Enriched uranium

Enriched uranium is a type of uranium in which the percent composition of uranium-235 (written ²³⁵U) has been increased through the process of isotope separation. Naturally occurring uranium is composed primarily of three isotopes: uranium-238 (²³⁸U, 99.2732–99.2752% natural abundance), uranium-235 (²³⁵U, 0.7198–0.7210%), and uranium-234 (²³⁴U, 0.0049–0.0059%). ²³⁵U is the only primordial nuclide present in nature in appreciable quantities that is fissile with thermal neutrons.thumb|Proportions of uranium-238 (blue) and uranium-235 (red) found naturally versus enriched grades

Enriched uranium is used in both civil nuclear power generation and military nuclear weapons. Low-enriched uranium (LEU), containing less than 20% ²³⁵U, is used as fuel in light-water reactors, which make up most nuclear power reactors worldwide. Highly enriched uranium (HEU), containing 20% or more ²³⁵U, has been used in nuclear weapons, naval propulsion reactors, some research reactors, and certain specialized reactor designs. There are about 2,000 tonnes of highly enriched uranium in the world.

Large-scale enrichment was first developed during the Manhattan Project, which used gaseous diffusion and electromagnetic isotope separation to produce enriched uranium. Historically, gaseous diffusion and the gas centrifuge are the two enrichment methods that have operated on a commercial scale, but gaseous diffusion is now obsolete and has been replaced by centrifuge technology in commercial use. Laser isotope separation has also been developed as a possible third-generation enrichment technology; Global Laser Enrichment has applied to license a laser-based uranium enrichment facility in the United States, but centrifugation remains the enrichment process used commercially.

The ²³⁸U 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 in armor-penetrating weapons.

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.

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". Naturally occurring uranium is made of a mixture of ²³⁵U and ²³⁸U. The ²³⁵U is fissile, meaning it is easily split with neutrons while the remainder is ²³⁸U, but in nature, more than 99% of the extracted ore is ²³⁸U. Most nuclear reactors require enriched uranium, which is uranium with higher concentrations of ²³⁵U ranging between 3.5% and 4.5% (although a few reactor designs using a graphite or heavy water 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.

thumb|A drum of [[yellowcake (a mixture of uranium precipitates)]]

Reprocessed uranium (RepU)

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 cycles involving nuclear reprocessing of spent fuel. RepU recovered from light water reactor (LWR) spent fuel typically contains slightly more ²³⁵U 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 ²³⁵U 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)

Low-enriched uranium (LEU) has a lower than 20% concentration of ²³⁵U; for instance, in commercial LWR, the most prevalent power reactors in the world, uranium is enriched to 3 to 5% ²³⁵U. <!-- Slightly enriched uranium redirects here-->Slightly enriched uranium (SEU) has a concentration of under 2% ²³⁵U.

High-assay LEU (HALEU)

High-assay LEU (HALEU) is enriched between 5% and 20% and is called for in many small modular reactor (SMR) designs. Fresh LEU used in research reactors is usually enriched between 12% and 19.75% ²³⁵U; the latter concentration is used to replace HEU fuels when converting to LEU.

Highly enriched uranium (HEU)

thumb|A [[billet (bar stock)|billet of highly enriched uranium metal]]

Highly enriched uranium (HEU) has a 20% or higher concentration of ²³⁵U. 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 ²³⁵U known as weapons grade, though theoretically for an 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"; 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.

thumb|A graph showing the calculated critical masses of uranium (in kilograms, left axis) as a function of the thickness of a beryllium reflector (in centimeters, bottom axis) and U-235 enrichment level (line labels).

The first uranium bomb, Little Boy, dropped by the United States on Hiroshima in 1945, used 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 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 , which at normal density would be a sphere about in diameter. 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% along with the 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 ²³⁸U is not said to be fissile but still is fissionable by fast neutrons (>2 MeV) such as the ones produced during D–T fusion.

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%. 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 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. 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 isotopes of the same element have nearly identical chemical properties and differ only slightly in mass. In the case of uranium, ²³⁵U is about 1.26% lighter than ²³⁸U. Commercial enrichment normally uses uranium hexafluoride (UF₆) rather than atomic uranium, which further reduces the relative mass difference between the molecules containing the two isotopes.

In the mid-2020s, commercial uranium enrichment is performed by gas centrifuge plants. Centrifuge enrichment uses UF₆ gas and separates isotopes by exploiting the small mass difference between molecules containing ²³⁵U and ²³⁸U. Modern centrifuge plants require far less electricity than gaseous diffusion plants; the World Nuclear Association estimates roughly 40–50 kWh per separative work unit for modern centrifuge plants, compared with about 2,400–2,500 kWh per SWU for gaseous diffusion. 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. In 2013, the Paducah facility in the U.S. ceased operating; it was the last commercial ²³⁵U gaseous diffusion plant in the world.

Thermal diffusion

Thermal diffusion uses the transfer of heat across a thin liquid or gas to accomplish isotope separation. 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

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 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,

Zippe centrifuge

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 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 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 More than 20 countries worked with laser separation during the 1990s and 2000s, though all achieved very limited success.

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 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. 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 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 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. It includes a detailed analysis of how the technology works and the challenges it presents for detecting clandestine uranium enrichment facilities.

Other techniques

Aerodynamic processes

thumb|Schematic diagram of an aerodynamic nozzle. Many thousands of these small foils would be combined in an enrichment unit.

right|thumb| The X-ray-based [[LIGA manufacturing process was originally developed at the Forschungszentrum Karlsruhe, Germany, to produce nozzles for isotope enrichment.]]

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. All methods have high energy consumption and substantial requirements for removal of waste heat; none is currently still in use.

Electromagnetic isotope separation

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 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, 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 ²³⁵U 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.

==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.

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: ²³⁴U is a minor isotope contained in natural uranium (primarily as a product of alpha decay of —because the half-life of is much larger than that of , it is produced and destroyed at the same rate in a constant steady state equilibrium, bringing any sample with sufficient content to a stable ratio of to over long enough timescales); 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. is produced primarily when absorbs a neutron and does not fission. The production of is thus unavoidable in any thermal neutron reactor with fuel. HEU reprocessed from nuclear weapons material production reactors (with an ²³⁵U assay of approximately 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. While also absorbs neutrons, it is a fertile material that is turned into fissile upon neutron absorption. If absorbs a neutron, the resulting short-lived beta decays to , 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 (for use in 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% ²³⁵U 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. 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

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
• Nuclear fuel cycle in France
• Orano
• Uranium market
• Uranium mining

References

External links

• Annotated bibliography on enriched uranium from the Alsos Digital Library for Nuclear Issues
• Silex Systems Ltd
• Uranium Enrichment , World Nuclear Association
• Overview and history of U.S. HEU production
• News Resource on Uranium Enrichment
• Nuclear Chemistry-Uranium Enrichment
• 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