Enriched uranium

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% 238U isotope, with 235U only constituting about 0.711% of its mass. 235U is the only nuclide existing in nature (in any appreciable amount) that is fissile with thermal neutrons.[1]

Enriched uranium is a critical component for both civil nuclear power generation and military nuclear weapons. 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 weapons proliferation.

During the Manhattan Project 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. There are about 2,000 tonnes (t, Mg) of highly enriched uranium in the world,[2] produced mostly for nuclear power, nuclear weapons, naval propulsion, and smaller quantities for research reactors.

The 238U 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 armor-penetrating weapons and radiation shielding. At present, 95 percent of the world's stocks of depleted uranium remain in secure storage.

Uranium enrichment proportions
Proportions of uranium-238 (blue) and uranium-235 (red) found naturally versus enriched 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. 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 60% uranium whereas the ore typically contains less than 1% uranium and as little as 0.1% uranium (Henderson 2000). 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". Naturally-occurring uranium is made of a mixture of U-235 and U-238. The U-235 is fissile meaning it is easily split with neutrons while the remainder is U-238, but in nature, more than 99% of the extracted ore is U-238. Most nuclear reactors require enriched uranium, which is uranium with higher concentrations of U-235 ranging between 3.5% and 4.5%. 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.

A drum of yellowcake (a mixture of uranium precipitates)

Reprocessed uranium (RepU)

Reprocessed uranium (RepU) is a product of nuclear fuel cycles 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 reactors. 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.

Low enriched uranium (LEU)

Low enriched uranium (LEU) has a lower than 20% concentration of 235U; for instance, in commercial light water reactors (LWR), the most prevalent power reactors in the world, uranium is enriched to 3 to 5% 235U. Fresh LEU used in research reactors is usually enriched 12% to 19.75% U-235, the latter concentration being used to replace HEU fuels when converting to LEU.[3]

Highly enriched uranium (HEU)

A billet of highly enriched uranium metal

Highly enriched uranium (HEU) has a 20% or higher concentration of 235U. The fissile uranium in nuclear weapon primaries usually contains 85% or more of 235U known as weapons-grade, though theoretically for an 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";[4][5] even lower enrichment is hypothetically possible, but as the enrichment percentage decreases the critical mass for unmoderated fast neutrons rapidly increases, with for example, an infinite mass of 5.4% 235U being required.[4] For criticality experiments, enrichment of uranium to over 97% has been accomplished.[6]

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 designs that use less than what would be one bare-sphere critical mass at normal density. The presence of too much of the 238U 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 50 kilograms (110 lb), which at normal density would be a sphere about 17 centimetres (6.7 in) in diameter.

Later US 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%[7] along with the 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 238U is not fissile but still fissionable by fusion neutrons.

HEU is also used in fast neutron reactors, whose cores require about 20% or more of fissile material, as well as in naval reactors, where it often contains at least 50% 235U, but typically does not exceed 90%. The Fermi-1 commercial fast reactor prototype used HEU with 26.5% 235U. Significant quantities of HEU are used in the production of medical isotopes, for example molybdenum-99 for technetium-99m generators.[8]

Enrichment methods

Isotope separation is difficult because two isotopes of the same element have very nearly identical chemical properties, and can only be separated gradually using small mass differences. (235U is only 1.26% lighter than 238U.) This problem is compounded by the fact that uranium is rarely separated in its atomic form, but instead as a compound (235UF6 is only 0.852% lighter than 238UF6.) A cascade of identical stages produces successively higher concentrations of 235U. 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 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%[9] as much energy as gaseous diffusion (at least a "factor of 20" more efficient).[10] Some work is being done that would use nuclear resonance; however there is no reliable evidence that any nuclear resonance processes have been scaled up to production.

Diffusion techniques

Gaseous diffusion

Gaseous diffusion is a technology used to produce enriched uranium by forcing gaseous uranium hexafluoride (hex) through semi-permeable membranes. This produces a slight separation between the molecules containing 235U and 238U. 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,[11] 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.[12] In 2013, the Paducah facility in the US ceased operating, it was the last commercial 235U gaseous diffusion plant in the world.[13]

Thermal diffusion

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 235U gas molecules will diffuse toward a hot surface, and the heavier 238U gas molecules will diffuse toward a cold surface. The S-50 plant at Oak Ridge, Tennessee was used during World War II to prepare feed material for the EMIS process. It was abandoned in favor of gaseous diffusion.

