with 60 cm diameter, 12 m height and 900 m/s peripheral speed. A countercurrent of the gas is stimulated either mecanically or (less preferred) by a temperature gradient between the top and bottom of the rotor. With a countercurrent-to-feed ratio of 4, Glaser calculates a separation factor of 1,74 for a TC-21 centrifuge of 5 m height. Lowering this ratio (by increasing the feed) decreases the separation factor but increases the throughput and thus the productivity. To reduce friction, the rotor spins in a vacuum. Part of the rotor with the near-by housing acts as a molecular pump, which maintains the vacuum. A magnetic bearing holds the top of the rotor steady, and the only physical contact (necessary only during start-up) is the conical jewel bearing on which the rotor sits. Both bearings contain measures for damping vibrations. The three gas lines enter the rotor on its axis. After the scientists were released from Soviet captivity in 1956, 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. Zippe left the United States when he was effectively barred from continuing his research: The Americans classified the work as secret, requiring him either to become a U.S. citizen (he refused), return to Europe, or abandon his research. He returned to Europe where, during the 1960s, he and his colleagues made the centrifuge more efficient by changing the material of the rotor from aluminium to maraging steel, an alloy with a longer fatigue life and longer breaking length, which allowed higher speed. This improved centrifuge design was long used by the commercial company Urenco to produce enriched uranium fuel for nuclear power stations. More recently, they use (e.g. in their model TC-21) carbon fiber reinforced walls. The exact details of advanced Zippe-type centrifuges are closely guarded secrets. For example, the efficiency of the centrifuges is improved by increasing their speed of rotation. To do so, stronger materials, such as carbon fiber-reinforced composite materials, are used; but details of the material and its protection against chemical attacks are proprietary. 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. But it also decreases