Gas centrifuge technology again makes use of the mass difference between 235UF6 and “8UF6 to promote separation. The process entails UFi gas being fed into a centrifuge unit spinning at very high speed, with the wall of the centrifuge acting as the rotor. The rotation of the gas applies an acceleration to the gas molecules in the direction of the centrifuge wall, with the greater force exerted on the molecule with the higher mass so that the more massive molecules concentrate at the centrifuge walls, whilst the less massive molecules concentrate closer to the central axis of the unit. The partially separated gas is then encouraged to circulate along the centrifuge axis using a thermal gradient. ‘Scoops’ are used to draw off an enriched product stream and a depleted tails stream. A diagram of a single centrifuge unit is shown in Fig. 7.5 . The centrifuge sits above the motor which drives the rotor. There is also a magnetic bearing at the top of the centrifuge through which the inlet and outlet pipes pass. The whole centrifuge is housed in a casing, with the space evacuated to reduce friction to a minimum.

The separation factor achieved by a single centrifuge unit is much greater than for a single diffuser unit, in the range 1.2 to 2.0 depending on the sophistication of the centrifuge. The enrichment required for nuclear fuel can still not be achieved with a single machine, however, and the feed rate is quite low so that a cascade containing many machines linked in series and parallel (see Fig. 7.4) is needed to achieve the desired output and to reduce the 235U level in the tails to economically viable levels. A commercial scale gas centrifuge plant is likely to have tens of thousands of separate units in a number of cascades. The greater separation factor that can be achieved by a single centrifuge still means that the footprint is likely to be smaller than for a gaseous diffusion plant with the same SWU rating. Crucially, the power consumption of a centrifuge plant, although still significant, is in the region of 2% of that required for gaseous diffusion.

The performance of a centrifuge increases with its height (axial length) and rotational speed. Construction materials are therefore required that are both strong and resistant to UF. . Older models used aluminium alloys or maraging steel, whilst later models use carbon fibre. Centrifuge units spin at speeds that can

approach the speed of sound, putting considerable stress on the machine, particularly the rotors and bearings. These parts of the centrifuge require advanced design and manufacturing capability, as does the electrical drive system, which must provide very precise speed control. The vast majority of the world’s centrifuge enrichment capacity is based on either Russian or ETC designed machines. The American Centrifuge being developed by USEC is designed to have a higher output than existing machines, although only time will prove whether it has comparable reliability.

The basic philosophy for running a centrifuge is to vacuum it down, start it spinning, feed it with UF6 and leave it. The units are kept at constant temperature and protected from impact and other physical interference and allowed to run 24 hours a day, 365 days a year without maintenance or interference. Any adjustment to the performance of a cascade in terms of product and tails enrichment is usually made by altering the feed and bleed between machines within the cascade, rather than changing the way that the machines operate. Should a machine fail, for example if a drive motor fails, then operation of the cascade is adjusted to accommodate it rather than attempting repair.

Modern centrifuges are an engineering marvel. They operate continuously and without maintenance yet rarely fail, despite the extreme stresses they endure. ETC machines on UEC sites run with a failure rate of less than 1% per year and many centrifuges have been operating continuously since they were first brought on line, some as long ago as the early 1980s.