Centrifuge techniques

Gas centrifuge

Gas centrifuge cascade
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 238U move tangentially toward the outside of the cylinder and the lighter gas molecules rich in 235U 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,[11] which translates to about one-fiftieth of the energy requirements. Gas centrifuge techniques produce close to 100% of the world's enriched uranium.

Zippe centrifuge

Zippe-type gas centrifuge
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 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 235U 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 advanced and licensed for commercial operation in 2012.

Atomic vapor laser isotope separation (AVLIS)

Atomic vapor laser isotope separation employs specially tuned lasers[14] to separate isotopes of uranium using selective ionization of hyperfine transitions. The technique uses lasers tuned to frequencies that ionize 235U atoms and no others. The positively charged 235U 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 235U 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.[15] GEH has since built a demonstration test loop and announced plans to build an initial commercial facility.[16] 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.[11] 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.[17] 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.[18]

Other techniques

Aerodynamic processes

Aerodynamic enrichment nozzle
Schematic diagram of an aerodynamic nozzle. Many thousands of these small foils would be combined in an enrichment unit.
The X-ray based LIGA manufacturing process was originally developed at the Forschungszentrum Karlsruhe, Germany, to produce nozzles for isotope enrichment.[19]

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.[20] 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 are currently still in use.

Electromagnetic isotope separation

Electromagnetic separation
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 235U 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, utilising 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 235U isotope in a plasma containing a mix of ions. 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.

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 235U that ends up in the depleted uranium. However, unlike the number of SWUs required during enrichment, which increases with decreasing levels of 235U in the depleted stream, the amount of NU needed will decrease with decreasing levels of 235U 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% 235U (as compared to 0.7% in NU) while the depleted stream contains 0.2% to 0.3% 235U. 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% 235U. On the other hand, if the depleted stream had only 0.2% 235U, 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 235U to be left in the DU stream whereas if NU is more expensive and enrichment is less so, then they would choose the opposite.


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: 234U is a minor isotope contained in natural uranium; during the enrichment process, its concentration increases but remains well below 1%. High concentrations of 236U 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 235U assay of approx. 50%) may contain 236U concentrations as high as 25%, resulting in concentrations of approximately 1.5% in the blended LEU product. 236U is a neutron poison; therefore the actual 235U concentration in the LEU product must be raised accordingly to compensate for the presence of 236U.

The blendstock can be NU, or DU, however depending on feedstock quality, SEU at typically 1.5 wt% 235U 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.

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.[11]

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.[21]

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.[22][23] 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.[24] Australia has developed a laser enrichment process known as SILEX, which it intends to pursue through financial investment in a U.S. commercial venture by General Electric.[25] It has also been claimed that Israel has a uranium enrichment program housed at the Negev Nuclear Research Center site near Dimona.[26]

