View source for Enriched uranium

{{Short description|Uranium processed to increase the percentage of uranium-235}}
{{Use dmy dates|date=December 2020}}

'''Enriched uranium''' is a type of [[uranium]] in which the percent composition of [[uranium-235]] (written <sup>235</sup>U) has been increased through the process of [[isotope separation]]. Naturally occurring uranium is composed of three major [[Isotopes of uranium|isotopes]]:  [[uranium-238]] (<sup>238</sup>U with 99.2732–99.2752% [[natural abundance]]), [[uranium-235]] (<sup>235</sup>U, 0.7198–0.7210%), and [[uranium-234]] (<sup>234</sup>U, 0.0049–0.0059%).{{NUBASE2020|ref}} <sup>235</sup>U is the only [[primordial nuclide|nuclide existing in nature]] (in any appreciable amount) that is [[fissile]] with [[thermal neutron]]s.<ref>{{cite book|last=OECD Nuclear Energy Agency|title=Nuclear Energy Today|publisher=OECD Publishing|year=2003|isbn=978-92-64-10328-3|page=25|url=https://books.google.com/books?id=PvL7twdmK9sC&pg=PA25}}</ref>[[File:Uranium enrichment proportions.svg|thumb|Proportions of uranium-238 (blue) and uranium-235 (red) found naturally versus enriched grades]]

Enriched uranium is a critical component for both civil [[nuclear power]] generation and military [[nuclear weapon]]s. Low-enriched uranium (below 20% <sup>235</sup>U) is necessary to operate [[light water reactors]], which make up almost 90% of nuclear electricity generation.<ref name="c847">{{cite web |last=IAEA |date=2024-08-29 |title=In Operation & Suspended Operation |url=https://pris.iaea.org/PRIS/WorldStatistics/OperationalReactorsByType.aspx |access-date=2024-08-30 |website=PRIS}}</ref> Highly enriched uranium (above 20% <sup>235</sup>U) is used for the cores of many nuclear weapons, as well as compact reactors for [[Nuclear marine propulsion|naval propulsion]] and [[Research reactor|research]], as well as [[breeder reactors]]. There are about 2,000&nbsp;[[tonne]]s of highly enriched uranium in the world.<ref>{{cite web |last=Cochran ([[Natural Resources Defense Council]]) |first=Thomas B. |date=12 June 1997 |title=Safeguarding Nuclear Weapon-Usable Materials in Russia |url=http://docs.nrdc.org/nuclear/files/nuc_06129701a_185.pdf |archive-url=https://web.archive.org/web/20120722184311/http://docs.nrdc.org/nuclear/files/nuc_06129701a_185.pdf |archive-date=22 July 2012 |publisher=Proceedings of international forum on illegal nuclear traffic}}</ref>

Enrichment methods were first developed on a large scale by the [[Manhattan Project]]. Its gaseous diffusion method was used in the 1940s and 1950s, when the gas centrifuge method was developed in the Soviet Union, and became widespread.<ref>{{cite web |url=https://doi.org/10.1353/tech.2012.0046 |title=Project MUSE - The End of Manhattan: How the Gas Centrifuge Changed the Quest for Nuclear Weapons |format= |work= }}</ref>

The <sup>238</sup>U remaining after enrichment is known as [[depleted uranium]] (DU), and is considerably less [[radioactive]] than natural uranium, though still very dense. [[Depleted uranium]] is used as a [[radiation shielding]] material and for [[Armour-piercing ammunition|armor-penetrating weapons]].
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==Grades==
Uranium as it is taken directly from the Earth is not suitable as fuel for most nuclear reactors and requires additional processes to make it usable (the [[CANDU]] design is a notable exception). Uranium is mined either underground or in an open pit depending on the depth at which it is found. After the [[uranium ore]] is mined, it must go through a milling process to extract the uranium from the ore. 

