Enriched uranium

Enriched uranium is a special kind of uranium that has more of a rare part called uranium-235. Normally, uranium found in nature has only a tiny bit of uranium-235 mixed with other parts, mostly uranium-238. But scientists can increase the amount of uranium-235 by a process called isotope separation.

Uranium-235 is important because it can split easily when touched by certain particles, making it very useful. This special uranium is used to make energy in nuclear power plants.

The way to make enriched uranium was first created during the Manhattan Project in the 1940s. One common method uses gas to separate the parts, and another uses spinning machines called centrifuges. After enrichment, the leftover uranium-238 is called depleted uranium, which is very dense and used for things like protection in vehicles.

Grades

Uranium taken from the Earth needs special preparation before it can be used as fuel in most nuclear reactors. It is first mined, either underground or in open pits, and then processed to remove the uranium from the ore, creating a material called "yellowcake" that contains about 80% uranium.

To make nuclear fuel, this uranium is turned into either uranium dioxide for some reactors or uranium hexafluoride for most others. Natural uranium contains mostly a type called uranium-238, with only a small amount of uranium-235, which is needed to produce energy in reactors. Most reactors need uranium with more uranium-235 than nature provides, called enriched uranium, usually between 3.5% and 4.5% uranium-235. There are two main ways to enrich uranium: gaseous diffusion and gas centrifugation, both using uranium hexafluoride.

Main article: Reprocessed uranium

Reprocessed uranium (RepU) comes from spent nuclear fuel that has been treated chemically and physically to recover usable uranium. It often has a little more uranium-235 than natural uranium and can be used in certain reactors.

Low-enriched uranium (LEU) has less than 20% uranium-235. In most common reactors, it is enriched to about 3 to 5% uranium-235. Slightly enriched uranium (SEU) has under 2% uranium-235.

High-assay LEU (HALEU) is uranium enriched between 5% and 20% uranium-235. It is needed for many small modular reactor designs and research reactors.

Highly enriched uranium (HEU) has 20% or more uranium-235. This level of enrichment is used in some special reactors and for making medical isotopes.

Enrichment methods

Isolating one type of uranium from another is very hard because they behave almost the same way. They can only be separated slowly, using very small differences in weight. (²³⁵U is only 1.26% lighter than ²³⁸U.) This makes the process challenging, especially since uranium is usually handled as a compound, not in its pure form (²³⁵UF₆ is only 0.852% lighter than ²³⁸UF₆).

There are two main ways to enrich uranium used around the world: gaseous diffusion (an older method) and gas centrifuge (a newer method that uses much less energy). Some research is exploring nuclear resonance, but it has not yet been used for large-scale production.

Diffusion techniques

Gaseous diffusion

Main article: Gaseous diffusion

Gaseous diffusion is a way to make enriched uranium by pushing a gas form of uranium hexafluoride ('hex') through special membranes. This lets some molecules pass more easily than others, creating a small difference in concentration. During the Cold War, this was a major method for enriching uranium. By 2008, it made up about one-third of the world’s enriched uranium, but it is now considered old technology. By 2011, it was being replaced as the plants that used it were shutting down. In 2013, the Paducah facility in the U.S. closed, leaving no commercial plants using this method anywhere in the world.

Thermal diffusion

Thermal diffusion uses heat to separate the uranium types. It works by heating one side of a liquid or gas and cooling the other. Lighter ²³⁵U molecules move toward the hot side, while heavier ²³⁸U molecules move toward the cold side. During World War II, the S-50 plant in Oak Ridge, Tennessee used this method with liquid uranium hexafluoride. This method was used to help make material for another process but was later stopped in favor of better methods.

Centrifuge techniques

Gas centrifuge

Main article: Gas centrifuge

The gas centrifuge method uses many spinning cylinders set up in lines and groups. The spinning creates a force that pushes the heavier ²³⁸U molecules to the outer edge, while the lighter ²³⁵U molecules stay closer to the center. This uses much less energy than the older gaseous diffusion method and has mostly replaced it. It now makes almost all of the world’s enriched uranium.

Zippe centrifuge

Main article: Zippe-type centrifuge

The Zippe-type centrifuge improves on the basic gas centrifuge by adding heat. Heating the bottom of the spinning cylinder creates currents that lift the ²³⁵U up to be collected. This design is used by Urenco to make nuclear fuel and was also used by Pakistan in its nuclear program.

Laser techniques

Laser methods are being studied because they could use less energy, cost less, and need less space. Several laser methods are being developed or tested. Separation of isotopes by laser excitation (SILEX) is advanced and has been approved for commercial use since 2012. This method is very effective and cheap, needing much less energy and space than older methods. Many countries worked on laser separation in the 1990s and 2000s, but most had limited success.

Atomic vapor laser isotope separation (AVLIS)

Atomic vapor laser isotope separation uses special lasers to separate uranium isotopes by targeting hyperfine transitions. The lasers are tuned to affect only ²³⁵U atoms, which are then collected.

