Thermal Diffusion Uranium Enrichment
Thermal diffusion utilizes the transfer of heat across a thin liquid or gas to accomplish isotope separation. By cooling a vertical film on one side and heating it on the other side, the resultant convection currents will produce an upward flow along the hot surface and a downward flow along the cold surface. Under these conditions, the lighter 235 U gas molecules will diffuse toward the hot surface, and the heavier 238 U molecules will diffuse toward the cold surface. These two diffusive motions combined with the convection currents will cause the lighter 235 U molecules to concentrate at the top of the film and the heavier 238 U molecules to concentrate at the bottom of the film.
The thermal-diffusion process is characterized by its simplicity, low capital cost, and high heat consumption. Thermal diffusion in liquid UF 6 was used during World War II to prepare feed material for the EMIS process. A production plant containing 2,100 columns (each approximately 15 meters long) was operated in Oak Ridge for less than 1 year and provided a product assay of less than 1% 235 U. Each of these columns consisted of three tubes. Cooling water was circulated between the outer and middle tubes, and the inner tube carried steam. The annular space between the inner and middle tubes was filled with liquid UF 6.
The thermal-diffusion plant in Oak Ridge was dismantled when the much more energy-efficient (by a factor of 140) gaseous-diffusion plant began operation in the 1940's. Today, thermal diffusion remains a practical process to separate isotopes of noble gases (e.g., xenon) and other light isotopes (e.g., carbon) for research purposes.