Gaseous diffusion

Gaseous diffusion

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.

History

Gaseous diffusion was one of several uranium isotope separation technologies developed as part of the Manhattan Project. 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 Kazakhstan), 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.

The technique has largely been replaced by the newer gas centrifuge technology, which requires far less energy to produce the same separation.

Technology

This process is based on the principle that in a closed box, all molecules have the same energy; therefore 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 small hole 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, since rate of effusion (diffusion to outside the box) is proportional to the inverse square of the mass. The gas leaving the container is somewhat enriched in the lighter molecules, while the residual gas is somewhat depleted.

A single container where the enrichment process takes place through gaseous diffusion is called a diffuser. It does not contain one hole, however, but has a porous barrier.

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. Teflon was used to coat the valves and seals. 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.

There are two types of barriers on which information is available: Film-type and Aggregate. Film-type barriers are made by boring pores through an initially nonporous medium. One way this can be done is by removing one constituent in an alloy, for instance using HCl to remove the zinc from AgZn. Aggregate barriers are made by agglomerating or sintering powders: the barriers used in the USA are in the form of sintered nickel tubes.

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.

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