This is accomplished by a combination of chemical processes with the end product being concentrated uranium oxide, which is known as "[[yellowcake]]", contains roughly 80% uranium whereas the original ore typically contains as little as 0.1% uranium.<ref>[https://world-nuclear.org/information-library/nuclear-fuel-cycle/introduction/nuclear-fuel-cycle-overview.aspx Nuclear Fuel Cycle Overview], ''Uranium milling''. World Nuclear Association, update April 2021</ref>

This yellowcake is further processed to obtain the desired form of uranium suitable for [[nuclear fuel]] production. After the milling process is complete, the uranium must next undergo a process of conversion, "to either [[uranium dioxide]], which can be used as the fuel for those types of reactors that do not require enriched uranium, or into [[uranium hexafluoride]], which can be enriched to produce fuel for the majority of types of reactors".<ref>{{cite web|url=https://www.hsdl.org/?view&did=770258 |title=Radiological Sources of Potential Exposure and/or Contamination |publisher=U.S. Army Center for Health Promotion and Preventive Medicine |page=27 |date=June 1999 |access-date=1 July 2019}}</ref> Naturally occurring uranium is made of a mixture of <sup>235</sup>U and <sup>238</sup>U. The <sup>235</sup>U is [[fissile]], meaning it is easily split with [[neutron]]s while the remainder is <sup>238</sup>U, but in nature, more than 99% of the extracted ore is <sup>238</sup>U. Most nuclear reactors require enriched uranium, which is uranium with higher concentrations of <sup>235</sup>U ranging between 3.5% and 4.5% (although a few reactor designs using a [[graphite]] or [[heavy water]] [[Neutron moderator|moderator]], such as the [[RBMK]] and [[CANDU]], are capable of operating with natural uranium as fuel). There are two commercial enrichment processes: [[gaseous diffusion]] and [[gas centrifugation]]. Both enrichment processes involve the use of uranium hexafluoride and produce enriched uranium oxide.<ref name="auto">{{Cite journal |last=Olander |first=Donald R. |date=1981-01-01 |title=The theory of uranium enrichment by the gas centrifuge |journal=Progress in Nuclear Energy |volume=8 |issue=1 |pages=1–33 |doi=10.1016/0149-1970(81)90026-3 |issn=0149-1970}}</ref>

[[File:LEUPowder.jpg|thumb|A drum of [[yellowcake]] (a mixture of uranium precipitates)]]

===Reprocessed uranium (RepU)===
{{Main|Reprocessed uranium}}

Reprocessed uranium (RepU) undergoes a series of chemical and physical treatments to extract usable uranium from spent nuclear fuel. 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 <sup>235</sup>U 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 <sup>235</sup>U enrichment) and creating [[neptunium-237]], which would be one of the more mobile and troublesome radionuclides in [[deep geological repository]] disposal of nuclear waste. Reprocessed uranium often carries traces of other transuranic elements and fission products, necessitating careful monitoring and management during fuel fabrication and reactor operation.

===Low-enriched uranium (LEU){{Anchor|Low-enriched uranium}}===
Low-enriched uranium (LEU) has a lower than 20% concentration of <sup>235</sup>U; for instance, in commercial LWR, the most prevalent power reactors in the world, uranium is enriched to 3 to 5% <sup>235</sup>U. {{anchor|SEU}}<!-- [[Slightly enriched uranium]] redirects here-->'''Slightly enriched uranium''' ('''SEU''') has a concentration of under 2% <sup>235</sup>U.<ref>{{cite journal |last1=Carter |first1=John P. |last2=Borrelli |first2=R.A. |title=Integral molten salt reactor neutron physics study using Monte Carlo N-particle code |journal=Nuclear Engineering and Design |date=August 2020 |volume=365 |article-number=110718 |doi=10.1016/j.nucengdes.2020.110718 |s2cid=225435681 |doi-access=free }}</ref>

