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| {{Use dmy dates|date=August 2013}}
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| [[File:Uranium enrichment proportions.svg|thumb|Proportions of uranium-238 (blue) and uranium-235 (red) found naturally versus enriched grades]]
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| '''Enriched uranium''' is a type of [[uranium]] in which the percent composition of [[uranium-235]] has been increased through the process of [[isotope separation]]. [[Natural uranium]] is 99.284% [[uranium-238|U<sup>238</sup>]] [[isotope]], with U<sup>235</sup> only constituting about 0.711% of its weight. U<sup>235</sup> is the only [[primordial nuclide|nuclide existing in nature]] (in any appreciable amount) that is [[fissile]] with [[thermal neutron]]s.<ref>{{cite book|author=OECD Nuclear Energy Agency|title=Nuclear Energy Today|publisher=OECD Publishing|year=2003|isbn=9789264103283|page=25|url=http://books.google.com/books?id=PvL7twdmK9sC&pg=PA25}}</ref>
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| Enriched uranium is a critical component for both civil [[Nuclear power|nuclear power generation]] and military [[nuclear weapon]]s. The [[International Atomic Energy Agency]] attempts to monitor and control enriched uranium supplies and processes in its efforts to ensure nuclear power generation safety and curb [[nuclear proliferation|nuclear weapons proliferation]].
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| During the [[Manhattan Project]] enriched uranium was given the codename oralloy, a shortened version of [[Oak Ridge, Tennessee|Oak Ridge]] [[alloy]], after the location of the plants where the uranium was enriched.{{cn|date=May 2013}} The term oralloy is still occasionally used to refer to enriched uranium. There are about 2,000 [[tonnes]] (t, Mg) of highly enriched uranium in the world,<ref>{{cite web|url=http://docs.nrdc.org/nuclear/nuc_06129701a_185.pdf | title=Safeguarding Nuclear Weapon-Usable Materials in Russia | author=Thomas B. Cochran ([[Natural Resources Defense Council]]) | publisher=Proceedings of international forum on illegal nuclear traffic| date = 12 June 1997}}</ref> produced mostly for [[nuclear weapons]], [[Nuclear marine propulsion|naval propulsion]], and smaller quantities for [[research reactor]]s.
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| The U<sup>238</sup> remaining after enrichment is known as [[depleted uranium]] (DU), and is considerably less [[radioactive]] than even natural uranium, though still very dense and extremely hazardous in granulated form – such granules are a natural by-product of the shearing action that makes it useful for [[Vehicle armor|armor]]-[[Staballoy|penetrating weapons]] and [[radiation shielding]]. At present, 95% of the world's stocks of depleted uranium remain in secure storage.
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| ==Grades==
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| ===Slightly enriched uranium (SEU)===
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| [[File:LEUPowder.jpg|thumb|A drum of [[yellowcake]] (a mixture of uranium precipitates)]]
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| ''Slightly enriched uranium'' (SEU) has a <sup>235</sup>U concentration of 0.9% to 2%. This new grade can be used to replace [[natural uranium]] (NU) fuel in some [[heavy water reactor]]s like the [[CANDU]]. Fuel designed with SEU could provide additional benefits such as safety improvements or operational flexibility, normally the benefits were considered in safety area while retaining the same operational envelope. Safety improvements could lower positive reactivity feedback such as reactivity void coefficient. Operational improvements would consist in increasing the fuel burnup allowing fuel costs reduction because less uranium and fewer bundles are needed to fuel the reactor. This in turn reduces the quantity of used fuel and its subsequent management costs.{{cn|date=September 2012}}
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| ===Reprocessed uranium (RepU)===
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| {{Main|Reprocessed uranium}}
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| ''Reprocessed uranium'' (RepU) is a product of [[nuclear fuel cycle]]s involving [[nuclear reprocessing]] of [[spent fuel]]. RepU recovered from [[light water reactor]] (LWR) spent fuel typically contains slightly more U-235 than [[natural uranium]], and therefore could be used to fuel reactors that customarily use natural uranium as fuel, such as [[CANDU reactor]]s. It also contains the undesirable isotope [[uranium-236]] which undergoes [[neutron capture]], wasting neutrons (and requiring higher U-235 enrichment) and creating [[neptunium-237]] which would be one of the more mobile and troublesome radionuclides in [[deep geological repository]] disposal of nuclear waste.
