3.1 Principle

The gaseous diffusion process makes use of the phenomenon of molecular effusion to effect separation. In a vessel containing a mixture of two gases, molecules of the gas of lower molecular weight have higher speeds and strike the walls of the vessel more frequently, relative to their concentration, than do the molecules of the gas with higher molecular weight. If the walls of the vessel have holes just large enough to allow passage of molecules one by one without permitting flow of the gas as a continuous fluid, more of the lighter molecules flow through the wall, relative to their concentration, than the heavier molecules. The flow of individual molecules through minute holes is known as molecular effusion. The possibility of separating gases by effusion through porous media was discovered experimentally by Graham over a hundred years ago. Maxwell showed that this separation was due to the fact that the relative frequency with which molecules of different species enter a small hole is inversely proportional to the square root of their molecular weights. For a mixture of 235UF6 and 238UF6 this ratio, the ideal separation factor for gaseous diffusion a0, is

(14.1)

Because this value is so close to unity, to obtain a useful degree of separation the process must be repeated many times in a countercurrent cascade of gaseous diffusion stages, such as was shown in Fig. 12.2.

3.2 History

The first use of gaseous diffusion for isotope separation was by Aston [A4], who in 1920 effected a slight separation of the isotopes of neon in a single stage of gaseous diffusion through a porous clay tube. Hertz [H2, H5, H6] greatly increased the separation obtainable by this method by using a countercurrent recycle cascade of from 24 to 50 stages of the type shown in Fig. 12.2. This apparatus effected practically complete separation of the neon isotopes of mass 20 and 22 and completely separated hydrogen and deuterium. With a 34-stage cascade, Wooldridge, Jenkins, and Smythe [W3, W4] enriched 13CH4 from 1 to 16 percent.

When World War II created a demand for 235U, the proved ability of gaseous diffusion to effect isotope separation and the existence of a stable, volatile compound of uranium, UF6, led

to intensive development of this process in England and the United States. Because of greater security against attack and more abundant energy supplies, the two governments decided that the first gaseous diffusion uranium enrichment plant would be built in the United States. The Manhattan Project, under the leadership of General Leslie ft.. Groves, built the first gaseous diffusion plant, the K-25 plant, at Oak Ridge, Tennessee, which began operation in 1945. Partial descriptions of this plant and the demanding development effort that led to its successful operation have been given by Smyth [S6], Keith [Kl], Hogerton [H10], Groves [G5], and Groueff [G4], and the official U. S. history by Hewlett and Anderson [Н7]. The development effort in England and the construction of the British gaseous diffusion plant at Capenhurst in the 1950s has been described by Jay [J3]. The independent development of the gaseous diffusion process in France in the 1950s and the construction of the first French plant at Pierrelatte in 1964-1967 has been described by CEA [С7].