Editing Enriched uranium (section)

==Enrichment methods==
[[Isotope separation]] is difficult because two isotopes of the same element have 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 because 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>).
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.

There are currently two 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>{{cite web |url=http://www.world-nuclear.org/info/Nuclear-Fuel-Cycle/Conversion-Enrichment-and-Fabrication/Uranium-Enrichment/ |title=Uranium Enrichment |publisher=world-nuclear.org |access-date=14 April 2013 |archive-date=1 July 2013 |archive-url=https://web.archive.org/web/20130701071520/http://www.world-nuclear.org/info/Nuclear-Fuel-Cycle/Conversion-Enrichment-and-Fabrication/Uranium-Enrichment/ |url-status=dead }}</ref> as much energy as gaseous diffusion. 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.

===Diffusion techniques===

====Gaseous diffusion====
{{Main|Gaseous diffusion}}
[[File:Gaseous Diffusion (44021367082) (cropped).jpg|thumb|Gaseous diffusion uses semi-permeable membranes to separate enriched uranium]]

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 |archive-url=https://ghostarchive.org/archive/20221009/http://www.asx.com.au/asxpdf/20080410/pdf/318j6y3ctrzwqf.pdf |archive-date=2022-10-09 |url-status=live | title=Lodge Partners Mid-Cap Conference 11&nbsp;April 2008 | publisher=Silex Ltd |date=11 April 2008}}</ref> but in 2011 was deemed 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 |last=Adams |first=Rod |date=24 May 2011 |title=McConnell asks DOE to keep using 60-year-old enrichment plant to save jobs |url=http://atomicinsights.com/2011/05/mcconnell-asks-doe-to-keep-using-60-year-old-enrichment-plant-to-save-jobs.html |url-status=dead |archive-url=http://archive.wikiwix.com/cache/20130128020737/http://atomicinsights.com/2011/05/mcconnell-asks-doe-to-keep-using-60-year-old-enrichment-plant-to-save-jobs.html |archive-date=28 January 2013 |access-date=26 January 2013 |publisher=Atomic Insights}}</ref> In 2013, the [[Paducah Gaseous Diffusion Plant|Paducah]] facility in the U.S. ceased operating; it was the last commercial <sup>235</sup>U gaseous diffusion plant in the world.<ref>{{Cite web|url=https://www.world-nuclear-news.org/ENF_Paducah_enrichment_plant_to_be_closed_2805132.html|title=Paducah enrichment plant to be closed|website=World Nuclear News}}</ref>

====Thermal diffusion====
Thermal diffusion uses the transfer of heat across a thin liquid or gas to accomplish isotope separation.<ref name="Lei Science Direct"/>  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, explained later in this article. It was abandoned in favor of gaseous diffusion.

===Centrifuge techniques===

====Gas centrifuge====
{{Main|Gas centrifuge}}

[[File:Gas centrifuge cascade.jpg|thumb|A cascade of gas centrifuges at a U.S. enrichment plant]]
The gas centrifuge process uses a large number of rotating cylinders in series and parallel formations. Each cylinder's rotation creates a strong [[centripetal force]] so that the heavier gas molecules containing <sup>238</sup>U move tangentially 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 close to 100% of the world's enriched uranium. The cost per separative work unit is approximately 100 dollars per [[Separative Work Units]] (SWU), making it about 40% cheaper than standard gaseous diffusion techniques.<ref name=Weinberger2012>{{cite journal |last=Weinberger |first=Sharon |title=US grants licence for uranium laser enrichment |journal=Nature |date=28 September 2012 |pages=nature.2012.11502 |doi=10.1038/nature.2012.11502 |s2cid=100862135 |doi-access=free }}</ref>

====Zippe centrifuge====
{{main|Zippe-type centrifuge}}
[[File:Zippe-type gas centrifuge.svg|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]]

The Zippe-type 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 its nuclear weapons program.