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| ===Low-enriched uranium (LEU)===
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| ''Low-enriched uranium'' (LEU) has a lower than 20% concentration of <sup>235</sup>U. | |
| For use in commercial [[light water reactor]]s (LWR), the most prevalent power reactors in the world, uranium is enriched to 3 to 5% <sup>235</sup>U. Fresh LEU used in [[research reactor]]s is usually enriched 12% to 19.75% U-235, the latter concentration being used to replace HEU fuels when converting to LEU.
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| ===Highly enriched uranium (HEU)===
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| [[File:HEUraniumC.jpg|thumb|A [[billet (bar stock)|billet]] of highly enriched uranium metal]]
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| Highly enriched uranium (HEU) has a greater than 20% concentration of <sup>235</sup>U or <sup>233</sup>U. The fissile uranium in [[nuclear weapon]] primaries usually contains 85% or more of <sup>235</sup>U known as weapon(s)-grade, though theoretically for an [[nuclear weapon design|implosion design]], a minimum of 20% could be sufficient (called weapon(s)-usable) although it would require hundreds of kilograms of material and "would not be practical to design";<ref name="DefWpnsUsable">{{cite web |url= http://web.ornl.gov/info/reports/1998/3445606060721.pdf |title= Definition of Weapons-Usable Uranium-233 |last1= Forsberg |first1= C. W. |last2= Hopper |first2= C. M. |last3= Richter |first3= J. L. |last4= Vantine |first4= H. C. |date= March 1998 |work= ORNL/TM-13517 |publisher= Oak Ridge National Laboratories |accessdate=30 October 2013}}</ref><ref name="NWFAQ">{{cite web |url= http://www.nuclearweaponarchive.org/Nwfaq/Nfaq4-1.html#Nfaq4.1.7.1 |title= Nuclear Weapons FAQ, Section 4.1.7.1: Nuclear Design Principles – Highly Enriched Uranium |last1= Sublette |first1= Carey |date= 4 October 1996 |work= Nuclear Weapons FAQ |accessdate=2 October 2010}}</ref> even lower enrichment is hypothetically possible, but as the enrichment percentage decreases the [[Critical mass (nuclear)|critical mass]] for unmoderated [[fast neutron]]s rapidly increases, with for example, an [[infinite]] mass of 5.4% <sup>235</sup>U being required.<ref name="DefWpnsUsable">{{cite web |url= http://web.ornl.gov/info/reports/1998/3445606060721.pdf |title= Definition of Weapons-Usable Uranium-233 |last1= Forsberg |first1= C. W. |last2= Hopper |first2= C. M. |last3= Richter |first3= J. L. |last4= Vantine |first4= H. C. |date= March 1998 |work= ORNL/TM-13517 |publisher= Oak Ridge National Laboratories |accessdate=30 October 2013}}</ref> For criticality experiments, enrichment of uranium to over 97% has been accomplished.<ref>{{cite journal |last=Mosteller |first=R.D. |year=1994 |title=Detailed Reanalysis of a Benchmark Critical Experiment: Water-Reflected Enriched-Uranium Sphere |journal=Los Alamos technical paper |issue=LA–UR–93–4097 |page=2 |url=http://www.osti.gov/bridge/servlets/purl/10120434-rruwqp/native/10120434.PDF |accessdate=19 December 2007 |quote=The enrichment of the pin and of one of the hemispheres was 97.67 w/o, while the enrichment of the other hemisphere was 97.68 w/o.}}</ref>
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| The very first uranium bomb, [[Little Boy]] dropped by the [[United States]] on [[Hiroshima]] in 1945, used 64 kilograms of 80% enriched uranium. Wrapping the weapon's fissile core in a [[neutron reflector]] (which is standard on all nuclear explosives) can dramatically reduce the [[critical mass]]. Because the core was surrounded by a good neutron reflector, at explosion it comprised almost 2.5 critical masses. Neutron reflectors, compressing the fissile core via implosion, fusion boosting, and "tamping", which slows the expansion of the fissioning core with inertia, allow [[nuclear weapon design]]s that use less than what would be one bare-sphere critical mass at normal density. The presence of too much of the <sup>238</sup>U isotope inhibits the runaway [[nuclear chain reaction]] that is responsible for the weapon's power. The critical mass for 85% highly enriched uranium is about {{convert|50|kg}}, which at normal density would be a sphere about {{convert|17|cm}} in diameter.
