Gaseous diffusion

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Gaseous diffusion is a technology used to produce enriched uranium by forcing gaseous uranium hexafluoride, UF₆ through semi-permeable membranes. This produces a slight separation between the molecules containing uranium-235 and uranium-238. By use of a large cascade of many stages, high separations can be achieved. It was the first economic enrichment process to be successfully developed.

1 History
2 Technology
3 See also
4 External links

[edit] History

Gaseous diffusion was one of several uranium isotope separation technologies developed as part of the Manhattan Project. It has largely been replaced by the newer gas centrifuge technology, which requires far less energy to produce the same separation.

Of the several separation technologies ultimately used by the Manhattan Project, gaseous diffusion was probably the most significant. The process buildings built for the cascades were then the largest ever constructed. The preparation of uranium hexafluoride feedstock (known as hex in the trade) was the first ever application for commercially produced fluorine, and the problems addressed in handling both fluorine and hex as a corrosive gas were significant.

Large gaseous diffusion plants were constructed by the United States of America, the Soviet Union (including a plant now in Kazakstan), the United Kingdom, France and China. Most of these have now closed or are expected to close, unable to compete economically with newer enrichment techniques. However some of the technology used in pumps and membranes remains secret, and some of the materials used remain subject to export controls as part of the continuing effort to control nuclear proliferation.

[edit] Technology

This process is based on the observation that in a mixture of two gases, the lighter molecules, on average, travel faster than the heavier molecules. As a result, the lighter molecules strike the walls of a container more frequently than their heavier counterparts. If a portion of the container consists of a porous material having holes large enough to permit the passage of individual gas molecules, but not so large that a mass flow-through of the gas can occur, then more light molecules than heavy molecules will pass out of the container. This type of diffusion is now referred to as effusion. The gas leaving the container is somewhat enriched in the lighter molecules, while the residual gas is somewhat depleted.

A single container where enrichment process takes place through gaseous diffusion is called diffuser.

Uranium, of course, is not a gas, and the diffusion enrichment of uranium is carried out using uranium hexafluoride. While this substance is solid at room temperature, it is easily vaporized. However, this requires that all components of a diffusion plant be maintained at an appropriate temperature to assure that UF₆ remains in gaseous form. While UF₆ is a stable compound, it is highly reactive with water and corrosive to most common metals. As a consequence, internal gaseous pathways must be fabricated from nickel, or austenitic stainless steel, and the entire system must be leak tight. Despite its unpleasant characteristics, UF₆ is the only compound of uranium sufficiently volatile to be used in the gaseous diffusion process.

Fortunately, fluorine consists of only a single isotope ¹⁹F, so that the difference in molecular weights of different molecules of UF₆ is due only to the difference in weights of the uranium isotopes. However, because the molecular weights of ²³⁵UF₆ and ²³⁸UF₆ are so nearly equal, very little separation of the ²³⁵U and ²³⁸U is effected by a single pass through a barrier, that is, in one diffuser. It is necessary, therefore, to connect a great many diffusers together in a sequence of stages, using the outputs of each stage as the inputs for two adjoining stages. Such a sequence of stages is called a cascade. In practice, diffusion cascades require thousands of stages, depending on the desired product enrichment.

The gas must be compressed at each stage to make up for a loss in pressure across the diffuser. This leads to compression heating of the gas, which then must be cooled before entering the diffuser. The requirements for pumping and cooling make diffusion plants enormous consumers of electricity.