See also


  1. ^ OECD Nuclear Energy Agency (2003). Nuclear Energy Today. OECD Publishing. p. 25. ISBN 9789264103283.
  2. ^ Thomas B. Cochran (Natural Resources Defense Council) (12 June 1997). "Safeguarding Nuclear Weapon-Usable Materials in Russia" (PDF). Proceedings of international forum on illegal nuclear traffic. Archived from the original (PDF) on 22 July 2012.
  3. ^ Alexander Glaser (6 November 2005). "About the Enrichment Limit for Research Reactor Conversion : Why 20%?" (PDF). Princeton University. Retrieved 18 April 2014.
  4. ^ a b Forsberg, C. W.; Hopper, C. M.; Richter, J. L.; Vantine, H. C. (March 1998). "Definition of Weapons-Usable Uranium-233" (PDF). ORNL/TM-13517. Oak Ridge National Laboratories. Archived from the original (PDF) on 2 November 2013. Retrieved 30 October 2013.
  5. ^ Sublette, Carey (4 October 1996). "Nuclear Weapons FAQ, Section Nuclear Design Principles – Highly Enriched Uranium". Nuclear Weapons FAQ. Retrieved 2 October 2010.
  6. ^ Mosteller, R.D. (1994). "Detailed Reanalysis of a Benchmark Critical Experiment: Water-Reflected Enriched-Uranium Sphere" (PDF). Los Alamos technical paper (LA–UR–93–4097): 2. Retrieved 19 December 2007. 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.
  7. ^ "Nuclear Weapons FAQ". Nuclearweaponarchive.org. Retrieved 26 January 2013.
  8. ^ Frank N. Von Hippel; Laura H. Kahn (December 2006). "Feasibility of Eliminating the Use of Highly Enriched Uranium in the Production of Medical Radioisotopes". Science & Global Security. 14 (2 & 3): 151–162. doi:10.1080/08929880600993071. Retrieved 26 March 2010.
  9. ^ "Uranium Enrichment". world-nuclear.org.
  10. ^ Economic Perspective for Uranium Enrichment (PDF), 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
  11. ^ a b c d "Lodge Partners Mid-Cap Conference 11 April 2008" (PDF). Silex Ltd. 11 April 2008.
  12. ^ Rod Adams (24 May 2011). "McConnell asks DOE to keep using 60 year old enrichment plant to save jobs". Atomic Insights. Archived from the original on 28 January 2013. Retrieved 26 January 2013.
  13. ^ "Paducah enrichment plant to be closed. The 1950s facility is the last remaining gaseous diffusion uranium enrichment plant in the world.".
  14. ^ F. J. Duarte and L.W. Hillman (Eds.), Dye Laser Principles (Academic, New York, 1990) Chapter 9.
  15. ^ [1] Archived 23 July 2015 at the Wayback Machine
  16. ^ "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.
  17. ^ Broad, William J. (20 August 2011). "Laser Advances in Nuclear Fuel Stir Terror Fear". The New York Times. Retrieved 21 August 2011.
  18. ^ "Uranium Plant Using Laser Technology Wins U.S. Approval". New York Times. September 2012.
  19. ^ 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.
  20. ^ Smith, Michael; Jackson A G M (2000). "Dr". South African Institution of Chemical Engineers – Conference 2000: 280–289.
  21. ^ [2] Archived 6 April 2001 at the Wayback Machine
  22. ^ Arjun Makhijani; Lois Chalmers; Brice Smith (15 October 2004). Uranium enrichment (PDF). Institute for Energy and Environmental Research. Retrieved 21 November 2009.
  23. ^ 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. Retrieved 3 April 2015.
  24. ^ BBC (1 September 2006). "Q&A: Uranium enrichment". BBC News. Retrieved 3 January 2010.
  25. ^ "Laser enrichment could cut cost of nuclear power". The Sydney Morning Herald. 26 May 2006.
  26. ^ "Israel's Nuclear Weapons Program". Nuclear Weapon Archive. 10 December 1997. Retrieved 7 October 2007.

External links

Gaseous diffusion

Gaseous diffusion is a technology used to produce enriched uranium by forcing gaseous uranium hexafluoride (UF6) through semipermeable membranes. This produces a slight separation between the molecules containing uranium-235 (235U) and uranium-238 (238U). By use of a large cascade of many stages, high separations can be achieved. It was the first process to be developed that was capable of producing enriched uranium in industrially useful quantities.

Gaseous diffusion was devised by Francis Simon and Nicholas Kurti at the Clarendon Laboratory in 1940, tasked by the MAUD Committee with finding a method for separating uranium-235 from uranium-238 in order to produce a bomb for the British Tube Alloys project. The prototype gaseous diffusion equipment itself was manufactured by Metropolitan-Vickers (MetroVick) at Trafford Park, Manchester, at a cost of £150,000 for four units, for the M. S. Factory, Valley. This work was later transferred to the United States when the Tube Alloys project became subsumed by the later Manhattan Project.