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| Later US nuclear weapons usually use [[plutonium-239]] in the primary stage, but the secondary stage which is compressed by the primary nuclear explosion often uses HEU with enrichment between 40% and 80%<ref>{{cite web|url=http://nuclearweaponarchive.org/Nwfaq/Nfaq6.html#nfaq6.2 |title=Nuclear Weapons FAQ |publisher=Nuclearweaponarchive.org |date= |accessdate=2013-01-26}}</ref>
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| along with the [[nuclear fusion|fusion]] fuel [[lithium deuteride]]. For the secondary of a large nuclear weapon, the higher critical mass of less-enriched uranium can be an advantage as it allows the core at explosion time to contain a larger amount of fuel. The <sup>238</sup>U is not [[fissile]] but still fissionable by [[fusion neutron]]s.
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| HEU is also used in [[fast neutron reactor]]s, whose cores require about 20% or more of fissile material, as well as in [[Nuclear marine propulsion|naval reactors]], where it often contains at least 50% <sup>235</sup>U, but typically does not exceed 90%. The Fermi-1 commercial fast reactor prototype used HEU with 26.5% <sup>235</sup>U. Significant quantities of HEU are used in the production of [[medical isotopes]], for example [[molybdenum-99]] for [[technetium-99m generator]]s.<ref>{{cite journal |url=http://www.informaworld.com/smpp/content~content=a769414426~db=all |title=Feasibility of Eliminating the Use of Highly Enriched Uranium in the Production of Medical Radioisotopes |author=Frank N. Von Hippel, Laura H. Kahn |journal=Science & Global Security |volume=14 |issue= 2 & 3|date=December 2006 |pages=151–162 |doi=10.1080/08929880600993071 |accessdate=26 March 2010}}</ref>
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| ==Enrichment methods==
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| [[Isotope separation]] is difficult because two isotopes of the same elements have very nearly identical chemical properties, and can only be separated gradually using small mass differences. (<sup>235</sup>U is only 1.26% lighter than <sup>238</sup>U.) This problem is compounded by the fact that uranium is rarely separated in its atomic form, but instead as a compound (<sup>235</sup>UF<sub>6</sub> is only 0.852% lighter than <sup>238</sup>UF<sub>6</sub>.)
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| A [[cascade (chemical engineering)|cascade]] of identical stages produces successively higher concentrations of <sup>235</sup>U. Each stage passes a slightly more concentrated product to the next stage and returns a slightly less concentrated residue to the previous stage.
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| There are currently two generic commercial methods employed internationally for enrichment: [[gaseous diffusion]] (referred to as ''first'' generation) and [[gas centrifuge]] (''second'' generation) which consumes only 2% to 2.5%<ref>http://www.world-nuclear.org/info/Nuclear-Fuel-Cycle/Conversion-Enrichment-and-Fabrication/Uranium-Enrichment/#.UWrver-IRAs</ref> as much energy as gaseous diffusion. Later generation methods will become established because they will be more efficient in terms of the energy input for the same degree of enrichment and the next method of enrichment to be commercialized will be referred to as ''third'' generation. Some work is being done that would use [[Nuclear magnetic resonance|nuclear resonance]]; however there is no reliable evidence that any nuclear resonance processes have been scaled up to production.
