in the vortex separation process a mixture of uranium hexafluoride gas and hydrogen is injected tangentially into a tube at one end through nozzles or holes, at velocities close to the speed of sound. the tube tapers to a small exit aperture at one or both ends. this tangential injection of gas results in a spiral or vortex motion within the tube, and two gas streams are withdrawn at opposite ends of the vortex tube; centrifugal force providing the isotopic separation. the spiral swirling flow decays downstream of the feed inlet due to friction at the tube wall. consequently, the inside diameter of the tube is typically tapered to reduce decay in the swirling flow velocity. this process is characterized by a separating element with a very small stage cut (the ratio of product flow to feed flow) of about 1/20, and high process-operating pressures.