KLT-40 reactor

The KLT-40 and KLT-40M reactors are nuclear fission reactors used to power the Taymyr-class icebreakers (KLT-40M, 171 MW) and the LASH carrier Sevmorput (KLT-40, 135 MW). They are pressurized water reactors (PWR) fueled by either 30–40% or 90 % enriched uranium-235 fuel to produce 135 to 171 MW of thermal power.The KLT-40S variant is used in the Russian floating nuclear power station Akademik Lomonosov. It was developed by OKBM Afrikantov and produced by NMZ. The KLT-40S produces 150 MW thermal (about 52 MWe at 35% efficiency). The KLT-40S also uses low-enriched uranium at 14.1% enrichment to meet international proliferation standards.

Little Boy

"Little Boy" was the code name for the type of atomic bomb dropped on the Japanese city of Hiroshima on 6 August 1945 during World War II. It was the first nuclear weapon used in warfare. The bomb was dropped by the Boeing B-29 Superfortress Enola Gay piloted by Colonel Paul W. Tibbets, Jr., commander of the 509th Composite Group of the United States Army Air Forces. It exploded with an energy of approximately 15 kilotons of TNT (63 TJ) and caused widespread death and destruction throughout the city. The Hiroshima bombing was the second nuclear explosion in history, after the Trinity test, and the first uranium-based detonation.

Little Boy was developed by Lieutenant Commander Francis Birch's group at the Manhattan Project's Los Alamos Laboratory during World War II, a development of the unsuccessful Thin Man nuclear bomb. Like Thin Man, it was a gun-type fission weapon, but it derived its explosive power from the nuclear fission of uranium-235, whereas Thin Man was based on fission of plutonium-239. Fission was accomplished by shooting a hollow cylinder of enriched uranium (the "bullet") onto a solid cylinder of the same material (the "target") by means of a charge of nitrocellulose propellant powder. It contained 64 kg (141 lb) of enriched uranium, although less than a kilogram underwent nuclear fission. Its components were fabricated at three different plants so that no one would have a copy of the complete design.

After the war ended, it was not expected that the inefficient Little Boy design would ever again be required, and many plans and diagrams were destroyed. However, by mid-1946, the Hanford Site reactors began suffering badly from the Wigner effect, the dislocation of atoms in a solid caused by neutron radiation, and plutonium became scarce, so six Little Boy assemblies were produced at Sandia Base. The Navy Bureau of Ordnance built another 25 Little Boy assemblies in 1947 for use by the Lockheed P2V Neptune nuclear strike aircraft which could be launched from the Midway-class aircraft carriers. All the Little Boy units were withdrawn from service by the end of January 1951.

MR 41

The MR 41 is a French-built thermonuclear warhead to be launched with the M1 and M2 missiles in Redoutable class ballistic missile submarines.

It had a yield of 500 kilotons and was boosted fission warhead based on highly enriched uranium combined with deuterium and tritium.

Entering service in 1972 it was withdrawn from service by the end of 1979, being replaced by TN 60 warheads.

Megatons to Megawatts Program

The Megatons to Megawatts Program, successfully completed in December 2013, is the popular name given to the program which is also called the United States-Russia Highly Enriched Uranium Purchase Agreement. The official name of the program is the "Agreement between the Government of the Russian Federation and the Government of the United States of America Concerning the Disposition of Highly-Enriched Uranium Extracted from Nuclear Weapons", dated February 18, 1993. Under this Agreement, Russia agreed to supply the United States with low-enriched uranium (LEU) obtained from high-enriched uranium (HEU) found to be in excess of Russian defense purposes. The United States agreed to purchase the low-enriched uranium fuel.

The original proposal for this program was made by Thomas Neff, a physicist at MIT, in an October 24, 1991 Op-Ed in The New York Times. On August 28, 1992, in Moscow, U.S. and Russian negotiators initialed the 20-year agreement and President George H. W. Bush announced the agreement on August 31, 1992. In 1993, the agreement was signed and initiated by President Clinton and the commercial implementing contract was then signed by both parties.

Mexico and weapons of mass destruction

Mexico is one of the few countries which has technical capabilities to manufacture nuclear weapons. However it has renounced them and pledged to only use its nuclear technology for peaceful purposes following the Treaty of Tlatelolco in 1968. In the 1970s Mexico's national institute for nuclear research successfully achieved the creation of highly enriched uranium which is used in nuclear power plants and in the construction of nuclear weapons. However the country agreed in 2012 to downgrade the high enriched uranium used on its nuclear power plants to low enriched uranium, the process was realised with the assistance of the International Atomic Energy Agency. It is unknown if Mexico ever created or possessed nuclear or any other kind of mass destruction weapons.