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| ===Diffusion techniques===
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| ====Gaseous diffusion====
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| {{Main|Gaseous diffusion}}
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| Gaseous diffusion is a technology used to produce enriched uranium by forcing gaseous [[uranium hexafluoride]] (''hex'') through [[semi-permeable membrane]]s. This produces a slight separation between the molecules containing <sup>235</sup>U and <sup>238</sup>U. Throughout the [[Cold War]], gaseous diffusion played a major role as a uranium enrichment technique, and as of 2008 accounted for about 33% of enriched uranium production,<ref name="Lodge">{{cite web|url=http://www.asx.com.au/asxpdf/20080410/pdf/318j6y3ctrzwqf.pdf | title=Lodge Partners Mid-Cap Conference 11 April 2008 | publisher=Silex Ltd | date = 11 April 2008}}</ref> but is now an obsolete technology that is steadily being replaced by the later generations of technology as the diffusion plants reach their ends-of-life.<ref>{{cite web|author=Rod Adams|url=http://atomicinsights.com/2011/05/mcconnell-asks-doe-to-keep-using-60-year-old-enrichment-plant-to-save-jobs.html |title=McConnell asks DOE to keep using 60 year old enrichment plant to save jobs |publisher=Atomic Insights |date=2011-05-24 |accessdate=2013-01-26}}</ref>
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| ====Thermal diffusion====
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| Thermal diffusion utilizes the transfer of heat across a thin liquid or gas to accomplish isotope separation. The process exploits the fact that the lighter <sup>235</sup>U gas molecules will diffuse toward a hot surface, and the heavier <sup>238</sup>U gas molecules will diffuse toward a cold surface. The [[S-50 (Manhattan Project)|S-50]] plant at [[Oak Ridge, Tennessee]] was used during [[World War II]] to prepare feed material for the [[#Electromagnetic isotope separation|EMIS]] process. It was abandoned in favor of gaseous diffusion.
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| ===Centrifuge techniques===
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| ====Gas centrifuge====
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| {{Main|Gas centrifuge}}
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| [[File:Gas centrifuge cascade.jpg|thumb|A cascade of gas centrifuges at a U.S. enrichment plant]]
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| The gas centrifuge process uses a large number of rotating cylinders in series and parallel formations. Each cylinder's rotation creates a strong [[centrifugal force]] so that the heavier gas molecules containing <sup>238</sup>U move toward the outside of the cylinder and the lighter gas molecules rich in <sup>235</sup>U collect closer to the center. It requires much less energy to achieve the same separation than the older gaseous diffusion process, which it has largely replaced and so is the current method of choice and is termed ''second generation''. It has a separation factor per stage of 1.3 relative to gaseous diffusion of 1.005,<ref name="Lodge" /> which translates to about one-fiftieth of the energy requirements. Gas centrifuge techniques produce about 54% of the world's enriched uranium.
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| ====Zippe centrifuge====
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| [[File:Zippe-type gas centrifuge.svg|left|thumb|upright|Diagram of the principles of a Zippe-type gas centrifuge with U-238 represented in dark blue and U-235 represented in light blue]]
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| The [[Zippe centrifuge]] is an improvement on the standard gas centrifuge, the primary difference being the use of heat. The bottom of the rotating cylinder is heated, producing convection currents that move the <sup>235</sup>U up the cylinder, where it can be collected by scoops. This improved centrifuge design is used commercially by [[Urenco Group|Urenco]] to produce nuclear fuel and was used by [[Pakistan]] in their nuclear weapons program.
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| ===Laser techniques===
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| Laser processes promise lower energy inputs, lower capital costs and lower tails assays, hence significant economic advantages. Several laser processes have been investigated or are under development. Separation of Isotopes by Laser Excitation ([[SILEX]]) is well advanced and licensed for commercial operation in 2012.
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| ====Atomic vapor laser isotope separation (AVLIS)====
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| ''[[AVLIS|Atomic vapor laser isotope separation]]'' employs specially tuned lasers<ref>[[F. J. Duarte]] and L.W. Hillman (Eds.), ''Dye Laser Principles'' (Academic, New York, 1990) Chapter 9.</ref> to separate isotopes of uranium using selective ionization of hyperfine transitions. The technique uses [[laser]]s which are tuned to frequencies that ionize <sup>235</sup>U atoms and no others. The positively charged <sup>235</sup>U ions are then attracted to a negatively charged plate and collected.
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| ====Molecular laser isotope separation (MLIS)====
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| ''[[Molecular laser isotope separation]]'' uses an infrared laser directed at [[Uranium hexafluoride|UF<sub>6</sub>]], exciting molecules that contain a <sup>235</sup>U atom. A second laser frees a [[fluorine]] atom, leaving [[uranium pentafluoride]] which then precipitates out of the gas.