===High-assay LEU (HALEU)===
High-assay LEU (HALEU) is enriched between 5% and 20%<ref>{{Cite web|url=https://www.energy.gov/sites/prod/files/2019/04/f61/HALEU%20Report%20to%20NEAC%20Committee%203-28-19%20%28FINAL%29.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.energy.gov/sites/prod/files/2019/04/f61/HALEU%20Report%20to%20NEAC%20Committee%203-28-19%20%28FINAL%29.pdf |archive-date=2022-10-09 |url-status=live|title=High-assay low enriched uranium|last=Herczeg|first=John W.|date=28 March 2019|website=energy.gov}}</ref> and is called for in many [[small modular reactor]] (SMR) designs.<ref name=nei-20240630>{{cite news |url=https://www.neimagazine.com/analysis/haleu-uf6-and-smr-fuel-fabrication/?cf-view |title=HALEU UF6 and SMR fuel fabrication |publisher=Nuclear Engineering International |date=30 June 2024 |access-date=16 July 2024}}</ref> Fresh LEU used in [[research reactor]]s is usually enriched between 12% and 19.75% <sup>235</sup>U; the latter concentration is used to replace HEU fuels when converting to LEU.<ref>{{cite conference |last=Glaser |first=Alexander |date=6 November 2005 |title=About the Enrichment Limit for Research Reactor Conversion: Why 20%? |url=http://www.princeton.edu/~aglaser/2005aglaser_why20percent.pdf |url-status=live |conference=The 27th International Meeting on Reduced Enrichment for Research and Test Reactors (RERTR |publisher=Princeton University |archive-url=https://ghostarchive.org/archive/20221009/http://www.princeton.edu/~aglaser/2005aglaser_why20percent.pdf |archive-date=2022-10-09 |access-date=18 April 2014}}</ref>

===Highly enriched uranium (HEU)===
[[File:HEUraniumC.jpg|thumb|A [[billet (bar stock)|billet]] of highly enriched uranium metal]]

Highly enriched uranium (HEU) has a 20% or higher concentration of <sup>235</sup>U. This high enrichment level is essential for nuclear weapons and certain specialized reactor designs. The fissile uranium in [[nuclear weapon]] primaries usually contains 85% or more of <sup>235</sup>U known as [[Weapons grade uranium|weapons grade]], though theoretically for an [[nuclear weapon design|implosion design]], a minimum of 20% could be sufficient (called weapon-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 |access-date= 30 October 2013 |archive-url= https://web.archive.org/web/20131102011417/http://web.ornl.gov/info/reports/1998/3445606060721.pdf |archive-date= 2 November 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&nbsp;– Highly Enriched Uranium |last= Sublette |first= Carey |date= 4 October 1996 |work= Nuclear Weapons FAQ |access-date=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 [[infinity|infinite]] mass of 5.4% <sup>235</sup>U being required.<ref name="DefWpnsUsable"/> For [[Criticality (status)|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 |archive-url=https://ghostarchive.org/archive/20221009/http://www.osti.gov/bridge/servlets/purl/10120434-rruwqp/native/10120434.PDF |archive-date=2022-10-09 |url-status=live |access-date=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.|doi=10.2172/10120434 }}</ref>

The first uranium bomb, [[Little Boy]], dropped by the United States on [[Hiroshima]] in 1945, used {{Convert|64|kg}} 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 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.<ref name="auto" /> The highly enriched uranium core used in the first Chinese nuclear test, [[Project 596]], weighed about 15 kilograms,<ref name="b963">{{cite web |date=2018-12-08 |title=你以为原子弹都是科学家造出来的吗？错！_澎湃号·政务_澎湃新闻-The Paper |url=https://www.thepaper.cn/newsDetail_forward_2719375 |access-date=2025-06-21 |website=thepaper.cn |language=zh}}</ref> corresponding to a diameter of 11.4 cm.

Later U.S. nuclear weapons usually use [[plutonium-239]] in the primary stage, but the jacket or tamper 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 |access-date=26 January 2013}}</ref> along with the [[nuclear fusion|fusion]] fuel [[lithium deuteride]]. This multi-stage design enhances the efficiency and effectiveness of nuclear weapons, allowing for greater control over the release of energy during detonation. 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. This design strategy optimizes the explosive yield and performance of advanced nuclear weapons systems. The <sup>238</sup>U is not said to be fissile but still is fissionable by fast neutrons (>2 MeV) such as the ones produced during [[deuterium–tritium fusion|D–T fusion]].<ref name="Lei Science Direct">{{Cite journal |last1=Lei |first1=Jia |last2=Liu |first2=Huanhuan |last3=Zhou |first3=Li |last4=Wang |first4=Yazhou |last5=Yu |first5=Kaifu |last6=Zhu |first6=Hui |last7=Wang |first7=Bo |last8=Zang |first8=Mengxuan |last9=Zhou |first9=Jian |last10=He |first10=Rong |last11=Zhu |first11=Wenkun |date=2023-09-01 |title=Progress and perspective in enrichment and separation of radionuclide uranium by biomass functional materials |url=https://www.sciencedirect.com/science/article/pii/S138589472303317X |journal=Chemical Engineering Journal |volume=471 |article-number=144586 |doi=10.1016/j.cej.2023.144586 |issn=1385-8947|url-access=subscription }}</ref>