Natural uranium

Natural uranium (NU, Unat) refers to uranium with the same isotopic ratio as found in nature. It contains 0.711% uranium-235, 99.284% uranium-238, and a trace of uranium-234 by weight (0.0055%). Approximately 2.2% of its radioactivity comes from uranium-235, 48.6% from uranium-238, and 49.2% from uranium-234.

Natural uranium can be used to fuel both low- and high-power nuclear reactors. Historically, graphite-moderated reactors and heavy water-moderated reactors have been fueled with natural uranium in the pure metal (U) or uranium dioxide (UO2) ceramic forms. However, experimental fuelings with uranium trioxide (UO3) and triuranium octaoxide, (U3O8) have shown promise.The 0.72% uranium-235 is not sufficient to produce a self-sustaining critical chain reaction in light water reactors or nuclear weapons; these applications must use enriched uranium. Nuclear weapons take a concentration of 90% uranium-235, and light water reactors require a concentration of roughly 3% uranium-235. Unenriched natural uranium is appropriate fuel for a heavy-water reactor, like a CANDU reactor.

In rare occasions, earlier in geologic history when uranium-235 was more abundant, uranium ore was found to have naturally engaged in fission, forming natural nuclear fission reactors. Uranium-235 decays at a faster rate (half-life of 700 million years) compared to uranium-238, which decays extremely slowly (half-life of 4.5 billion years). Therefore, a billion years ago, there was more than double the uranium-235 compared to now.

During the Manhattan Project, the name Tuballoy was used to refer to natural uranium in the refined condition; this term is still in occasional use. Uranium was also called codenamed "X-Metal" during World War II. Similarly, enriched uranium was referred to as Oralloy (Oak Ridge alloy), and depleted uranium was referred to as Depletalloy (depleted alloy).

Nuclear Control Institute

The Nuclear Control Institute is a research and advocacy center for preventing nuclear proliferation and nuclear terrorism. The non-profit organization was founded by Paul Leventhal in 1981. It went under a reorganization in 2003 to make it a web-based program. The institute is supported by the donations of philanthropic foundations and individuals.

The Nuclear Control Institute is particularly focused on the elimination of plutonium and highly enriched uranium, which can be used to create nuclear weapons, from nuclear power plants and research reactors, and preventing plutonium and highly enriched uranium from dismantled nuclear weapons from being disposed of in commercial reactors. This means that they are strongly opposed to the use of mixed oxide fuel.

Tom Clements is the executive director.

Nuclear energy in Kazakhstan

As of 2015, Kazakhstan has no active nuclear power generation capacity. The country's only nuclear power plant, the BN-350 sodium-cooled fast reactor located near Aktau in Mangystau Region, ceased generating in June 1999 after 26 years of operation, and was decommissioned in 2001. However, the plant's primary purpose was desalinization, not electricity generation, so its power output was limited. The country's National Nuclear Center (NNC) also operates three research reactors at the former Semipalatinsk Test Site.In 2003, the Kazakh Minister of Energy and Mines announced plans for the construction of a new nuclear power plant by 2018. The two- or three-unit plant was to be established on the shores of Lake Balkhash in the Karaganda region of central Kazakhstan. However, these plans were later amended – in January 2013, President Nursultan Nazarbayev gave the government one month to submit new proposals for the construction of a nuclear power plant. In September 2013, the Director General of the NNC, Erlan Batyrbekov, recommended the construction of a nuclear power plant to ensure Kazakh energy security. In May 2014, Russia and Kazakhstan signed a preliminary cooperation agreement regarding the construction of a new nuclear power plant with a generating capacity of between 300 and 1,200 MW.Kazakhstan's thermal neutron pool-type reactor WWR-K was commissioned in 1967. In order to decrease proliferation risks, Kazakhstan started the program to convert WWR-K to LEU. Kazakhstan's Institute of Nuclear Physics developed and implemented comprehensive safety programs for the reactor. The International Atomic Energy Agency (IAEA) also provided recommendation on how ensure further continuous safety improvements on WWR-K.