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| ====Separation of Isotopes by Laser Excitation (SILEX)====
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| ''[[SILEX|Separation of isotopes by laser excitation]]'' is an Australian development that also uses [[Uranium hexafluoride|UF<sub>6</sub>]]. After a protracted development process involving U.S. enrichment company [[USEC]] acquiring and then relinquishing commercialization rights to the technology, [[GE Hitachi Nuclear Energy]] (GEH) signed a commercialization agreement with Silex Systems in 2006.<ref>[http://www.ge-energy.com/about/press/en/2006_press/052206b.htm ]{{dead link|date=January 2013}}</ref>{{dl|date=September 2012}} GEH has since built a demonstration test loop and announced plans to build an initial commercial facility.<ref>{{cite web|title=GE Hitachi Nuclear Energy Selects Wilmington, N.C. as Site for Potential Commercial Uranium Enrichment Facility|url=http://www.businesswire.com/portal/site/ge/index.jsp?ndmViewId=news_view&ndmConfigId=1004554&newsId=20080430006101&newsLang=en&vnsId=681|publisher=Business Wire|accessdate=30 September 2012|date=30 April 2008}}</ref> Details of the process are classified and restricted by intergovernmental agreements between United States, Australia, and the commercial entities. SILEX has been projected to be an order of magnitude more efficient than existing production techniques but again, the exact figure is classified.<ref name="Lodge" /> In August, 2011 Global Laser Enrichment, a subsidiary of GEH, applied to the U.S. [[Nuclear Regulatory Commission]] (NRC) for a permit to build a commercial plant.<ref>{{cite news |last=Broad |first=William J. |title=Laser Advances in Nuclear Fuel Stir Terror Fear |url=http://www.nytimes.com/2011/08/21/science/earth/21laser.html |accessdate=21 August 2011 |newspaper=[[The New York Times]] |date=20 August 2011}}</ref> In September 2012, the NRC issued a license for GEH to build and operate a commercial SILEX enrichment plant, although the company had not yet decided whether the project would be profitable enough to begin construction, and despite concerns that the technology could contribute to [[nuclear proliferation]].<ref>[http://www.nytimes.com/2012/09/28/business/energy-environment/uranium-plant-using-laser-technology-wins-us-approval.html?ref=science&_r=0 New York Times, Uranium Plant Using Laser Technology Wins U.S. Approval, September 2012]</ref>
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| ===Other techniques===
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| ====Aerodynamic processes====
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| [[File:Aerodynamic enrichment nozzle.svg|thumb|Schematic diagram of an aerodynamic nozzle. Many thousands of these small foils would be combined in an enrichment unit.]]
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| Aerodynamic enrichment processes include the Becker jet nozzle techniques developed by E. W. Becker and associates using the [[LIGA]] process and the [[vortex tube]] separation process. These [[aerodynamic]] separation processes depend upon diffusion driven by pressure gradients, as does the gas centrifuge. They in general have the disadvantage of requiring complex systems of cascading of individual separating elements to minimize energy consumption. In effect, aerodynamic processes can be considered as non-rotating centrifuges. Enhancement of the centrifugal forces is achieved by dilution of [[Uranium hexafluoride|UF<sub>6</sub>]] with [[hydrogen]] or [[helium]] as a carrier gas achieving a much higher flow velocity for the gas than could be obtained using pure uranium hexafluoride. The [[NECSA|Uranium Enrichment Corporation of South Africa]] (UCOR) developed and deployed the continuous Helikon vortex separation cascade for high production rate low enrichment and the substantially different semi-batch Pelsakon low production rate high enrichment cascade both using a particular vortex tube separator design, and both embodied in industrial plant.<ref name="The Pelsakon Cascade for Uranium Enrichment">{{cite journal|last=Smith|first=Michael|coauthors=Jackson A G M|title=Dr|journal=S a Institution of Chemical Engineers – Conference 2000|year=2000|pages=280–289}}</ref> A demonstration plant was built in [[Brazil]] by NUCLEI, a consortium led by Industrias Nucleares do Brasil that used the separation nozzle process. However all methods have high energy consumption and substantial requirements for removal of waste heat; none is currently still in use.
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| ====Electromagnetic isotope separation====
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| {{Main|Calutron}}
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| [[File:Electromagnetic separation.svg|thumb|Schematic diagram of uranium isotope separation in a [[calutron]] shows how a strong magnetic field is used to redirect a stream of uranium ions to a target, resulting in a higher concentration of uranium-235 (represented here in dark blue) in the inner fringes of the stream.]]
