Filtration Materials for Separating Isotopes
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Filtration Materials for Separating Isotopes Certain porous materials or layers hâve been used for filtering substances since the first human tried to strain muddy drinking water. Thin filter média, such as filter cloth, filter screens, or laboratory filter paper, hâve mesh sizes smaller than the particles to be trapped. "En masse" filter beds, such as a thick layer of sand, or a coke bed, porous ceramics, or porous metals operate on the principle that, though the individual pores in the filter média are much larger than the particulates to be removed, the tortuous path ensures that particulates will be trapped by attaching to the médium before they can émerge on the other side of the filter. A third séparation technique uses the thermal motions of molécules through the tangled "maze" of filtration média. Lighter molécules will diffuse through the material faster than heavier molécules; with several such filtration stages, the fraction of lighter molécules can be increased with each pass through the média. Perhaps the most extraordinary use of this filtration technique was for separating uranium isotopes in the 1940s. Doing so required the development of exceptional new membrane materials and extremely high quality-control standards. Natural uranium contains about 99.3% of the isotope with an atomic weight of 238, and 0.7% of the 235 isotope. Séparation of the isotopes was not possible using chernical methods because the uranium chemistry remains the same for both isotopes. The only way to separate them had to be physical. Isotope séparation had already been achieved in 1932 by Gustav Hertz in Germany. Hertz isolated isotopes of néon by cycling the gas through 12 mercury diffusion pumps in a cascade, allowing the néon to diffuse through a stream of mercury vapor. By 1938, this technique had been used to enrich C-13, N-15, and 0-18 ten-to twentyfold. However, the mass différence between isotopes of lighter éléments is about 5%, while for uranium isotopes U-238 and U-235 it is doser to 1%. A différent séparation scheme was required. In the early 1940s, researchers attempted various physical methods, such as electromagnetic séparation using the new cyclotrons at Berkeley and thermal diffusion
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using a heated tube inside a cooled concentric tube. Thermal diffusion caused the heavier fraction to concentrate at the cooler interface, and the lighter isotope to concentrate at the hotter interface; convection then allowed a greater fraction of the lighter isotope to rise toward the top. The "gaseous diffusion" method using filtration média, however, required the development of a new kind of material with pores so fine that they would preferentially allow one isotope of uranium to pass through more rapidly than another. Since the mean molecular velocity in a gas is inversely proportional to the square root of its molecular weight, a light isotope will diffuse through filtration média slightly faster than a heavier isotope.
Uranium hexafluoride feed gas destroyed most typical materials, including steel piping and any org
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