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%. These specialized reactor systems rely on highly enriched uranium for their unique operational requirements, including high neutron flux and precise control over reactor dynamics. The [[Fermi 1|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 |last1=Von Hippel |first1=Frank N. |last2=Kahn |first2=Laura H. |date=December 2006 |title=Feasibility of Eliminating the Use of Highly Enriched Uranium in the Production of Medical Radioisotopes |journal=Science & Global Security |volume=14 |issue=2 & 3 |pages=151–162 |bibcode=2006S&GS...14..151V |doi=10.1080/08929880600993071 |s2cid=122507063}}</ref> The medical industry benefits from the unique properties of highly enriched uranium, which enable the efficient production of critical isotopes essential for diagnostic imaging and therapeutic applications.

==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% as much energy as gaseous diffusion.<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/ }}</ref> 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 |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. At the [[S-50 (Manhattan Project)|S-50]] plant in [[Oak Ridge, Tennessee]], during [[World War II]], liquid [[uranium hexafluoride]] was placed between two concentric vertical pipes, with the inner pipe heated and the outer pipe cooled. This caused lighter 235U molecules to migrate toward the hot inner wall and heavier 238U molecules toward the cold outer wall, with [[Convection current|convection currents]] carrying the enriched uranium upward for collection, and the height of the columns directly influenced the degree of separation.<ref>{{Cite web |title=Manhattan Project: Processes > Uranium Isotope Separation > THERMAL DIFFUSION |url=https://www.osti.gov/opennet/manhattan-project-history/Processes/UraniumSeparation/thermal-diffusion.html |access-date=2025-08-15 |website=www.osti.gov}}</ref> The facility was used to prepare feed material for the Electromagnetic Isotope Separation (EMIS) process but was later 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 |article-number=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 1990s and 2000s, 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. |author-link=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.

==Separative work unit==
{{Further|Separative work units}}"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 that 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.<ref name="Lei Science Direct" /> Separative work is measured in ''Separative work units'' SWU, kg SW, or kg UTA (from the German ''Urantrennarbeit'' – literally ''uranium separation work''). Efficient utilization of separative work is crucial for optimizing the economic and operational performance of uranium enrichment facilities. 
* 1 SWU = 1&nbsp;kg SW = 1&nbsp;kg UTA
* 1 kSWU = 1 tSW = 1 t UTA
* 1 MSWU = 1 ktSW = 1 kt UTA

==Cost issues==

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.

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.

When converting uranium ([[hexafluoride]], hex for short) to metal, 0.3% is lost during manufacturing.<ref>{{cite web |
url=https://inis.iaea.org/collection/NCLCollectionStore/_Public/02/014/2014451.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://inis.iaea.org/collection/NCLCollectionStore/_Public/02/014/2014451.pdf |archive-date=2022-10-09 |url-status=live |
title=Economics of blending, a case study | 
last=Balakrishnan |
first=M. R. |
publisher=Government of India, Atomic Energy Commission |
location=Bombay, India |
page=6 |
date=1971 |
access-date=7 November 2021 }}</ref><ref>{{cite web |
url=https://books.google.com/books?id=PteWDwOTK08C&q=conversion+losses&pg=PA29 |
title=Costs of nuclear power | 
last=US Atomic Energy Commission |
publisher=Office of Technical Services, Dept of Commerce |
location=Washington DC |
page=29 |
date=January 1961 |
access-date=7 November 2021 }}</ref>