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| In the [[electromagnetic isotope separation]] process (EMIS), metallic uranium is first vaporized, and then ionized to positively charged ions. The cations are then accelerated and subsequently deflected by magnetic fields onto their respective collection targets. A production-scale [[mass spectrometer]] named the [[Calutron]] was developed during World War II that provided some of the <sup>235</sup>U used for the [[Little Boy]] nuclear bomb, which was dropped over [[Hiroshima]] in 1945. Properly the term 'Calutron' applies to a multistage device arranged in a large oval around a powerful electromagnet. Electromagnetic isotope separation has been largely abandoned in favour of more effective methods.
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| ====Chemical methods====
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| One chemical process has been demonstrated to pilot plant stage but not used. The French CHEMEX process exploited a very slight difference in the two isotopes' propensity to change [[Valence (chemistry)|valency]] in [[redox|oxidation/reduction]], utilising immiscible aqueous and organic phases. An ion-exchange process was developed by the Asahi Chemical Company in [[Japan]] which applies similar chemistry but effects separation on a proprietary resin [[ion-exchange]] column.
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| ====Plasma separation====
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| Plasma separation process (PSP) describes a technique that makes use of [[superconducting magnet]]s and [[plasma physics]]. In this process, the principle of [[ion cyclotron resonance]] is used to selectively energize the <sup>235</sup>U isotope in a [[Plasma (physics)|plasma]] containing a mix of [[ion]]s. The French developed their own version of PSP, which they called RCI. Funding for RCI was drastically reduced in 1986, and the program was suspended around 1990, although RCI is still used for stable isotope separation.
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| ==Separative work unit==
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| "Separative work" – the amount of separation done by an enrichment process – is a function of the concentrations of the feedstock, the enriched output, and the depleted tailings; and is expressed in units which are so calculated as to be proportional to the total input (energy / machine operation time) and to the mass processed. Separative work is ''not'' energy. The same amount of separative work will require different amounts of energy depending on the efficiency of the separation technology. Separative work is measured in ''Separative work units'' SWU, kg SW, or kg UTA (from the German ''Urantrennarbeit'' – literally ''uranium separation work'')
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| * 1 SWU = 1 kg SW = 1 kg UTA
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| * 1 kSWU = 1 tSW = 1 t UTA
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| * 1 MSWU = 1 ktSW = 1 kt UTA
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| The work <math>W_\mathrm{SWU}</math> necessary to separate a mass <math>F</math> of feed of assay <math>x_{f}</math> into a mass <math>P</math> of product assay <math>x_{p}</math>, and tails of mass <math>T</math> and assay <math>x_{t}</math> is given by the expression
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| :<math>W_\mathrm{SWU} = P \cdot V\left(x_{p}\right)+T \cdot V(x_{t})-F \cdot V(x_{f})</math>
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| where <math>V\left(x\right)</math> is the [[value function]], defined as
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| :<math>V(x) = (1 - 2x) \ln \left(\frac{1 - x}{x}\right)</math>
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| The feed to product ratio is given by the expression
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| :<math>\frac{F}{P} = \frac{x_{p} - x_{t}}{x_{f} - x_{t}}</math>
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| whereas the tails to product ratio is given by the expression
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| :<math>\frac{T}{P} = \frac{x_{p} - x_{f}}{x_{f} - x_{t}}</math>
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| For example, beginning with {{convert|102|kg}} of NU, it takes about 62 SWU to produce {{convert|10|kg}} of LEU in <sup>235</sup>U content to 4.5%, at a tails assay of 0.3%.
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| The number of separative work units provided by an enrichment facility is directly related to the amount of energy that the facility consumes. Modern gaseous diffusion plants typically require 2,400 to 2,500 [[kilowatt-hour]]s (kW·h), or 8.6–9 [[gigajoule]]s, (GJ) of electricity per SWU while gas centrifuge plants require just 50 to 60 kW·h (180–220 MJ) of electricity per SWU.