Nuclear material

Nuclear material refers to the metals uranium, plutonium, and thorium, in any form, according to the IAEA. This is differentiated further into "source material", consisting of natural and depleted uranium, and "special fissionable material", consisting of enriched uranium (U-235), uranium-233, and plutonium-239. Uranium ore concentrates are considered to be a "source material", although these are not subject to safeguards under the Nuclear Non-Proliferation Treaty.Different countries may use different terminology: in the United States of America, "nuclear material" most commonly refers to "special nuclear materials" (SNM), with the potential to be made into nuclear weapons as defined in the Atomic Energy Act of 1954. The "special nuclear materials" are also plutonium-239, uranium-233, and enriched uranium (U-235).

Note that the 1980 Convention on the Physical Protection of Nuclear Material definition of nuclear material does not include thorium.

Nuclear reactor core

A nuclear reactor core is the portion of a nuclear reactor containing the nuclear fuel components where the nuclear reactions take place and the heat is generated. Typically, the fuel will be low-enriched uranium contained in thousands of individual fuel pins. The core also contains structural components, the means to both moderate the neutrons and control the reaction, and the means to transfer the heat from the fuel to where it is required, outside the core.

OK-150 reactor

The OK-150 reactor (1st generation) and its successor, the OK-900 reactor (2nd generation) are Soviet marine nuclear fission reactors used to power ships at sea. They are pressurized water reactors (PWRs) that use enriched uranium-235 fuel. They have been used in various Russian nuclear-powered icebreaker ships. The reactor was developed by OKBM.

OK-150 specifications:

Fuel: 5% enriched uranium in the form of ceramic uranium dioxide (UO2) fuel elements with a cladding. Different cladding materials were used; initially zirconium, later on, stainless steel as well as a zirconium-niobium alloy were tried.

Fuel load: 75 to 85 kilograms

Power production: 90 megawattsDistilled water was used for heat transfer and as a moderator.

The core was 1.6 m high by 1 m diameter. It consisted of 219 fuel assemblies, totalling 7,704 fuel pins. There was a biological shield made of concrete mixed with metal shavings.

OK-900A specifications:

Fuel: 90% enriched uranium in the form of metallic uranium-zirconium alloy fuel elements

Fuel load: 150.7 kg

Power production: 171 megawattsThree OK-150s were used to power the Soviet icebreaker Lenin at the time of its launch in 1957. Later, after damage caused by nuclear accidents in 1965 and 1967, these were removed and replaced with two OK-900s.

Oleg Khinsagov

Oleg Khinsagov (Russian: Олег Хинсагов) is a Russian citizen from Vladikavkaz, North Ossetia–Alania. On January 25, 2007 he was sentenced by a Georgian court for 8.5 years for smuggling 100 grams of highly enriched uranium.According to the Georgian authorities, in January 2006, Khinsagov together with a few Georgian citizens from the separatist region of South Ossetia was trying to sell 100 grams of highly enriched uranium. He claimed that the material is only a sample and he has more than 3 kilograms of the substance in his Vladikavkaz garage. Georgian police arranged meeting of Khinsagov with their Turkish-speaking agent introduced as a representative of a rich Muslim organization willing to buy the sample for $1 million US. At the meeting held on February 1, 2006 Khinsagov was arrested with 100 grams of a substance in two plastic pouches. The chemical analysis performed by an American Department of Energy Lab confirmed the substance as being a U-235 purity of 89.451 percent enriched Uranium that makes it a weapons-grade material.The Georgian side accused Russian investigators in the lack of cooperation with the investigation. According to Shota Utiashvili, the head of the Georgian Interior Ministry's analytical department: "We received the test results from Russian specialists. They confirmed that the substance was high-enriched uranium, but did not say anything about its origin.". According to Rosatom the Georgian side did not provided enough material to pinpoint its origin, still the FSB report provided for the Georgian investigators confirmed the substance as being the highly enriched uranium and indicated it was processed more than ten years ago.

