Uranium thermal diffusion

A number of special processes have been developed for difficult separations, such as the separation of the stable isotopes of uranium and those of other elements (see Nuclear reactors Uraniumand uranium compounds). Two of these processes, gaseous diffusion and gas centrifugation, are used by several nations on a multibillion doUar scale to separate partially the uranium isotopes and to produce a much more valuable fuel for nuclear power reactors. Because separation in these special processes depends upon the different rates of diffusion of the components, the processes are often referred to collectively as diffusion separation methods. There is also a thermal diffusion process used on a modest scale for the separation of heflum-group gases (qv) and on a laboratory scale for the separation of various other materials. Thermal diffusion is not discussed herein. 

Consider a large uranium plate of thickness L = 4 cm, thermal conductivity k -- 28 W/m °C, and thermal diffusivity a = 12.5 X 10 m /s that is initially at a uniform temperature of 2C0 C. Heat is generated uniformly in the plate at a constant rate of e = 5 x 10 W/m. At lime t = 0, one side of llie plate is brought into contact with iced water and is maintained at 0°C at all times, while the other side is subjected to convection to an environment at f, = 30°C with a heat transfer coefficient of / = 45 W/m °C, as shov/n in Fig. 5 44. Considering a total of three equally spaced nodes in the medium, two at the boundaries and one at the middle, estimate the exposed surface temperature of the plate 2.5 min after the start of cooling using (a) the explicit method and (6) the implicit metliod. 

With the development of specific equipment and processes by which thermal be diffusion is now carried out, separation of substances from their mixtures can often carried out more cheaply by this method, if applicable, than by other separation techniques. Amongst the successful separations effected by thermal diffusion are those of the isotopes of helium and the isotopes of chlorine gas. The method had also been used to effect separation of the isotopes of uranium during the years of World War II in the U.S.A. Constituent hydrocarbons can easily be separated from their mixture by liquid-phase thermal diffusion, because interaction between molecules of different hydrocarbons is practically non-existent and, consequently, each hydrocarbon molecule of the mixture acts independently under the influence of the applied temperature gradient. Another use of thermal diffusion of special interest is its applicability to the separation of mixtures of liquids of close boiling points and of mixtures of isomers, into their respective components. 

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... 

Al. Abelson, P. H., and J. I. Hoover Separation of Uranium Isotopes by Liquid Thermal Diffusion, Proceedings of the International Symposium on Isotope Separation, Interscience, New York, 1958, p. 483. 

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... 

Although its very poor power utilization compared with gaseous diffusion and the gas centrifuge precludes use of thermal diffusion for large-scale uranium isotope separation, the simplicity of the equipment, the absence of moving parts, and the large separation attainable in... 

Figure 14.40 shows the most accurate measurements of the thermal diffusion effect in UF vapor at low pressure, by Kirch and Schutte [K2]. Results are plotted both as k, for comparison with other gases in Fig. 14.39, and as the thermal diffusion constant y. The very low values, under 0.00005, explain Nier s [N3] inability to detect a thermal diffusion effect in UF5 vapor. The thermal diffusion coefficient 7 is so much smaller than the analogous parameter in gaseous diffusion, Qq 1 = 0.0043, that vapor-phase thermal diffusion caimot compete economically with gaseous diffusion for uranium enrichment. 

Suppose that all 2100 columns of the S-50 thermal diffusion plant, with individual column characteristics as given in row 3 of Table 14.25, were operated in parallel as an enridiing section, at very high natural UF feed rate, without stripping section. At the product rate at which separative capacity is a maximum, what would be the content of product The product flow rate in kilograms of uranium per year ... 

In-pile self-diffusion of uranium in stoichiometric UO2 and UC has been measured by Hoh and Matzke 3J6). The diffusion coefficients obtained at a nominal irradiation temperature of 900°C and a fission rate of 1 x 10 //cm indicated that radiation-enhanced diffusion was higher by a factor of 10 to lO than determined by extrapolation of thermal diffusion coefficients. They suggested that the data are of immediate relevance to the understanding and the prediction of such quantities as in-pile sintering and densification, diffusion-controlled creep, and fission gas behavior in the outer zones of the fuel. 

William S. Deke Parsons and Philip Abelson. Parsons directed ordnance development at Los Alamos Abelson pioneered liquid thermal diffusion for uranium enrichment. 

The foregoing discussion fairly well demonstrates that on a small scale any desired Isotope can be separated either by the electromagnetic or the thermal diffusion method. In contrast to these laboratory-scale processes, the separations of the heavier Isotope D, of the lightest element, hydrogen, and of the lighter Isotope 235u, of the heaviest natural element uranium, are carried out on a literally enormous Industrial scale. 

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