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What is a Gas Centrifuge?1

In nature, uranium contains less than 1 percent of the fissile isotope uranium 235 (U-235). A nuclear explosive needs uranium enriched to at least 20 percent U-235. Ideally greater than 90% U-235 is used. In order to increase the percentage of U-235 in relation to the more prevalent U-238 the uranium must be processed, or “enriched.” One technique to enrich uranium uses gas centrifuges.

A gas centrifuge (diagrams below) comprises an evacuated casing containing a cylindrical rotor which rotates at high speed in an almost friction-free environment. The uranium is fed into the rotor as gaseous uranium hexafluoride (UF6)2 which also rotates.

The centrifugal forces push the heavier uranium 238 (U-238) closer to the wall of the rotor than the lighter U-235. The gas closer to the wall becomes depleted in U-235 whereas the gas nearer the rotor axis is enriched in U-235.

The arrows in the first illustration depict the gas flowing within the rotor. The gas flow can be produced by a temperature gradient over the length of the centrifuge. UF6 depleted in U-235 flows upwards adjacent to the rotor wall, while UF6 enriched in U-235 flows downwards close to the axis. The two gas streams are removed through small scooped pipes, called "scoops."

The enrichment effect of a single centrifuge is small, so they are linked together by pipes into cascades. Passing through the successive centrifuges of a cascade, the U-235 is gradually enriched to the required level. For civil applications, natural uranium containing about 0.7 percent U-235 is enriched to about 3-5 percent U-235 and the depleted uranium contains typically about 0.2-0.3 percent U-235. For military applications, highly enriched uranium (HEU) containing greater than 20 percent U-235 is usually produced.

Once started, a modern centrifuge runs for more than 10 years with no maintenance. An advantage of the centrifuge process is its low energy consumption.

Click on any of the above images to see a larger display


2 Uranium hexafluoride (UF6) is a solid white material at room temperature, which evaporates into gaseous material at elevated temperatures.

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