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
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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