Thermal diffusion enrichment process

The thermal diffusion enrichment process was developed by Dr. Philip H. Abelson for naval applications. He was asked to oversee the engineering and operations of a thermal diffusion plant, S-50, at K-25 in Oak Ridge, Tennessee, during World War II as a back-up to the other enrichment processes, which were experiencing problems. 

The thermal diffusion method requires large quantities of power and is therefore primarily of interest for preparation of laboratory scale samples. As such, it has been developed by Clusius among others, and is a very effective separation process. Overall separations as high as 10,000,000 have been achieved by the Clusius group. A summary of the evolution of the thermal diffusion column in Clusius laboratory is given in Table III (JO). Of particular note is the enrichment of Ar, a middle isotope, from a natural abundance of 0.064% to a final isotopic purity of 99.984%. 

Specific equipment and processes have been developed to bring about the desired enrichment with respect to the individual components of a mixture by thermal diffusion. 

Thermal diffusion of UF Small amount of slightly enriched UF produced in United States in 1945 process abandoned... 

Thermal diffusion of UF . The thermal diffusion process makes use of the small difference in 23su/Js u ratio that is established when heat flows through a mixture of UFj and UF. The principle of the process is described in Chap. 14. The process was used [Al] in 1945 in the United States by the Manhattan Prqect to enrich uranium to 0.86 percent U. This slightly enriched material was used as feed for an electromagnetic separation plant. Although the process could be put into production quickly because of the simplicity of the equipment, it... 

The degree of separation obtainable in thermal diffusion (the difference in composition between hot and cold walls) is much less than in other diffusion processes, so that use of a column to multiply the composition difference is practically essential. The stage type of thermal diffusion has been used only to measure the thermal diffusion coefficient and is never used for practical separations. In some thermal diffusion columns, htu s are as low as 1.5 cm, and as many as 800 stages of separation have been obtained from a sin e column. Even with such a great increase in separation, it is often necessary to use a tapered cascade of thermal diffusion columns for isotopic mixtures, to minimize hold-up of partially enriched isotopes and to reduce equilibrium time. 

Abelson and Hoover [Al], working in the U.S. Naval Research Laboratory, found that thermal diffusion in UF at pressures above the critical (4.6 MPa) resulted in small but measurable enrichment of at the hot waU. Because of the simplicity of thermal diffusion equipment compared with the advanced technology needed for the gaseous diffusion process, the Manhattan District in the United States in 1944-1945 used thermal diffusion of UF to raise its content to 0.86 percent, to serve as partially enriched feed for the Y-12... 

The maximum separative capacity, A , and the power consumed per unit separative capacity, G/ max, given in the last two columns of Table 14.25 have been calculated from Abelson s parameters Y and 0 to permit comparison with the other processes for enriching uranium treated in this chapter. Because the thermal diffusion column operates with constant reflux ratio, its steady-state separation performance as an enricher is given by Eq. (14.237), expressed here in the form... 

A number of other processes for separating isotopes are documented. A partial list includes membrane pervaporation, thermal diffusion of liquids, mass diffusion, electrolysis and electro migration, differential precipitation, solvent extraction, biological microbial enrichment, and more. Although not discussed in this chapter, some are suitable for small-scale laboratory separations, and others have been applied on reasonably large scale to D/T, H/T, and i/ Li and presumably to other pairs of isotopes. 

The plant operated for 1 year but was shut down and abandoned in favor of the gaseous diffusion process. Clusius and Dickel in Germany experimented with thermal diffusion in 1938 for other isotopes, and the Japanese tried to use the process for uranium enrichment without success during World War n. There is no known interest in thermal diffusion at this time. 

Meanwhile, other Manhattan Project scientists were enriching uranium through the thermal-diffusion process. By heating a thin, vertically held film of uranium on one side and cooling it on the other, they were able to draw the U-235 molecules to the top of the film. 

GREEN SALT. Some processes for enriching uranium involve an intermediate step in which uranium dioxide is converted to uranium tetrafluoride (UF4), a green crystalline compound commonly called green salt. This compound is then converted to uranium hexafluoride (UFg), a volatile gas used in isotopic separation processes to yield uranium-235. See also GAS CENTRIFUGE GASEOUS DIFFUSION THERMAL DIFFUSION. 

Enrichment, Isotopic—An isotopic separation process by which the relative abundances of the isotopes of a given element are altered, thus producing a form of the element that has been enriched in one or more isotopes and depleted in others. In uranium enrichment, the percentage of uranium-235 in natural uranium can be increased from 0.7% to >90% in a gaseous diffusion process based on the different thermal velocities of the constituents of natural uranium (234U, 235U, 238U) in the molecular form UF6. 

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