Table of Contents
Enrichment & Reconversion
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.
Why is enrichment necessary?
Uranium exists as a range of different isotopes, each with the same number of protons and electrons but a different number of neutrons.
So, each isotope has a slightly different mass and different nuclear properties:
- Natural Uranium – approximately 99.33 % 238U + 0.7 % 235U.
Uranium-235 (235U) is the fissile isotope required for the nuclear fission chain reaction inside a nuclear reactor. Uranium-238 (238U), the most common isotope in naturally occurring uranium, is non-fissile. Hence, natural uranium must be enriched to sufficiently increase the proportion of the fissile 235U isotope:
- Fuel-Grade Uranium – typically requires a higher proportion of 235U.
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.
What level of enrichment is needed?
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
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.
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.
Where is enrichment carried out?
What happens in the UK?
What happens in the UK?
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.
Adam Piatt
