Principles of ultracentrifugiation:
Uranium hexafluoride (UF6) gas is injected into a centrifuge which spins at twice the speed of sound. The heaviest of these isotopes, uranium 238, is pushed against the outer wall of the cylinder while uranium 235, the lightest of these, stays closer to the axis of rotation. This uranium compound must pass through several thousand centrifuges before it is enriched to the required percentage. These procedures can, of course, be misappropriated to manufacture atomic bombs.
IN2P3/Urenco