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Isotope separation is difficult because two isotopes of the same element have nearly identical chemical properties, and can only be separated gradually using small mss differences. (U is only 1.26% lighter than U). This problem is compounded because uranium is rarely separated in its atomic form, but instead as a cmpound (UF₆ is only 0.852% lighter than UF₆). A cascade of identical stages produces successively higher concentrations of U. Each stage passesaslightly more concentrated product to the next stage and returns a slightly less concentrated residue to the previous stage.

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% as much energy as gaseous diffusion (at least a "factor of 20" more efficient). Some work is being done that would use nu

Gas centrifuge

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 U move tangentially toward the outside of the cylinder and the lighter gas molecules rich in 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, 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 SWU, making it about 40% cheaper than standard gaseous diffusion techniques.

Zippe centrifuge

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 U up the cylinder, where it can be collected by scoops. This improved centrifuge design is used commercially by Urenco to produce nuclear fuel and was used by Pakistan in their 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 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. 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 to separate isotopes of uranium using selective ionization of hyperfine transitions. The technique uses lasers tuned to frequencies that ionize U atoms and no others. The positively charged 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 UF₆, exciting molecules that contain a 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 UF₆. 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. GEH has since built a demonstration test loop and announced plans to build an initial commercial facility. 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. 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. 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. 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 only requiring the amount of 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. Due to these concerns the American Physical Society filed a petition with the U.S. Nuclear Regulatory Commission, 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" due to the ability for it to be hidden from any type of detection.

Other techniques

Aerodynamic processes

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 UF₆ 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 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. 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.

Electromagnetic isotope separation

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 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 valency in 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 magnets and plasma physics. In this process, the principle of ion cyclotron resonance is used to selectively energize the U isotope in a plasma containing a mix of ions. 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

"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. Separative work is measured in Separative work units SWU, kg SW, or kg UTA (from the German Urantrennarbeit – literally uranium separation work)

• 1 SWU = 1 kg SW = 1 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 U that ends up in the depleted uranium. However, unlike the number of SWUs required during enrichment, which increases with decreasing levels of U in the depleted stream, the amount of NU needed will decrease with decreasing levels of 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% U (as compared to 0.7% in NU) while the depleted stream contains 0.2% to 0.3% 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% U. On the other hand, if the depleted stream had only 0.2% 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 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.

Downblending

The opposite of enriching is downblending; surplus HEU can be downblended to LEU to make it suitable for use in commercial nuclear fuel.

The HEU feedstock can contain unwanted uranium isotopes: U is a minor isotope contained in natural uranium (primarily as a product of alpha decay of
U - because the half-life of
U is much larger than that of
U, it'll be produced and destroyed at the same rate in a constant steady state equilibrium, bringing any sample with sufficient
U content to a stable ratio of
U to
U over long enough timescales); during the enrichment process, its concentration increases but remains well below 1%. High concentrations of U are a byproduct from irradiation in a reactor and may be contained in the HEU, depending on its manufacturing history.
U is produced primarily when
U absorbs a neutron and does not fission. The production of
U is thus unavoidable in any thermal neutron reactor with
U fuel. HEU reprocessed from nuclear weapons material production reactors (with an U assay of approx. 50%) may contain U concentrations as high as 25%, resulting in concentrations of approximately 1.5% in the blended LEU product. U is a neutron poison; therefore the actual U concentration in the LEU product must be raised accordingly to compensate for the presence of U. While
U also absorbs neutrons, it is a fertile material that is turned into fissile
U upon neutron absorption. If
U absorbs a neutron, the resulting short-lived
U beta decays to
Np, 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
Pu (for use in nuclear batteries) in special reactors.

The blendstock can be NU, or DU, however depending on feedstock quality, SEU at typically 1.5 wt% 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. At present, 95 percent of the world's stocks of depleted uranium remain in secure storage.

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.

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.

Global enrichment facilities

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. Belgium, Iran, Italy, and Spain hold an investment interest in the French Eurodif enrichment plant, with 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. The Australian company Silex Systems has developed a 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, 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. It has also been claimed that Israel has a uranium enrichment program housed at the Negev Nuclear Research Center site near Dimona.

Codename

During the Manhattan Project, weapons-grade highly enriched uranium was given the codename oralloy, a shortened version of Oak Ridge alloy, after the location of the plants where the uranium was enriched. The term oralloy is still occasionally used to refer to enriched uranium.