Paducah Gaseous Diffusion Plant

The Paducah Gaseous Diffusion Plant (PGDP) is a facility located in McCracken County, Kentucky, near Paducah, Kentucky that produced enriched uranium 1952–2013. It is owned by the U.S. Department of Energy (DOE). The PGDP was the only operating uranium enrichment facility in the United States in the period 2001–2010. The Paducah plant produced low-enriched uranium, originally as feedstock for military reactors, weapons and later for nuclear power fuel.

The gaseous diffusion plant covers 750 acres (300 ha) of a 3,556 acres (1,439 ha) site. The four process buildings cover 74 acres (30 ha), and consumed a peak electrical demand of 3,040 megawatts.DOE leased the facility to a publicly held company, USEC, from the mid 1990s. USEC ceased operations in 2013 and return the facility to the Department of Energy for decontamination and decommissioning.

Pool Test Reactor

Pool Test Reactor (PTR) was a 10 kWt ordinary (light) water moderated pool-type reactor fueled with highly enriched uranium built at Chalk River in 1957. It used 93% enriched uranium-aluminum plate-type fuel. The reactor, was used for burnup measurement of fissile samples from NRX. The reactor was designed and built by Canadair, now a division of Bombardier.

S-50 (Manhattan Project)

The S-50 Project was the Manhattan Project's effort to produce enriched uranium by liquid thermal diffusion during World War II. It was one of three technologies for uranium enrichment pursued by the Manhattan Project.

The liquid thermal diffusion process was not one of the enrichment technologies initially selected for use in the Manhattan Project, and was developed independently by Philip H. Abelson and other scientists at the United States Naval Research Laboratory. This was primarily due to doubts about the process's technical feasibility, but inter-service rivalry between the United States Army and United States Navy also played a part.

Pilot plants were built at the Anacostia Naval Air Station and the Philadelphia Navy Yard, and a production facility at the Clinton Engineer Works in Oak Ridge, Tennessee. This was the only production-scale liquid thermal diffusion plant ever built. It could not enrich uranium sufficiently for use in an atomic bomb, but it could provide slightly enriched feed for the Y-12 calutrons and the K-25 gaseous diffusion plants. It was estimated that the S-50 plant had sped up production of enriched uranium used in the Little Boy bomb employed in the atomic bombing of Hiroshima by a week.

The S-50 plant ceased production in September 1945, but it was reopened in May 1946, and used by the United States Army Air Forces Nuclear Energy for the Propulsion of Aircraft (NEPA) project. The plant was demolished in the late 1940s.

Steam-generating heavy water reactor

Steam Generating Heavy Water Reactor (SGHWR) is a United Kingdom design for commercial nuclear reactors. It is similar to the Canadian CANDU reactor designs in that it uses a low-pressure reactor vessel containing high-pressure piping for the coolant, which reduces construction costs and complexity.

SGHWR was a heavy water moderated reactor, which used ordinary (light) water as coolant, in contrast with earlier UK designs that used graphite moderators which led to very large reactor sizes. Unlike CANDU, the SGHWR uses slightly enriched uranium fuel, which allows for higher burnup and more economical fuel cycles. The modern CANDU ACR-1000 reactor design uses a similar concept, as does the Italian CIRENE, hosted at Latina Nuclear Power Plant.

Only a single SGHWR was ever built, the small 100 MW prototype reactor at Winfrith, often known simply as the "Winfrith Reactor". It was connected to the grid in 1967 and ceased operation in 1990 after 23 successful years. [1] It was owned by the United Kingdom Atomic Energy Authority. Decommissioning is now being carried out by Magnox Ltd on behalf of the Nuclear Decommissioning Authority.

A similar design was the Gentilly Nuclear Generating Station in Quebec, but this was not successful and shut down after a short lifetime.

The Apollo Affair

The Apollo Affair was a 1965 incident in which a US company, Nuclear Materials and Equipment Corporation (NUMEC), in the Pittsburgh suburbs of Apollo and Parks Township, Pennsylvania was investigated for losing 200–600 pounds (91–272 kg) of highly enriched uranium, with suspicions that it had gone to Israel's nuclear weapons program.

