the top end. Separative work units The separative work unit (SWU) is a measure of the amount of work done by the centrifuge and has units of mass (typically kilogram separative work unit). The work necessary to separate a mass of feed of assay into a mass of product assay , and tails of mass and assay is expressed in terms of the number of separative work units needed, given by the expression where is the value function, defined as Practical application of centrifugation Separating uranium-235 from uranium-238 The separation of uranium requires the material in a gaseous form; uranium hexafluoride (UF6) is used for uranium enrichment. Upon entering the centrifuge cylinder, the UF6 gas is rotated at a high speed. The rotation creates a strong centrifugal force that draws more of the heavier gas molecules (containing the U-238) toward the wall of the cylinder, while the lighter gas molecules (containing the U-235) tend to collect closer to the center. The stream that is slightly enriched in U-235 is withdrawn and fed into the next higher stage, while the slightly depleted stream is recycled back into the next lower stage. Separation of zinc isotopes For some uses in nuclear technology, the content of zinc-64 in zinc metal has to be lowered in order to prevent formation of radioisotopes by its neutron activation. Diethyl zinc is used as the gaseous feed medium for the centrifuge cascade. An example of a resulting material is depleted zinc oxide, used as a corrosion inhibitor. History Suggested in 1919, the centrifugal process was first successfully performed in 1934. American scientist Jesse Beams and his team at the University of Virginia developed the process by separating two chlorine isotopes through a vacuum ultracentrifuge. It was one of the initial isotopic separation means pursued during the Manhattan Project, more particularly by Harold Urey and Karl P. Cohen, but research was discontinued in 1944 as it was felt that the method would not produce results by the end of the war, and that other means of uranium enrichment (gaseous diffusion and electromagnetic separation) had a better chance of success in the short term. This method was successfully used in the Soviet nuclear program, making the Soviet Union the most effective supplier of enriched uranium. In the long term, especially with the development of the Zippe-type centrifuge, the gas centrifuge has become a very economical mode of separation, using