left|[[Henri Becquerel|Antoine Henri Becquerel discovered the phenomenon of radioactivity by exposing a photographic plate to uranium (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.<ref name="BuildingBlocks477">Emsley, ''Nature's Building Blocks'' (2001), page 477</ref> Klaproth mistakenly 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).<ref name="BuildingBlocks477"/><ref>{{cite journal
| title = Chemische Untersuchung des Uranits, einer neuentdeckten metallischen Substanz
| author = M. H. Klaproth
| journal = Chemische Annalen
| volume = 2
| issue =
| year = 1789
| pages = 387–403}}</ref> He named the newly discovered element after the planet Uranus, which had been discovered eight years earlier by William Herschel.<ref></ref>

In 1841, Eugène-Melchior Péligot, who was 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.<ref>{{cite journal
| title = Recherches Sur L'Uranium
| author = E.-M. Péligot
| journal = Annales de chimie et de physique
| volume = 5
| issue = 5
| year = 1842
| pages = 5–47
| url =http://gallica.bnf.fr/ark:/12148/bpt6k34746s/f4.table}}</ref><ref name="BuildingBlocks477"/> <!-- NEEDS CITE In 1850 the first commercial use of uranium in glass was developed by Lloyd & Summerfield of Birmingham, England. /NEEDS CITE --> 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.

Antoine Henri Becquerel discovered radioactivity by using uranium in 1896.<ref name="ColumbiaEncy"/> Becquerel made the discovery in Paris by leaving a sample of a uranium salt on top of an unexposed photographic plate in a drawer and noting that the plate had become 'fogged'.<ref name="BuildingBlocks478"/> He determined that a form of invisible light or rays emitted by uranium had exposed the plate.

===Fission research===
right|thumb|left|[[Enrico Fermi (bottom left) and the rest of the team that initiated the first artificial nuclear chain reaction (1942).]]
A team led by beta rays (fission (break apart) into lighter elements and release binding energy were conducted by Otto Hahn and Fritz Strassmann<ref name="EncyChem773">Seaborg, ''Encyclopedia of the Chemical Elements'' (1968), page 773</ref> in Hahn's laboratory in Berlin. Lise Meitner and her nephew, physicist Otto Robert Frisch, published the physical explanation in February 1939 and named the process 'nuclear fission'.<ref>{{cite journal
| title = Disintegration of Uranium by Neutrons: a New Type of Nuclear Reaction
| author = L. Meitner, O. Frisch
| journal = Nature
| volume = 143
| issue =
| year = 1939
| pages = 239–240
| doi = 10.1038/224466a0
| url = http://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Meitner-Fission-1939.html}}</ref> 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 1/2 neutrons are released by each fission of the rare uranium isotope transmuted into plutonium, which, like uranium-235, is also fissionable by thermal neutrons.

On chemical explosives.

===Bombs===
thumb|left|The [[mushroom cloud over Hiroshima after the dropping of the uranium-based atomic bomb nicknamed 'Little Boy' (1945)]]
Two major types of atomic bomb were developed in the 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).<ref name="BuildingBlocks478"/><!-- NEEDS CITE 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. /NEEDS CITE -->
===Reactors===
thumb|Four light bulbs lit with electricity generated from the first artificial electricity-producing [[nuclear reactor, EBR-I (1951)]]
Idaho National Laboratory(INL) near Obninsk in the Calder Hall in USS ''Nautilus'', in 1954.<ref name="EncyChem773"/>

===Naturally Occurring Nuclear Fission===

Fifteen ancient and no longer active Oklo Fossil Reactors. The ore they exist in is 1.7 billion years old; at that time, uranium-235 constituted about three percent of the total uranium on Earth.<ref name="OCRWM"></ref> This is high enough to permit a sustained nuclear fission chain reaction to occur, providing other conditions are right. The ability of the surrounding sediment to contain the nuclear waste products in less than ideal conditions has been cited by the U.S. federal government as evidence of their claim that the Yucca Mountain facility could safely be a repository of waste for the nuclear power industry.<ref name="OCRWM"/>

===Cold War legacy and waste===
thumb|U.S. and USSR/Russian nuclear weapons stockpiles, 1945–2006
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&nbsp;tons (540&nbsp;tonnes) 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.<ref name="EncyIntel"/> Police in intercepted shipments of smuggled bomb-grade uranium or plutonium, most of which was from ex-Soviet sources.<ref name="EncyIntel"/> From 1993 to 2005 the US $550 million to help safeguard uranium and plutonium stockpiles in Russia.<ref name="EncyIntel"/> The improvements made provided repairs and security enhancements at research and storage facilities. ''Scientific American'' reported in February of 2006 that some of the facilities had been protected only by 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 prior to the improvement project; another had been keeping track of its stock of nuclear warheads using index cards kept in a shoe box.<ref name="thwarting"> Glaser, Alexander and von Hippel, Frank N. "Thwarting Nuclear Terrorism" Scientific American Magazine, February 2006</ref><!--Commentary: I can't believe that they kept track of nuclear warheads using index cards in a shoe box! The inspector said he almost had a heart attack when he saw their "cataloguing system"-->