From 1965 to 1980, the Federal Bureau of Investigation (FBI) investigated Zalman Shapiro, the company's president, over the loss of 206 pounds (93 kg) of highly enriched uranium. The Atomic Energy Commission, the Central Intelligence Agency, other government agencies, and inquiring reporters conducted similar investigations, and no charges were ever filed. A General Accounting Office study of the investigations declassified in May 2010 stated "We believe a timely, concerted effort on the part of these three agencies would have greatly aided and possibly solved the NUMEC diversion questions, if they desired to do so."In February 1976 the CIA briefed senior staff at the Nuclear Regulatory Commission (NRC) about the matter, stating that the CIA believed the missing highly enriched uranium went to Israel. The NRC informed the White House, leading to President-elect Carter being briefed about the investigation. Carter asked for an assessment by his National Security Advisor, whose staff concluded "The CIA case is persuasive, though not conclusive."Some remain convinced that Israel received 206 pounds (93 kg) or more of highly enriched uranium from NUMEC, particularly given the visit of Rafi Eitan, later revealed as an Israeli spy and who was later involved in the Jonathan Pollard incident. In June 1986, analyst Anthony Cordesman told United Press International:

There is no conceivable reason for Eitan to have gone [to the Apollo plant] but for the nuclear material.”

In his 1991 book, The Samson Option, Seymour Hersh concluded that Shapiro did not divert any uranium; rather "it ended up in the air and water of the city of Apollo as well as in the ducts, tubes, and floors of the NUMEC plant." He also wrote that Shapiro's meetings with senior Israeli officials in his home were related to protecting the water supply in Israel rather than any diversion of nuclear material or information.A later investigation was conducted by the Nuclear Regulatory Commission (successor to the AEC) regarding an additional 198 pounds (90 kg) of uranium that was found to be missing between 1974 and 1976, after the plant had been purchased by Babcock & Wilcox and Shapiro was no longer associated with the company. That investigation found that more than 110 pounds (50 kg) of it could be accounted for by what was called "previously unidentified and undocumented loss mechanisms", including "contamination of workers' clothes, losses from scrubber systems, material embedded in the flooring, and residual deposits in the processing equipment." Hersh further quoted one of the main investigators, Carl Duckett, as saying "I know of nothing at all to indicate that Shapiro was guilty."In 1993, Glenn T. Seaborg, former head of the Atomic Energy Commission wrote a book, The Atomic Energy Commission under Nixon, Adjusting to Troubled Times which devoted a chapter to Shapiro and NUMEC, the last sentence of which states:

Distinguished as Shapiro's career has been, one cannot but wonder whether it might not have been even more illustrious had these unjust charges not been leveled against him.

Later U.S. Department of Energy records show that NUMEC had the largest highly enriched uranium inventory loss of all U.S. commercial sites, with a 269 kilograms (593 lb) inventory loss before 1968, and 76 kilograms (168 lb) thereafter.At the prompting of Zalman Shapiro's lawyer, senator Arlen Specter asked the Nuclear Regulatory Commission (NRC) to clear him of any suspicion of diversion in August 2009. The NRC refused, stating:

NRC found no documents that provided specific evidence that the diversion of nuclear materials occurred. However, consistent with previous Commission statements, NRC does not have information that would allow it to unequivocally conclude that nuclear material was not diverted from the site, nor that all previously unaccounted for material was accounted for during the decommissioning of the site.

In 2014, further documents about the investigation were declassified, though still heavily redacted.The U.S. Army Corps of Engineers is overseeing a cleanup of contaminated land at the site of NUMEC's waste disposal, currently scheduled to be completed in 2015.

W33 (nuclear warhead)

The W33 was an American nuclear artillery shell, fired from an eight-inch (203 mm) M110 howitzer and M115 howitzer.

A total of 2,000 W33 projectiles were produced, the first of which was manufactured in 1957. The W33 remained in service until 1992. The warhead used enriched uranium (code named oralloy) as its nuclear fissile material and could be used in two different yield configurations. This required the assembly and insertion of different pits, with the amount of fissile materials used controlling whether the destructive yield was low or high. The highest-yield version of the W33 may have been a boosted fission weapon.

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