==Downblending==<!-- This section is linked from [[Radioactive waste]] -->
The opposite of enriching is downblending; surplus HEU can be downblended to LEU to make it suitable for use in commercial nuclear fuel. Downblending is a key process in nuclear non-proliferation efforts, as it reduces the amount of highly enriched uranium available for potential weaponization while repurposing it for peaceful purposes.
[[File:U.S. Department of Energy - Science - 089 045 001 (10314669725).jpg|thumb|Enriched uranium produced at LLNL plant was collected as nuggets the size and thickness of several quarters.]]
The HEU feedstock can contain unwanted uranium isotopes: [[Uranium-234|<sup>234</sup>U]] is a minor isotope contained in natural uranium (primarily as a product of [[alpha decay]] of {{chem|238|U| link= uranium-238}}—because the [[half-life]] of {{chem|238|U}} is much larger than that of {{chem|234|U}}, it is produced and destroyed at the same rate in a constant steady state equilibrium, bringing any sample with sufficient {{chem|238|U}} content to a stable ratio of {{chem|234|U}} to {{chem|238|U}} over long enough timescales); 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. {{Chem|236|U}} is produced primarily when {{chem|235|U}} absorbs a neutron and does not fission. The production of {{chem|236|U}} is thus unavoidable in any thermal neutron reactor with {{chem|235|U}} fuel. HEU reprocessed from nuclear weapons material production reactors (with an <sup>235</sup>U assay of approximately 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. While {{chem|234|U}} also absorbs neutrons, it is a [[fertile material]] that is turned into fissile {{chem|235|U}} upon [[neutron absorption]]. If {{chem|236|U}} absorbs a neutron, the resulting short-lived {{chem|237|U}} [[beta decay]]s to {{chem|237|Np| link=Neptunium-237}}, which is not usable in thermal neutron reactors but can be chemically separated from spent fuel to be disposed of as waste or to be transmutated into {{chem|238|Pu|link= Plutonium-238}} (for use in [[nuclear battery| nuclear batteries]]) in special reactors. Understanding and managing the isotopic composition of uranium during downblending processes is essential to ensure the quality and safety of the resulting nuclear fuel, as well as to mitigate potential radiological and proliferation risks associated with unwanted isotopes.

The blendstock can be NU or DU; however, depending on feedstock quality, SEU at typically 1.5 wt% <sup>235</sup>U may be 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. Effective management and disposition strategies for depleted uranium are crucial to ensure long-term safety and environmental protection. Innovative approaches such as reprocessing and recycling of depleted uranium could offer sustainable solutions to minimize waste and optimize resource utilization in the nuclear fuel cycle.

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"/>  This ambitious initiative not only addresses nuclear disarmament goals but also serves as a significant contributor to global energy security and environmental sustainability, effectively repurposing material once intended for destructive purposes into a resource for peaceful energy production.

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>{{cite web|url = http://www.usec.com/v2001_02/HTML/Megatons_DOEstatus.asp|date = 1 May 2000 |title = Status Report: USEC-DOE Megatons to Megawatts Program|archive-url=https://web.archive.org/web/20010406102039/http://www.usec.com/v2001_02/HTML/Megatons_DOEstatus.asp |archive-date=6 April 2001|publisher = USEC.com }}</ref><ref>{{cite web|title =Megatons to Megawatts|website = centrusenergy.com|date = December 2013|url= https://www.centrusenergy.com/who-we-are/history/megatons-to-megawatts/}}</ref> This innovative program not only facilitated the safe and secure elimination of excess weapons-grade uranium but also contributed to the sustainable operation of civilian nuclear power plants, reducing reliance on newly enriched uranium and promoting non-proliferation efforts globally