Above-ground nuclear tests by the Soviet Union and the United States in the 1950s and early 1960s and by fallout from uranium daughter isotopes around the world.<ref>{{cite journal 
| author = T. Warneke, I. W. Croudace, P. E. Warwick, R. N. Taylor
| title = A new ground-level fallout record of uranium and plutonium isotopes for northern temperate latitudes
| journal = Earth and Planetary Science Letters
| year = 2002
| volume = 203
| issue = 3–4
| pages = 1047–1057
| doi = 10.1016/S0012-821X(02)00930-5
| url = }}</ref> Additional fallout and pollution occurred from several nuclear accidents.  

The Windscale fire at the Sellafield nuclear plant in 1957 spread iodine-131, a short lived radioactive isotope, over much of Northern England. 

The Three Mile Island accident in 1979 released a small amount of iodine-131.  The amounts released by the partial meltdown of the Three Mile Island power plant were minimal, and an environmental survey only found trace amounts in a few field mice dwelling nearby.  As I-131 has a half life of slightly more than eight days, any danger posed by the radioactive material has long since passed for both of these incidents.

The Chernobyl disaster in 1986, however, was a complete core breach meltdown and partial detonation of the reactor, which ejected iodine-131 and strontium-90 over a large area of Europe.  The 28 year half-life of strontium-90 means that only recently has some of the surrounding countryside around the reactor been deemed safe enough to be habitable.<ref name="BuildingBlocks480"/> Since this is less than one half life after the accident, more than half the original release of strontium-90 will still be present. Many other radio active elements with half lives of many thousands of years were also released so use of the term "safe" is curious.

==Occurrence==
===Biotic and abiotic===

thumb|left|[[Uraninite, also known as Pitchblende, is the most common ore mined to extract uranium.]]
Uranium is a naturally occurring element that can be found in low levels within all rock, soil, and water. Uranium is also the highest-numbered element to be found naturally in significant quantities on earth and is always found combined with other elements.<ref name="LANL"/> Along with all elements having potassium-40 in the Earth's mantle is thought to be the main source of heat<ref></ref><ref></ref> that keeps the outer core liquid and drives mantle convection, which in turn drives plate tectonics.

Its average concentration in the crust is (depending on the reference) 2 to 4 parts per million,<ref name="SciTechEncy"></ref><ref name="BuildingBlocks480"/> or about 40 times as abundant as silver.<ref name="ColumbiaEncy"/> The Earth's crust from the surface to 25&nbsp;km (15&nbsp;mi) down is calculated to contain 10<sup>17</sup>&nbsp;kg (2&nbsp;lb) of uranium while the oceans may contain 10<sup>13</sup>&nbsp;kg (2&nbsp;lb).<ref name="SciTechEncy"/> 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 3 parts per billion of sea water is composed of the element.<ref name="BuildingBlocks480"/>

It is more plentiful than mercury, or silver, and it is about as abundant as arsenic or molybdenum.<ref name="LANL"/><ref name="BuildingBlocks480"/> It is found in hundreds of minerals including uraninite (the most common uranium ore), autunite, uranophane, torbernite, and coffinite.<ref name="LANL"/> 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<ref name="LANL"/> (it is recovered commercially from these sources with as little as 0.1% uranium<ref name="ColumbiaEncy"/>).

thumb|''[[Citrobacter'' species can have concentrations of uranium in their bodies 300 times higher than in the surrounding environment.]]
<!-- NEEDS CITE
It has been shown in some recent work at Manchester that bacteria can reduce and fix uranium in soils. This research is continuing at the University of Plymouth by Dr Keith Roach and S Handley.
/NEEDS CITE -->
Some organisms, such as the lichen ''decontaminate uranium-polluted water.<ref name="BuildingBlocks477"/><ref>{{cite journal
| title = Uranium bioaccumulation by a Citrobacter sp. as a result of enzymically mediated growth of polycrystalline 
| author = L. E. Macaskie, R. M. Empson, A. K. Cheetham, C. P. Grey, A. J. Skarnulis
| journal = Science
| volume = 257
| issue =
| year = 1992
| pages = 782–784
| doi = 10.1126/science.1496397
| pmid = 1496397}}</ref> 

Plants absorb some uranium from the soil they are rooted in. 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.<ref name="BuildingBlocks477"/> Dry weight concentrations of uranium in food plants are typically lower with one to two micrograms per day ingested through the food people eat.<ref name="BuildingBlocks477"/>

===Production and mining===

The worldwide production of uranium in 2003 amounted to 41&nbsp;429 tonnes, of which 25% was mined in Canada. Other important uranium mining countries are Australia, Russia, Niger, Namibia, Kazakhstan, Uzbekistan, South Africa, USA and Portugal.

