Enrichment
This vital step in nuclear fuel production involves increasing the concentration of fissile uranium-235.
What is enrichment?
Enrichment is used to increase the proportion of the fissile uranium-235 isotope (235U) to the quantity necessary to sustain a chain reaction in a nuclear reactor.
How is enrichment carried out?
Enrichment can be carried out via gas diffusion or using a gas centrifuge, with the latter being the current commercial method of choice. Research into alternative methods, such as laser separation is ongoing.
Gas Diffusion
Gas Diffusion was the first-generation method for uranium enrichment.
It is no longer commercially viable.
- Lighter 235UF6 passes through a membrane more readily than heavier 238UF6, according to Graham’s Law (rate of diffusion of a gaseous species is inversely proportional to the square root of its mass).
Gas Centrifuge
Gas Centrifuge is the current commercial method for uranium enrichment.
This includes being used in the UK, at the Urenco site in Capenhurst.
- Natural UF6 is spun in a cylindrical rotor at 50,000-70,000 rpm.
- By centrifugal force, heavier 238UF6 is pushed to the edges.
- Lighter 235UF6 collects at the centre, allowing collection of enriched uranium.
Laser Separation
Lasers can also be used for enrichment.
One example is the SILEX process – Separation of Isotopes by Laser EXcitation – which is currently under research.
- Lasers preferentially excite vibrations in lighter 235UF6 dimers, causing these to break apart, allowing them to be collected.
What happens at the end of the enrichment process?
Enrichment is followed by reconversion – uranium hexafluoride (UF6, hex) is reconverted to uranium metal (U) or uranium dioxide (UO2), depending on the type of fuel.
Why is enrichment necessary?
Enrichment is necessary because naturally uranium contains only 0.7% of the fissile uranium-235 isotope (235U). Under most conditions, this concentration is insufficient to sustain a chain reaction.
Did you know?
Within a uranium enrichment facility, there are careful controls to avoid an unintended criticality incident (like happened in Tokaimura in 1997). The facility will also be inspected by safeguards personnel from the nuclear regulator for accountancy purposes – keeping track of what uranium is where and at what enrichment.
What level of enrichment is needed?
The extent of enrichment required depends on the use of the fuel and the type of reactor technology:
- Canadian Deuterated Uranium (CANDU) and Magnox reactors do not require enriched uranium.
- Pressurised Water Reactors (PWRs) require a minimum enrichment of 3% 235U.
- Fast breeder reactors, or those producing medical isotopes, typically require > 50% 235U.
- Atomic bombs typically require HEU, > 90% 235U.
Enrichment levels can be categorised, with different proportions of 235U:.
Depleted Uranium
Depleted Uranium
Low Enriched Uranium
Low Enriched Uranium
High Enriched Uranium
High Enriched Uranium
Where does enrichment take place?
The enrichment stage of the nuclear fuel cycle requires specialist facilities, primarily in countries with large nuclear sectors. Today, gas centrifuge is the enrichment technology of choice.
Key locations include China, France, Netherlands, Russia, UK and USA.
What happens in the UK?
What happens in the UK?
Explore Further
Choose from the articles below to continue learning about nuclear.
Nuclear Fuel – The source of incredible energy
Reactor Fuel – how does nuclear fuel power reactors?
The Nuclear Fuel Cycle Explained – An Open and Closed Case?
Reprocessing – How Spent Nuclear Fuel is Recycled for Reuse
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Below you can find references to the information and images used on this page.
Content References
- IAEA – Nuclear Fuel Cycle
- NSAN – Graduate Awareness in Nuclear (GAIN) Courses
- Nuclear-Power.com – Uranium Enrichment
- US NRC – Stages of the Nuclear Fuel Cycle
- World Nuclear Association – Uranium Enrichment
- World Nuclear Association – Nuclear Fuel Cycle Overview
- World Nuclear News – A guide: Uranium and the nuclear fuel cycle