Aerodynamic Process Uranium Enrichment
Aerodynamic uranium enrichment processes include the separation nozzle process and the vortex tube separation process. These aerodynamic separation processes depend upon diffusion driven by pressure gradients, as does the gas centrifuge. In effect, aerodynamic processes can be considered as nonrotating centrifuges. Enhancement of the centrifugal forces is achieved by dilution of UF 6 with a carrier gas (i.e., hydrogen or helium). This achieves a much higher flow velocity for the gas than could be obtained using pure UF 6 .
The separation nozzle process was developed by E.W. Becker and associates at the Karlsruhe Nuclear Research Center in Germany. In this process, a mixture of gaseous UF 6 and H 2 (or helium) is compressed and then directed along a curved wall at high velocity. The heavier 238 U-bearing molecules move preferentially out to the wall relative to those containing 235 U. At the end of the deflection, the gas jet is split by a knife edge into a light fraction and a heavy fraction, which are withdrawn separately.
Economic considerations drive process designers to select separation nozzles with physical dimensions as small as manufacturing technology will allow. The curved wall of the nozzle may have a radius of curvature as small as 10 mm (0.0004 in.). Production of these tiny nozzles by such processes as stacking photo-etched metal foils is technically demanding. A typical stage consists of a vertical cylindrical vessel containing the separation elements, a cross piece for gas distribution, a gas cooler to remove the heat of compression, and a centrifugal compressor driven by a electric motor.
The South African nuclear program used an aerodynamic separation technique in an indigenously designed and built device called a vortex tube. In the vortex a mixture of UF 6 gas and hydrogen is injected tangentially into a tube, which tapers to a small exit aperture at one or both ends; centrifugal force provides the separation. The Becker Nozzle Process, also an aerodynamic separation technique, was developed in Germany. The Becker process is not in common use; the vortex tube was used in South Africa for producing reactor fuel with a 235 U content of around 3-5 percent in addition to making 80-93 percent 235 U for the weapons program. Aerodynamic enrichment processes require large amounts of electricity and are not generally considered economically competitive; even the South African enrichment plant has apparently been closed.
The Uranium Enrichment Corporation of South Africa, Ltd. (UCOR) developed and deployed its own aerodynamic process characterized as an "advanced vortex tube" or "stationary-walled centrifuge" at the so called "Y" plant at Valindaba to produce hundreds of kilograms of HEU. In this process, a mixture of UF 6 and H 2 is compressed and enters a vortex tube tangentially at one end through nozzles or holes at velocities close to the speed of sound. 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. 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 the decay in the swirling flow velocity. This process is characterized by a separating element with very small stage cut (ratio of product flow to feed flow) of about 1/20 and high process-operating pressures.
Due to the very small cut of the vortex tube stages and the extremely difficult piping requirements that would be necessary based on traditional methods of piping stages together, the South Africans developed a cascade design technique, called Helikon. In essence, the Helikon technique permits 20 separation stages to be combined into one large module, and all 20 stages share a common pair of axial-flow compressors. A basic requirement for the success of this method is that the axial-flow compressors successfully transmit parallel streams of different isotopic compositions without significant mixing. A typical Helikon module consists of a large cylindrical steel vessel that houses a separating element assembly, two axial-flow compressors (one mounted on each end), and two water-cooled heat exchangers.
For both of these aerodynamic processes, the high proportion of carrier gas required in relation to UF 6 process gas results in high specific-energy consumption and substantial requirements for removal of waste heat.
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