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| ''Example:''
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| A large nuclear power station with a net electrical capacity of 1300 MW requires about 25 tonnes per year (25 [[Tonne|t]]/[[Julian year (astronomy)|a]]) of LEU with a <sup>235</sup>U concentration of 3.75%. This quantity is produced from about 210 t of NU using about 120 kSWU. An enrichment plant with a capacity of 1000 kSWU/a is, therefore, able to enrich the uranium needed to fuel about eight large nuclear power stations. | |
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| ==Cost issues==
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| In addition to the separative work units provided by an enrichment facility, the other important parameter to be considered is the mass of natural uranium (NU) that is needed to yield a desired mass of enriched uranium. As with the number of SWUs, the amount of feed material required will also depend on the level of enrichment desired and upon the amount of <sup>235</sup>U that ends up in the depleted uranium. However, unlike the number of SWUs required during enrichment which increases with decreasing levels of <sup>235</sup>U in the depleted stream, the amount of NU needed will decrease with decreasing levels of <sup>235</sup>U that end up in the DU.
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| For example, in the enrichment of LEU for use in a light water reactor it is typical for the enriched stream to contain 3.6% <sup>235</sup>U (as compared to 0.7% in NU) while the depleted stream contains 0.2% to 0.3% <sup>235</sup>U. In order to produce one kilogram of this LEU it would require approximately 8 kilograms of NU and 4.5 SWU if the DU stream was allowed to have 0.3% <sup>235</sup>U. On the other hand, if the depleted stream had only 0.2% <sup>235</sup>U, then it would require just 6.7 kilograms of NU, but nearly 5.7 SWU of enrichment. Because the amount of NU required and the number of SWUs required during enrichment change in opposite directions, if NU is cheap and enrichment services are more expensive, then the operators will typically choose to allow more <sup>235</sup>U to be left in the DU stream whereas if NU is more expensive and enrichment is less so, then they would choose the opposite.
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| * [http://www.wise-uranium.org/nfcue.html Uranium enrichment calculator designed by the WISE Uranium Project]
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| ==Downblending==<!-- This section is linked from [[Radioactive waste]] -->
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| The opposite of enriching is downblending; surplus HEU can be downblended to LEU to make it suitable for use in commercial nuclear fuel.
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| The HEU feedstock can contain unwanted uranium isotopes: [[Uranium-234|<sup>234</sup>U]] is a minor isotope contained in [[natural uranium]]; during the enrichment process, its concentration increases but remains well below 1%. High concentrations of [[Uranium-236|<sup>236</sup>U]] are a byproduct from irradiation in a reactor and may be contained in the HEU, depending on its manufacturing history. HEU reprocessed from nuclear weapons material production reactors (with an <sup>235</sup>U assay of approx. 50%) may contain <sup>236</sup>U concentrations as high as 25%, resulting in concentrations of approximately 1.5% in the blended LEU product. [[Uranium-236|<sup>236</sup>U]] is a [[neutron poison]]; therefore the actual <sup>235</sup>U concentration in the LEU product must be raised accordingly to compensate for the presence of <sup>236</sup>U. | |
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| The blendstock can be NU, or DU, however depending on feedstock quality, SEU at typically 1.5 wt% <sup>235</sup>U may used as a blendstock to dilute the unwanted byproducts that may be contained in the HEU feed. Concentrations of these isotopes in the LEU product in some cases could exceed [[ASTM]] specifications for nuclear fuel, if NU, or DU were used. So, the HEU downblending generally cannot contribute to the waste management problem posed by the existing large stockpiles of depleted uranium.
