Gaseous Diffusion Uranium Enrichment
One of the earliest successful enrichment technique was gaseous diffusion, in which the gas uranium hexafluoride (UF 6 ) is passed through a porous barrier material; the lighter molecules containing 235 U penetrate the barrier slightly more rapidly, and with enough stages significant separation can be accomplished.
The gaseous-diffusion process has been highly developed and employed to produce both HEU and commercial reactor-grade LEU. The United States first employed gaseous diffusion during WWII and expanded its capacity after the war to produce HEU. Since the late 1960's, the U.S. facilities have been used primarily to produce commercial LEU, with the last remaining HEU capacity being shut down in 1992. China and France currently have operating diffusion plants. Russia's enrichment facilities have been converted from diffusion to centrifuge technology. Britain's diffusion facility was shut down and dismantled.
The gaseous-diffusion process depends on the separation effect arising from molecular effusion (i.e., the flow of gas through small holes). On average, lighter gas molecules travel faster than heavier gas molecules and consequently tend to collide more often with the porous barrier material. Thus, lighter molecules are more likely to enter the barrier pores than are heavier molecules. For UF 6 , the difference in velocities between molecules containing 235 U and 238 U is small (0.4 percent), and, consequently, the amount of separation achieved by a single stage of gaseous diffusion is small. Therefore, many cascade stages are required to achieve even LEU assays.
UF6 is a solid at room temperature but becomes a gas when heated above 135 degrees Fahrenheit. The solid UF6 is heated to form a gas, and the gaseous diffusion enrichment process begins. The process separates the lighter U-235 isotopes from the heavier U-238. The gas is forced through a series of porous membranes with microscopic openings. Because the U-235 is lighter, it moves through the barriers more easily. As the gas moves, the two isotopes are separated, increasing the U-235 concentration and decreasing the concentration of U-238.
Diffusion equipment tends to be rather large and consumes significant amounts of energy. The main components of a single gaseous-diffusion stage are (1) a large cylindrical vessel, called a diffuser or converter, that contains the barrier; (2) a compressor used to compress the gas to the pressures needed for flow through the barrier; (3) an electric motor to drive the compressor; (4) a heat exchanger to remove the heat of compression; and (5) piping and valves for stage and interstage connections and process control. The entire system must be essentially leak free, and the compressors require special seals to prevent both out-leakage of UF 6 and in-leakage of air. The chemical corrosiveness of UF 6 requires use of metals such as nickel or aluminum for surfaces exposed to the gas (e.g., piping and compressors). In addition to the stage equipment, auxiliary facilities for a gaseous-diffusion plant could include a large electrical power distribution system, cooling towers to dissipate the waste process heat, a fluorination facility, a steam plant, a barrier production plant, and a plant to produce dry air and nitrogen.
The production of a sustainable, efficient separating membrane (barrier) is the key to the successful operation of a diffusion plant. To obtain an efficient porous barrier, the holes must be very small (on the order of one-millionth of an inch in diameter) and of uniform size. The porosity of the barrier must be high to obtain high flow rates through the barrier. The barrier must also be able to withstand years of operation while exposed to corrosive UF 6 gas. Typical materials for the barrier are nickel and aluminum oxide.
Gaseous diffusion is unlikely to be the preferred technology of a proliferator due to difficulties associated with making and maintaining a suitable barrier, large energy consumption, the requirement for procuring large quantities of specialized stage equipment, large in-process inventory requirements, and long equilibrium times.
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