Uranium Enrichment

Uranium enrichment is the physical process of increasing the proportion of uranium-235, the isotope capable of sustaining a nuclear fission chain reaction, relative to uranium-238. Natural uranium contains about 0.7 per cent uranium-235; enrichment separates isotopes rather than creating or chemically changing them.

Basis of separation

Uranium-235 and uranium-238 have nearly identical chemical behaviour but slightly different atomic masses. Uranium is therefore converted into gaseous uranium hexafluoride, UF6, allowing equipment to exploit the small mass difference between molecules containing the two isotopes.

• Enrichment raises uranium-235's percentage by separating out part of the more abundant uranium-238.
• The enriched stream is called the product, while the remaining uranium with a lower uranium-235 concentration is called depleted uranium or tails.

Gas centrifuge process

A gas centrifuge spins UF6 at very high speed. Centrifugal effects concentrate the heavier uranium-238-bearing molecules slightly nearer the rotor wall, while the lighter uranium-235-bearing molecules remain slightly closer to the axis.

• The separation achieved by one centrifuge is small, so many centrifuges are connected in a cascade.
• Successive stages repeatedly feed the slightly enriched stream forward, progressively increasing its uranium-235 concentration.
• Centrifuges arranged in parallel increase processing capacity, while those arranged in series increase the degree of enrichment.

Other method and strategic significance

In gaseous diffusion, UF6 passes through porous barriers; lighter uranium-235-bearing molecules pass marginally faster than heavier ones. Repetition across many stages produces enrichment, but centrifuges require substantially less energy than gaseous diffusion.

• Because the same basic technology can produce reactor fuel or, with much further enrichment, weapons-usable material, enrichment is a dual-use technology.
• Applicable IAEA safeguards use accountancy, inspections and monitoring to verify that declared nuclear material is not diverted from peaceful activities.

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