Gaseous diffusion method

The gaseous diffusion method of isotope separation is based upon the difference in the rate of diffusion of gases that differ in density. Since the rate of diffusion of a gas is inversely proportionate to the square root of its density, die lighter of two gases will diffuse more rapidly than the heavier. Therefore, die result of a partial diffusion process will be an enrichment of the partial product in die lighter component. 

Mack, Jr., E., Average cross-sectional areas of molecules by gaseous diffusion methods. J. Am. Chem. Soc. 1925, 47, 2468. 

The values of the specific activity include the activity of U-234, which is concentrated during the enrichment process these values do not include any daughter product contribution. The values are for the material originating from natural uranium enriched by a gaseous diffusion method. 

In the gaseous diffusion method, the determination of acetic acid is based on the fact that acetic acid diffuses through the membrane and dissolves into the acceptor solution. 

Uranium oxide [1344-57-6] from mills is converted into uranium hexafluoride [7783-81-5] FJF, for use in gaseous diffusion isotope separation plants (see Diffusion separation methods). The wastes from these operations are only slightly radioactive. Both uranium-235 and uranium-238 have long half-Hves, 7.08 x 10 and 4.46 x 10 yr, respectively. Uranium enriched to around 3 wt % is shipped to a reactor fuel fabrication plant (see Nuclear REACTORS, NUCLEAR FUEL reserves). There conversion to uranium dioxide is foUowed by peUet formation, sintering, and placement in tubes to form fuel rods. The rods are put in bundles to form fuel assembHes. Despite active recycling (qv), some low activity wastes are produced. 

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. 

In 1996, a field demonstration of horizontal recirculation wells was conducted at the X-701B site of the Portsmouth Gaseous Diffusion Plant in Piketon, Ohio. Using directional drilling methods, two horizontal wells 234 ft long were installed to a depth of 32 ft. Design and construction costs were estimated to be 1.43 million (D188709, p. 16). 

UF4,is combined with fluorine gas to yield uranium hexafluoride, UF6 UF4(s) + F2fe) —> UF6(g). Uranium hexafluoride is a white crystalline solid at standard temperature and pressure, but it sublimes to a gas at 57°C. The U-235 in uranium hexafluoride can be enriched by several methods based on the difference in masses of the uranium isotopes. Two common methods are gaseous diffusion and gas centrifuge. 

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... 

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