Uranium hexafluoride (UF6) gas is pulled into a vacuum centrifuge. Gas centrifuges are made up of a tubular container constructed from highly advanced materials, which contains a cylindrical rotor. This rotates at an extremely high speed in a vacuum on an almost frictionless bearing. The centrifuge motor produces heat at the bottom of the centrifuge. This creates a temperature gradient beside the rotor, through which gas flows, intensifying the separation process.
The rotating motion hurls the heavier U-238 molecules towards the wall of the centrifuge, while the lighter U-235 molecules congregate in the middle. A single centrifuge does not enrich uranium enough for use as nuclear fuel. Centrifuges are therefore used in series, to raise the level of enrichment. Enriched UF6 is piped to another centrifuge, while the depleted gas can be returned into the system. To increase the throughput of a system, centrifuges are also set up in parallel. These serial and parallel systems of centrifuges are known as cascades. Enriched uranium travels from one centrifuge to the next, for as long as is needed until it is sufficiently enriched to be used as pellets in nuclear fuel rods: the fuel for nuclear power stations..