Thermal diffusion separation isotopes

As detailed earlier, Aston had been faced with this problem for the case of neon where he was not convinced by Thomsons s conclusion that there are two neon isotopes of masses 20 and 22. He attempted to show that indeed neon consists of two isotopes by trying to separate the two isotopes using thermal diffusion. The result proved unsatisfactory and he then proceeded to invent the mass spectrograph. 

Tamborini G, Betti M (2000) Mikrochim Acta 132 411 Vasaru G (1975) Separation of isotopes by thermal diffusion. Rumanian Academy, 1972, Bucharest, distributed by USERDA, report ERDA-tr-32 Vickerman JC (1998) Surface analysis - the principal techniques. Wiley, Chichester Villani S (ed) (1979) Uranium enrichment. Topics in Applied Physics, Vol 35. Springer, Berlin VoTskii AN, Sterlin YAM (1970) Metallurgy of plutonium. Israel Program for Scientific Translation, Jerusalem... 

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. 

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. 

Ironically, our current plans call for the reverse linkage of the above enrichment procedures. That is, we shall use an electromagnetic isotope separator to enrich argon isotopes for a mass spectrometry experiment, and we shall enrich radiocarbon via thermal diffusion for improved mini-gas proportional counting. 

Clusius A process for separating isotopes by a combination of thermal diffusion and thermal siphoning. Invented in 1938 by K. Clusius and G. Dickel. 

Fig. 8.9 (a) A four stage thermal diffusion cascade for argon isotope separation (Modified from Spindel, W. ACS Symp. Ser. 11, 82 (1975)). (b) The thermal diffusion cascade operated by K. Clusius and collaborators at the University of Zurich during the 1950s (Photo credit Archives of the Institute of Physical Chemistry, University of Zurich)... 

Table 8.2 Isotopes separated by K. Clusius and coworkers by thermal diffusion (Clusius, K. and Dickel, G.,Naturwissenschaften 26,546 (1938) Clusius, K. and Starke, K., Z Naturforsch. 4A, 549 (1949))...

Many other methods for separating isotopes have been described. A partial list includes membrane and 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... 

Although thermal diffusion equipment is simple in construction and operation, the thermal requirements are so high that this method of separation is useful only for laboratory investigations or for recovery of isotopes on a small scale, which is being done currently. 

The thermal diffusion method of isotope separation has broad application to liquid-phase as well as gaseous-phase separations. The apparatus widely used for this purpose consists of a vertical tube provided with an electrically heated central wire. The gaseous or liquid mixture containing the isotopes to be separated is placed in the tube, and heated by means of the wire. In such an apparatus two effects act to separate the isotopes. Thermal diffusion tends to concentrate the heavier isotopes in the cooler outer portions of the system, while the portions near the hot wire are enriched in die lighter isotopes. At the same time, thermal convection causes the hotter fluid near the hot wire to rise, while the cooler fluid in the outer portions of the system tends to fell. The overall result of these two effects causes die heavier isotopes to collect at the bottom of the tube and the lighter at the lop, whereby both fractions may be withdrawn... 

The separation of chemical isotopes is based on small differences in their physical and chemical properties. For the lower-mass isotopes, chemical exchange, distillation, and electrolysis have been used. For the higher-mass isotopes, techniques based on mass have been used, including gaseous diffusion, centrifugation, thermal diffusion, and ion activation.29,30 A newer method uses lasers that produce coherent light tuned to the specific wavelength of a vibration bond related to the desired isotope in an atom or molecule. This technique is still under development but... 

The thermal diffusion factor a is proportional to the mass difference, (mi — mo)/(mi + m2). The thermal diffusion process depends on the transport of momentum in collisions between unlike molecules. The momentum transport vanishes for Maxwellian molecules, particles which repel one another with a force which falls off as the inverse fifth power of the distance between them. If the repulsive force between the molecules falls off more rapidly than the fifth power of the distance, then the light molecule will concentrate in the high temperature region of the space, while the heavy molecule concentrates in the cold temperature region. When the force law falls off less rapidly than the fifth power of the distance, then the thermal diffusion separation occurs in the opposite sense. The theory of the thermal diffusion factor a is as yet incomplete even for classical molecules. A summary of the theory has been given by Jones and Furry 15) and by Hirschfelder, Curtiss, and Bird 14), Since the thermal diffusion factor a for isotope mixtures is small, of the order of 10", it remained for Clusius and Dickel (8) to develop an elegant countercurrent system which could multiply the elementary effect. 

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%. 

Table III. Isotopes Separated by K. Clusius by Thermal Diffusion...

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