See also

• List of laser articles
• MOX fuel
• Nuclear fuel bank
• Orano
• Uranium market
• Uranium mining

References

1. "Uranium Enrichment". world-nuclear.org.
2. Economic Perspective for Uranium Enrichment (PDF), archived (PDF) from the original on 2022-10-09, The throughput per centrifuge unit is very small compared to that of a diffusion unit so small, in fact, that it is not compensated by the higher enrichment per unit. To produce the same amount of reactor-grade fuel requires a considerably larger number (approximately 50,000 to 500,000) of centrifuge units than diffusion units. This disadvantage, however, is outweighed by the considerably lower (by a factor of 20) energy consumption per SWU for the gas centrifuge
3. ^ "Lodge Partners Mid-Cap Conference 11 April 2008" (PDF). Silex Ltd. 11 April 2008. Archived (PDF) from the original on 2022-10-09.
4. ^ Weinberger, Sharon (28 September 2012). "US grants licence for uranium laser enrichment". Nature: nature.2012.11502. doi:10.1038/nature.2012.11502. S2CID 100862135.
5. ^ Slakey, Francis; Cohen, Linda R. (March 2010). "Stop laser uranium enrichment". Nature. 464 (7285): 32–33. Bibcode:2010Natur.464...32S. doi:10.1038/464032a. PMID 20203589. S2CID 205053626. ProQuest 204555310.
6. F. J. Duarte and L.W. Hillman (Eds.), Dye Laser Principles (Academic, New York, 1990) Chapter 9.
7. "GE Signs Agreement With Silex Systems of Australia To Develop Uranium Enrichment Technology" (Press release). GE Energy. 22 May 2006. Archived from the original on 14 June 2006.
8. "GE Hitachi Nuclear Energy Selects Wilmington, N.C. as Site for Potential Commercial Uranium Enrichment Facility". Business Wire. 30 April 2008. Retrieved 30 September 2012.
9. Broad, William J. (20 August 2011). "Laser Advances in Nuclear Fuel Stir Terror Fear". The New York Times. Retrieved 21 August 2011.
10. "Uranium Plant Using Laser Technology Wins U.S. Approval". The New York Times. September 2012.
11. Becker, E. W.; Ehrfeld, W.; Münchmeyer, D.; Betz, H.; Heuberger, A.; Pongratz, S.; Glashauser, W.; Michel, H. J.; Siemens, R. (1982). "Production of Separation-Nozzle Systems for Uranium Enrichment by a Combination of X-Ray Lithography and Galvanoplastics". Naturwissenschaften. 69 (11): 520–523. Bibcode:1982NW.....69..520B. doi:10.1007/BF00463495. S2CID 44245091.
12. Smith, Michael; Jackson A G M (2000). "Dr". South African Institution of Chemical Engineers – Conference 2000: 280–289.
13. Balakrishnan, M. R. (1971). "Economics of blending, a case study" (PDF). Bombay, India: Government of India, Atomic Energy Commission. p. 6. Archived (PDF) from the original on 2022-10-09. Retrieved 7 November 2021.
14. US Atomic Energy Commission (January 1961). "Costs of nuclear power". Washington DC: Office of Technical Services, Dept of Commerce. p. 29. Retrieved 7 November 2021.
15. "Status Report: USEC-DOE Megatons to Megawatts Program". USEC.com. 1 May 2000. Archived from the original on 6 April 2001.
16. "Megatons to Megawatts". centrusenergy.com. December 2013.
17. Arjun Makhijani; Lois Chalmers; Brice Smith (15 October 2004). Uranium enrichment (PDF). Institute for Energy and Environmental Research. Archived (PDF) from the original on 2022-10-09. Retrieved 21 November 2009.
18. Australia's uranium - Greenhouse friendly fuel for an energy hungry world (PDF). Standing Committee on Industry and Resources (Report). The Parliament of the Commonwealth of Australia. November 2006. p. 730. Archived (PDF) from the original on 2022-10-09. Retrieved 3 April 2015.
19. "Q&A: Uranium enrichment". BBC News. BBC. 1 September 2006. Retrieved 3 January 2010.
20. "Laser enrichment could cut cost of nuclear power". The Sydney Morning Herald. 26 May 2006.
21. Weinberger, Sharon (2012-09-28). "US grants licence for uranium laser enrichment". Nature. doi:10.1038/nature.2012.11502. ISSN 1476-4687. S2CID 100862135.
22. "Israel's Nuclear Weapons Program". Nuclear Weapon Archive. 10 December 1997. Retrieved 7 October 2007.
23. William Burr (22 December 2015). "Strategic Air Command Declassifies Nuclear Target List from 1950s". nsarchive2.gwu.edu. Retrieved 27 November 2020. Oralloy was a term of art for highly-enriched uranium.

External links

• Annotated bibliography on enriched uranium from the Alsos Digital Library for Nuclear Issues
• Silex Systems Ltd
• Uranium Enrichment Archived 2 December 2010 at the Wayback Machine, World Nuclear Association
• Overview and history of U.S. HEU production
• News Resource on Uranium Enrichment
• Nuclear Chemistry-Uranium Enrichment Archived 15 October 2008 at the Wayback Machine
• A busy year for SWU (a 2008 review of the commercial enrichment marketplace), Nuclear Engineering International, 1 September 2008
• 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
• Poliakoff, Martyn (2009). "How do you enrich Uranium?". The Periodic Table of Videos. University of Nottingham.
• Gilinsky, V.; Hoehn, W. (December 1969). "The Military Significance of Small Uranium Enrichment Facilities Fed with Low-Enrichment Uranium (Redacted)" (Document). RAND Corporation. DTIC ADA613260. {{cite document}}: Unknown parameter |oclc= ignored (help); Unknown parameter |website= ignored (help)

• Isotope separation
• Nuclear fuels
• Nuclear materials
• Nuclear weapon design
• Uranium