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| A major downblending undertaking called the [[Megatons to Megawatts Program]] converts ex-Soviet weapons-grade HEU to fuel for U.S. commercial power reactors. From 1995 through mid-2005, 250 tonnes of high-enriched uranium (enough for 10,000 warheads) was recycled into low-enriched-uranium. The goal is to recycle 500 tonnes by 2013. The decommissioning programme of Russian nuclear warheads accounted for about 13% of total world requirement for enriched uranium leading up to 2008.<ref name="Lodge" />
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| The [[United States Enrichment Corporation]] has been involved in the disposition of a portion of the 174.3 tonnes of highly enriched uranium (HEU) that the U.S. government declared as surplus military material in 1996. Through the U.S. HEU Downblending Program, this HEU material, taken primarily from dismantled U.S. nuclear warheads, was recycled into low-enriched uranium (LEU) fuel, used by [[nuclear power plants]] to generate electricity.<ref>[http://www.usec.com/v2001_02/HTML/Megatons_DOEstatus.asp ]{{dead link|date=January 2013}}</ref>
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| * [http://www.wise-uranium.org/nfcub.html A uranium downblending calculator designed by the WISE Uranium Project]
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| ==Global enrichment facilities==
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| The following countries are known to operate enrichment facilities: Argentina, Brazil, China, France, Germany, India, Iran, Japan, the Netherlands, North Korea, Pakistan, Russia, the United Kingdom, and the United States.<ref name=IEER-2004>{{Cite book|url=http://www.ieer.org/reports/uranium/enrichment.pdf|title=Uranium enrichment|author=Arjun Makhijani, Lois Chalmers, Brice Smith|date=15 October 2004|publisher=Institute for Energy and Environmental Research|accessdate=21 November 2009}}</ref> Belgium, Iran, Italy, and Spain hold an investment interest in the French [[Eurodif]] enrichment plant, with [[Dominique Lorentz#Eurodif and Iran's nuclear program|Iran's holding]] entitling it to 10% of the enriched uranium output. Countries that had enrichment programs in the past include Libya and South Africa, although Libya's facility was never operational.<ref>{{cite news|url=http://news.bbc.co.uk/1/hi/world/middle_east/5278806.stm | title=Q&A: Uranium enrichment | author=BBC | date = 1 September 2006 | accessdate=3 January 2010 | work=BBC News}}</ref> Australia has developed a [[Atomic vapor laser isotope separation|laser enrichment]] process known as SILEX, which it intends to pursue through financial investment in a U.S. commercial venture by General Electric.<ref>{{cite news|url=http://www.smh.com.au/news/national/laser-enrichment-could-cut-cost-of-nuclear-power/2006/05/26/1148524888448.html|title=Laser enrichment could cut cost of nuclear power|publisher=The Sydney Morning Herald|date=26 May 2006}}</ref> It has also been claimed that Israel has a uranium enrichment program housed at the [[Negev Nuclear Research Center]] site near [[Dimona]].<ref name=nwa-19971210>{{cite web|url=http://nuclearweaponarchive.org/Israel/|date=10 December 1997|title=Israel's Nuclear Weapons Program|publisher=Nuclear Weapon Archive|accessdate=7 October 2007}}</ref>
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| ==See also==
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| * [[Areva]]
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| * [[List of laser articles]]
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| * [[Nuclear fuel bank]]
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| * [[Nuclear power]]
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| * [[Uranium market]]
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| * [[Uranium mining]]
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| ==References==
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| {{reflist|30em}}
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| ==External links==
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| {{Wiktionary}}
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| * [http://alsos.wlu.edu/qsearch.aspx?browse=science/Enriching+Uranium Annotated bibliography on enriched uranium from the Alsos Digital Library for Nuclear Issues]
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| * [http://www.silex.com.au Silex Systems Ltd]
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| * [http://world-nuclear.org/info/inf28.html Uranium Enrichment], World Nuclear Association
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| * [http://www.fas.org/sgp/othergov/doe/heu/index.html Overview and history of U.S. HEU production]
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| * [http://www.huliq.com/tags/uranium-enrichment News Resource on Uranium Enrichment]
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| * [http://www.chemcases.com/nuclear/nc-07.htm Nuclear Chemistry-Uranium Enrichment]
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| * [http://www.neimagazine.com/story.asp?storyCode=2050947 A busy year for SWU (a 2008 review of the commercial enrichment marketplace)], Nuclear Engineering International, 1 September 2008
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| * [http://books.sipri.org/product_info?c_product_id=286 ''Uranium Enrichment and Nuclear Weapon Proliferation,'' by Allan S. Krass, Peter Boskma, Boelie Elzen and Wim A. Smit, 296 pp., Published for SIPRI by Taylor and Francis Ltd, London, 1983]
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| * {{cite web|last=Poliakoff|first=Martyn|title=How do you enrich Uranium?|url=http://www.periodicvideos.com/videos/feature_uranium_enrichment.htm|work=[[The Periodic Table of Videos]]|publisher=[[University of Nottingham]]|authorlink=Martyn Poliakoff|year=2009}}
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| {{Nuclear Technology}}
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| {{DEFAULTSORT:Enriched Uranium}}
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| [[Category:Isotope separation]]
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| [[Category:Nuclear fuels]]
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| [[Category:Nuclear materials|Uranium, Enriched]]
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| [[Category:Nuclear weapon design]]
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| [[Category:Uranium]]
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| {{Link FA|he}}
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