Thermal diffusion columns - Courtesy of the National Archives
July 1944
Construction of the S-50 thermal diffusion plant at Oak Ridge begins.
Thermal diffusion had initially been rejected as a means of separating U-235 because it could not efficiently enrich natural uranium all the way up to bomb grade. However, by mid 1944, with both the electromagnetic and gaseous diffusion plants running behind schedule, Philip Abelson brought forth the idea of using thermal diffusion to slightly enrich a large amount of uranium (from 0.71% to 0.89%).
This enriched uranium could then be fed into the electromagnets and substantially increase their yield of U-235. With this in mind, Groves contracted with the firm of H.K. Ferguson to build a 2100 column thermal diffusion plant in 90 days! Each column was a double walled nickel-copper pipe 48 feet long. On Sept. 13, 1944, only 69 days after construction began, operations started in 320 of the columns. S-50 remained operational only until Sept. 1945.
Thermal diffusion operates on the principle that lighter particles move toward hot surfaces while heavy particles move toward cooler surfaces. The lighter U-235 moves toward the hot inner tube while the heavier U-238 moves toward the cooler outer tube. Convection currents cause U-235 to flow upwards and U-238 to flow down.
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