180px|left|thumb|[[Yellowcake is a concentrated mixture of uranium oxides that is further refined to extract pure uranium.]]
Uranium ore is mined in several ways: by open pit, underground, in-situ leaching, and borehole mining (see uranium mining).<ref name="BuildingBlocks479"/> Low-grade uranium ore typically contains 0.1 to 0.25% of actual uranium oxides, so extensive measures must be employed to extract the metal from its ore.<ref name="EncyChem774">Seaborg, ''Encyclopedia of the Chemical Elements'' (1968), page 774</ref> High-grade ores found in Athabasca Basin deposits in Saskatchewan, Canada can contain up to 70% uranium oxides, and therefore must be diluted with waste rock prior to milling, as the undilute stockpiled ore could become critical and start a nuclear reaction. Uranium ore is crushed and rendered into a fine powder and then leached with either an acid or alkali. The leachate is then subjected to one of several sequences of precipitation, solvent extraction, and ion exchange. The resulting mixture, called yellowcake, contains at least 75% uranium oxides. Yellowcake is then calcined to remove impurities from the milling process prior to refining and conversion. 

Commercial-grade uranium can be produced through the reduction of uranium alkali or alkaline earth metals.<ref name="LANL"/> Uranium metal can also be made through electrolysis of  or 
, dissolved in a molten NaCl) solution.<ref name="LANL"/> Very pure uranium can be produced through the thermal decomposition of uranium halides on a hot filament.<ref name="LANL"/>

===Resources and reserves===

Current economic uranium resources will last for over 100 years at current consumption rates, while it is expected there is twice that amount awaiting discovery. With reprocessing and recycling, the reserves are good for thousands of years.<ref name="autogenerated1">http://www.world-nuclear-news.org/ENF_Exploration_drives_uranium_resources_up_17_0206082.html</ref>. It is estimated that 5.5 million tonnes of uranium ore reserves are economically viable<ref name="autogenerated1" /> , while 35 million tonnes are classed as mineral resources (reasonable prospects for eventual economic
extraction).<ref name = IAEAResourcesDemand/> 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 feasible).<ref name = UseaWater></ref><ref name = stanfordCohen></ref> 

Exploration for uranium is continuing to increase with US$200 million being spent world wide in 2005, a 54% increase on the previous year.<ref name = IAEAResourcesDemand/>.  This trend has continued through 2006, when expenditure on exploration rocketed to total 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. <ref name="autogenerated1" />

Australia has 40% of the world's uranium ore reserves<ref>[http://www.abc.net.au/worldtoday/content/2006/s1723255.htm The World Today - NT Opposition in favour of uranium enrichment<!-- Bot generated title -->]</ref> and the world's largest single uranium deposit, located at the Olympic Dam Mine in South Australia.<ref></ref> Almost all of the uranium production is exported, under strict International Atomic Energy Agency safeguards against use in nuclear weapons.

===Supply===
thumb|right|Uranium output in 2005
In 2005, seventeen countries produced concentrated uranium oxides, with China (1.7%) also producing significant amounts.<ref></ref> Kazakhstan continues to increase production and may become the world's largest producer of uranium by the year 2009 with an expected production of 12 826 tonnes, compared to Canada with 11 100 tonnes and Australia with 9 430 tonnes.<ref>http://mithridates.blogspot.com/2008/07/kazakhstan-to-surpass-canada-as-worlds.html</ref><ref>http://www.zaman.com.tr/haber.do?haberno=717292</ref> The ultimate supply of uranium is believed to be very large and sufficient for at least the next 85 years<ref name = IAEAResourcesDemand></ref> although some studies indicate underinvestment in the late twentieth century may produce supply problems in the 21st century.<ref name = MITfuelSupply></ref>  

Some claim that production of uranium will peak similar to peak oil.  Kenneth S. Deffeyes and Ian D. MacGregor point out that 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." <ref>{{cite web
  |url=http://www.osti.gov/energycitations/product.biblio.jsp?osti_id=6665051<!-- abstract -->
  |title=World Uranium Resources
  |author=Kenneth S. Deffeyes and Ian D. MacGregor
  |publisher=Scientific American
  |date=1980-01
  |page=p 66
  |language=English
  |accessdate=2008-04-21
}}</ref>  In other words, there is very little high grade ore and proportionately much more low grade ore.

==Compounds==
===Oxidation states and oxides===
====Oxides====
thumb|[[Triuranium octaoxide (diagram pictured) and uranium dioxide 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 particles that have long retention times or are recovered in the stack scrubber. While uranium content is referred to for  content, to do so is inaccurate and dates to the days of the Manhattan project when  was used as an analytical chemistry reporting standard. 