Molecular laser isotope separation (MLIS)

Molecular laser isotope separation uses an infrared laser on UF₆, exciting molecules that contain a ²³⁵U atom. A second laser removes a fluorine atom, leaving behind uranium pentafluoride, which then settles out of the gas.

Separation of isotopes by laser excitation (SILEX)

Separation of isotopes by laser excitation is an Australian method that also uses UF₆. After years of development, including by U.S. company USEC, GE Hitachi Nuclear Energy (GEH) signed an agreement to develop it commercially in 2006. GEH built a test loop and planned to build a full-scale plant. Details of how it works are kept secret due to agreements between the United States, Australia, and the companies involved. SILEX could be much more efficient than current methods, but exactly how much better is also secret.

Other techniques

Aerodynamic processes

Aerodynamic methods include the Becker jet nozzle and the vortex tube. These depend on differences in pressure to separate the uranium types, similar to how centrifuges work but without spinning. They usually mix UF₆ with hydrogen or helium to make the gas move faster. The Uranium Enrichment Corporation of South Africa (UCOR) used a special vortex tube design in industrial plants. A demonstration plant was built in Brazil by NUCLEI, a group led by Industrias Nucleares do Brasil, using the separation nozzle process. These methods need a lot of energy and ways to deal with waste heat, and none are used today.

Electromagnetic isotope separation

Main article: Calutron

In electromagnetic isotope separation (EMIS), uranium is turned into a vapor and then into positively charged ions. These ions are pushed and bent by magnetic fields to collect the different types. During World War II, a large machine called the calutron was built and provided some of the ²³⁵U used for the Little Boy nuclear bomb dropped on Hiroshima in 1945. This method has mostly been replaced by better techniques.

Chemical methods

One chemical method was tested but never used for production. The French CHEMEX process used small differences in how the isotopes change form, using different liquids that do not mix. A similar method was developed by the Asahi Chemical Company in Japan, using a special material to separate the isotopes.

Plasma separation

Plasma separation uses very strong magnets and plasma physics. It works by using the natural movement of ions to affect only the ²³⁵U isotope. France created its own version called RCI. Funding for RCI was cut in 1986, and the project was stopped around 1990, though RCI is still used for separating stable isotopes.

Separative work unit

Further information: Separative work units

Separative work measures how much an enrichment process can separate materials. It depends on the amount of starting material, the enriched result, and what is left over. It is not energy, but it shows how much work the process does. The unit for separative work is called a Separative work unit, or SWU.

Here are some ways to measure it:

• 1 SWU = 1 kg SW = 1 kg UTA
• 1 kSWU = 1 tSW = 1 t UTA
• 1 MSWU = 1 ktSW = 1 kt UTA

Using separative work well helps make uranium enrichment facilities work better and cost less.

Cost issues

When making enriched uranium, there are two main things to think about: how much natural uranium you start with and how much work is needed to enrich it. The amount of natural uranium needed changes depending on how much enriched uranium you want to make and how much of a special part called ²³⁵U ends up in the leftover material.

For example, to make one kilogram of enriched uranium for a reactor, you usually need about 8 kilograms of natural uranium if the leftover material has 0.3% ²³⁵U. But if you want the leftover to have only 0.2% ²³⁵U, you need less natural uranium—about 6.7 kilograms—but it takes more work to enrich it.

hexafluoride

Downblending

Downblending is the process of changing highly enriched uranium into low-enriched uranium. This makes the uranium safe to use in nuclear power plants instead of for weapons.

This process helps keep the world safer by turning materials that could be used for weapons into fuel for peaceful energy.

One big program called the Megatons to Megawatts Program changed old Russian weapons uranium into fuel for American power plants. This turned material meant for war into energy for homes and businesses. Another program in the United States also turned old American weapons uranium into safe fuel for power plants, helping to keep nuclear materials safe and support clean energy production.

Global enrichment facilities

Several countries have facilities to enrich uranium. These include Argentina, Brazil, China, France, Germany, India, Iran, Japan, the Netherlands, North Korea, Pakistan, Russia, the United Kingdom, and the United States. Some countries, like Belgium, Iran, Italy, and Spain, have invested in France's Eurodif enrichment plant. Iran’s investment gives it a share of the enriched uranium produced there.

In the past, Libya and South Africa had plans for enrichment programs, but Libya’s facility was never used. Australia’s Silex Systems created a method called laser enrichment, aiming to build a plant in the U.S. with support from General Electric. However, this technology is still being tested and has not yet proven to be practical for widespread use. Israel is also thought to have a program at the Negev Nuclear Research Center near Dimona.

Codename

During the Manhattan Project, a big science project to build a special type of energy, highly enriched uranium was called oralloy. This name was a short way to say "Oak Ridge alloy," named after the place where the uranium was changed. Sometimes, people still use the word oralloy to talk about enriched uranium.