Zippe-type centrifuge
The Zippe design was originally developed in the Soviet Union by a team led by 60 Austrian and German scientists and engineers captured after World War II, working in detention.
[1][2] Natural uranium consists of three isotopes; the majority (99.274%) is U-238, while approximately 0.72% is U-235, fissile by thermal neutrons, and the remaining 0.0055% is U-234.
Centrifuges need to work with a fluid rather than a solid, and this process used gaseous uranium hexafluoride.
[7][8] A Zippe-type centrifuge[9] has a hollow, cylindrical rotor filled with gaseous uranium hexafluoride (UF6) A rotating magnetic field at the bottom of the rotor, as used in an electric motor, is able to spin it quickly enough that the UF6 is thrown towards the outer wall, with the 238UF6 enriched in the outermost layer and the 235UF6 enriched at the inside of this layer.
The centrifugal force creates a pressure gradient: On the axis of the centrifuge there is practically vacuum, so that no mechanical feedthrough or seal is needed for the gas inlet and outlets; near the wall the UF6 reaches its saturation pressure, which in turn limits the rotation speed, because condensation must be avoided.
In the so-called countercurrent centrifuge, the bottom of the gaseous mix can be heated, producing convection currents.
where p is the pressure, r the variable radius and R its maximum, M the molecular mass, ω the angular velocity, k the Boltzmann constant and T the temperature.
A countercurrent of the gas is stimulated either mechanically or (less preferred) by a temperature gradient between the top and bottom of the rotor.
After the scientists were released from Soviet captivity in 1956,[1] Gernot Zippe was surprised to find that engineers in the West were years behind in their centrifuge technology.
He was able to reproduce his design at the University of Virginia in the United States, publishing the results, even though the Soviets had confiscated his notes.
This improved centrifuge design was long used by the commercial company Urenco to produce enriched uranium fuel for nuclear power stations.
Such are also the various techniques that are used to avoid forces causing destructive (bending) vibrations: Lengthening of a (countercurrent) centrifuge improves the enrichment exponentially.
[9] But it also decreases the vibrational frequency of mechanical resonances, which increases the danger of catastrophic failure during start-up (as happened during the Stuxnet event in Iran).
However, compared to other enrichment methods, it is much cheaper and is faster to set up, consumes much less energy and requires little area for the plant.
This makes it ideal for covert nuclear-weapons programs and increases the risk of nuclear proliferation.
The Soviets credit Steenbeck, Isaac Kikoin and Evgeni Kamenev with originating different valuable aspects of the design.