==Global enrichment facilities==
{{See also|Georges-Besse plant}}
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 |last1=Makhijani |first1=Arjun |url=http://www.ieer.org/reports/uranium/enrichment.pdf |title=Uranium enrichment |last2=Chalmers |first2=Lois |last3=Smith |first3=Brice |date=15 October 2004 |publisher=Institute for Energy and Environmental Research |access-date=21 November 2009 |archive-url=https://ghostarchive.org/archive/20221009/http://www.ieer.org/reports/uranium/enrichment.pdf |archive-date=2022-10-09 |url-status=live}}</ref><ref>{{cite report |page=730 |url=http://www.aph.gov.au/binaries/house/committee/isr/uranium/report/fullreport.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.aph.gov.au/binaries/house/committee/isr/uranium/report/fullreport.pdf |archive-date=2022-10-09 |url-status=live |title=Australia's uranium - Greenhouse friendly fuel for an energy hungry world |work=Standing Committee on Industry and Resources |publisher=The Parliament of the Commonwealth of Australia |date=November 2006 |access-date=3 April 2015}}</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 | publisher=BBC| date = 1 September 2006 | access-date=3 January 2010 | work=BBC News}}</ref> The Australian company Silex Systems has developed a [[Atomic vapor laser isotope separation|laser enrichment]] process known as SILEX ([[separation of isotopes by laser excitation]]), 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|newspaper=The Sydney Morning Herald|date=26 May 2006}}</ref> Although SILEX has been granted a license to build a plant, the development is still in its early stages as laser enrichment has yet to be proven to be economically viable, and there is a petition being filed to review the license given to SILEX over nuclear proliferation concerns.<ref name=":1">{{Cite journal |last=Weinberger |first=Sharon |date=2012-09-28 |title=US grants licence for uranium laser enrichment |url=https://www.nature.com/articles/nature.2012.11502 |journal=Nature |language=en |doi=10.1038/nature.2012.11502 |s2cid=100862135 |issn=1476-4687|doi-access=free }}</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|access-date=7 October 2007}}</ref>

==Codename==
During the [[Manhattan Project]], weapons-grade highly 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.<ref>{{cite web |author1=Burr |first=William |date=22 December 2015 |title=Strategic Air Command Declassifies Nuclear Target List from 1950s |url=https://nsarchive2.gwu.edu/nukevault/ebb538-Cold-War-Nuclear-Target-List-Declassified-First-Ever/ |access-date=27 November 2020 |website=nsarchive2.gwu.edu |quote=Oralloy [Oak Ridge alloy] was a term of art for highly enriched uranium.}}</ref> The term ''oralloy'' is still occasionally used to refer to enriched uranium.

==See also==
* [[List of laser articles]]
* [[MOX fuel]]
* [[Nuclear fuel bank]]
* [[Nuclear fuel cycle in France]]
* [[Orano]]
* [[Uranium market]]
* [[Uranium mining]]

==References==
{{Reflist}}

==External links==
{{subject bar|auto=y|d=y}}
* [https://web.archive.org/web/20090531024234/http://alsos.wlu.edu/qsearch.aspx?browse=science%2FEnriching+Uranium Annotated bibliography on enriched uranium from the Alsos Digital Library for Nuclear Issues]
* [http://www.silex.com.au Silex Systems Ltd]
* [http://world-nuclear.org/info/inf28.html Uranium Enrichment] {{Webarchive|url=https://web.archive.org/web/20101202112400/http://world-nuclear.org/info/inf28.html |date=2 December 2010 }}, World Nuclear Association
* [https://fas.org/sgp/othergov/doe/heu/index.html Overview and history of U.S. HEU production]
* [https://web.archive.org/web/20070303234531/http://www.huliq.com/tags/uranium-enrichment News Resource on Uranium Enrichment]
* [http://www.chemcases.com/nuclear/nc-07.htm Nuclear Chemistry-Uranium Enrichment] {{Webarchive|url=https://web.archive.org/web/20081015004559/http://www.chemcases.com/nuclear/nc-07.htm |date=15 October 2008 }}
* [https://web.archive.org/web/20110613105504/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
* [https://web.archive.org/web/20110727073116/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]
* {{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]]|author-link=Martyn Poliakoff|year=2009}}
* {{cite web|last1=Gilinsky|first1=V.|last2=Hoehn|first2=W.|title=The Military Significance of Small Uranium Enrichment Facilities Fed with Low-Enrichment Uranium (Redacted)|url=https://apps.dtic.mil/sti/citations/ADA613260|id={{DTIC|ADA613260}}|publisher=[[RAND Corporation]]|date=December 1969|website=[[Defense Technical Information Center]] |oclc=913595660 }}

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[[Category:Isotope separation]]
[[Category:Nuclear fuels]]
[[Category:Nuclear materials|Uranium, Enriched]]
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