===Laser techniques===
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 developed and is licensed for commercial operation as of 2012. Separation of isotopes by laser excitation is a very effective and cheap method of uranium separation, able to be done in small facilities requiring much less energy and space than previous separation techniques. The cost of uranium enrichment using laser enrichment technologies is approximately $30 per SWU<ref name=Weinberger2012/> which is less than a third of the price of gas centrifuges, the current standard of enrichment. Separation of isotopes by laser excitation could be done in facilities virtually undetectable by satellites.<ref name="Slakey & Cohen 2010">{{cite journal |last1=Slakey |first1=Francis |last2=Cohen |first2=Linda R. |title=Stop laser uranium enrichment |journal=Nature |date=March 2010 |volume=464 |issue=7285 |pages=32–33 |id={{ProQuest|204555310}} |doi=10.1038/464032a |pmid=20203589 |bibcode=2010Natur.464...32S |s2cid=205053626 }}</ref> More than 20 countries have worked with laser separation over the past two decades, the most notable of these countries being Iran and North Korea, though all countries have had very limited success up to this point.  

====Atomic vapor laser isotope separation (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 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.

====Molecular laser isotope separation (MLIS)====
[[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.

====Separation of isotopes by laser excitation (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>{{cite press release|url =http://www.ge-energy.com/about/press/en/2006_press/052206b.htm |archive-url=https://web.archive.org/web/20060614092643/http://www.ge-energy.com/about/press/en/2006_press/052206b.htm|archive-date=14 June 2006|title = GE Signs Agreement With Silex Systems of Australia To Develop Uranium Enrichment Technology|date =22 May 2006|publisher = GE Energy }}</ref> 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|access-date=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. |authorlink=William Broad |date=20 August 2011 |title=Laser Advances in Nuclear Fuel Stir Terror Fear |newspaper=[[The New York Times]] |url=https://www.nytimes.com/2011/08/21/science/earth/21laser.html |access-date=21 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>{{cite news |last=Associated Press |date=27 September 2012 |title=Uranium Plant Using Laser Technology Wins U.S. Approval |work=The New York Times |url=https://www.nytimes.com/2012/09/28/business/energy-environment/uranium-plant-using-laser-technology-wins-us-approval.html}}</ref> The fear of nuclear proliferation arose in part due to laser separation technology requiring less than 25% of the space of typical separation techniques, as well as requiring only the energy that would power 12 typical houses, putting a laser separation plant that works by means of laser excitation well below the detection threshold of existing surveillance technologies.<ref name="Slakey & Cohen 2010"/> Due to these concerns the [[American Physical Society]] filed a petition with the NRC, asking that before any laser excitation plants are built that they undergo a formal review of proliferation risks. The APS even went as far as calling the technology a "game changer"<ref name=Weinberger2012/> due to the ability for it to be hidden from any type of detection.

===Other techniques===

====Aerodynamic processes====
[[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.]]
[[File:LIGA-Doppelumlenksystem.jpg|right|thumb| The X-ray-based [[LIGA]] manufacturing process was originally developed at the Forschungszentrum Karlsruhe, Germany, to produce nozzles for isotope enrichment.<ref name=Becker-1982>{{Cite journal | last1=Becker |first1=E. W. | last2=Ehrfeld | first2=W. | last3=Münchmeyer | first3=D. | last4=Betz | first4=H. | last5=Heuberger | first5=A. | last6=Pongratz | first6=S. | last7=Glashauser | first7=W. | last8=Michel | first8=H. J. | last9=Siemens | first9=R. | title=Production of Separation-Nozzle Systems for Uranium Enrichment by a Combination of X-Ray Lithography and Galvanoplastics | journal=Naturwissenschaften | volume=69 | pages=520–523 | year=1982 | doi=10.1007/BF00463495 | issue=11 | bibcode=1982NW.....69..520B | s2cid=44245091 }}</ref>]]
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|author2=Jackson A G M|title=Dr|journal=South African 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. All methods have high energy consumption and substantial requirements for removal of waste heat; none is currently still in use.

====Electromagnetic isotope separation====
{{Main|Calutron}}
[[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.]]

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.

====Chemical methods====
One chemical process has been demonstrated to pilot plant stage but not used for production. The French CHEMEX process exploited a very slight difference in the two isotopes' propensity to change [[Valence (chemistry)|valency]] in [[redox|oxidation/reduction]], using immiscible aqueous and organic phases. An ion-exchange process was developed by the [[Asahi Chemical Company]] in Japan that applies similar chemistry but effects separation on a proprietary resin [[ion-exchange]] column.

====Plasma separation====
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. France developed its own version of PSP, which it 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.