The Zippe-type centrifuge is a gas centrifuge designed to enrich the rare fissile isotope uranium-235 (235U) from the mixture of isotopes found in naturally occurring uranium compounds. The isotopic separation is based on the slight difference in mass of the isotopes. 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. In the West (and now generally) the type is known by the name of the man who recreated the technology after his return to the West in 1956, based on his recollection of his work in (and contributions to) the Soviet program, Gernot Zippe. To the extent that it might be referred to in Soviet/Russian usage by any one person's name, it was known (at least at a somewhat earlier stage of development) as a Kamenev centrifuge (after Evgeni Kamenev). Background 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. If natural uranium is enriched to 3 % U-235, it can be used as fuel for light water nuclear reactors. If it is enriched to 90 % uranium-235, it can be used for nuclear weapons. Centrifuge uranium enrichment Enriching uranium is difficult because the isotopes are practically identical in chemistry and very similar in weight: U-235 is only 1.26% lighter than U-238 (note this applies only to uranium metal). Centrifuges need to work with a gas rather than a solid, and the gas used here is uranium hexafluoride. The relative mass difference between 235UF6 and 238UF6 is less than 0.86%. On the other hand, separation efficiency in a centrifuge depends on absolute mass difference. Separation of uranium isotopes requires a centrifuge that can spin at 1,500 revolutions per second (90,000RPM). If we assume a rotor diameter of 20 cm (as in some modern centrifuges), this would correspond to a linear speed of greater than Mach2 in air (Mach1 = sound velocity, in air ca. 340 m/s) and much more in UF6. For comparison, automatic washing machines operate at only about 12 to 25 revolutions per second (720–1500RPM) during the spin cycle, while turbines in automotive turbochargers can run up to around 2500–3333 revolutions per second (150,000–200,000RPM). A Zippe-type centrifuge has a hollow, cylindrical rotor filled with gaseous uranium hexafluoride