Most of the commercial nuclear power plants today use uranium enriched to a higher uranium-235 (U-235) concentration—typically below 5%—instead of natural uranium, which contains only 0.7% U-235. The remaining 99.3% of naturally occurring uranium consists predominantly of uranium 238 (U-238), which has three more neutrons in its atomic nucleus than U-235.

The rationale for this enrichment is, firstly, to increase the number of neutrons available for nuclear fission so as to compensate for neutron absorption by neighbouring material used for structural purposes in the reactor core, and secondly, to extend the operating life of the nuclear fuel, thereby improving overall economy.

ntially, enrichment involves separating a uranium mixture into two streams: one with a higher U-235 concentration and the other with a lower concentration.

Industrial uranium enrichment was first achieved by converting uranium into uranium hexafluoride (UF6) gas and forcing it under pressure through a porous membrane, in a method known as the gas diffusion process. In this process, UF6 containing the lighter U-235 passes through the membrane slightly faster than that containing the heavier U-238. However, because the molecular mass of the UF6 differ only marginally between U-235 and U-238, each pass through a membrane achieves only minimal separation. More than a thousand diffusers therefore need to be connected in series to form a cascade, gradually increasing the U-235 concentration to the desired enrichment level. At each stage, the UF6 gas must be compressed and cooled, and the repetition cycles render the diffusion process highly energy-intensive.

oday, commercial uranium enrichment primarily uses the gas centrifuge process, in which UF6 gas is spun at high speed inside a long cylindrical rotor. The UF6 gas with the lighter U-235 molecules tend to concentrate near the centre, while the heavier U-238 molecules accumulate at the periphery, enabling separation. As with diffusion, the process is passed through a cascade of centrifuges to achieve the desired level of U-235 enrichment. The centrifuge process is approximately 40-50 times more energy-efficient than the diffusion method.

Other form methods of isotope separation are also being explored, for example by forcing UF6 gas at very high speed through a very narrow radius, or by using lasers to selectively ionise U-235 atoms in a vapour of uranium metal.

This article is contributed by Ir Richard Fung with the coordination of the Nuclear Division.