GASEOUS DIFFUSION

• Centrifuges
• Electromagnetic Separation
• Gaseous Diffusion
• Thermal Diffusion
• Separation Hazards and Wastes

Uranium enrichment by gaseous diffusion is based upon the principle that the lighter molecules in a gas will pass through a porous barrier more readily than heavier molecules. In a gas composed of both uranium-235 and uranium-238 isotopes, molecules with the slightly less massive uranium-235 pass through a porous barrier more easily than molecules containing uranium-238. Since the difference in the relative concentrations of each isotope is very small with only one barrier in place, the enrichment facilities needed to successively filter the gas through multiple barriers to achieve ever greater levels of uranium-235 concentration. Processing pure uranium, with its very high boiling point, was impractical. The Manhattan Project thus used a molecule called uranium hexafluoride (UF6), composed of one uranium atom and six fluoride atoms. Since fluoride has only one isotope, any difference in the mass of the uranium hexafluoride molecules is due to the difference between the uranium-238 and uranium-235 isotopes. Uranium hexafluoride, in addition, is a solid at room temperature but sublimes, or becomes a gas without first becoming a liquid, at 56.5 degrees Celsius.

In May 1940, the Harvard chemist George Kistiakowsky suggested to Vannevar Bush that the gaseous diffusion method should be pursued as a feasible way of enriching uranium, a suggestion that was approved by the S-1 Committee. Preliminary research in gaseous diffusion was based at Columbia University under the leadership of physicist John Dunning. Although the theory of gaseous diffusion was straightforward, there were many practical problems involved with implementing this enrichment method on an industrial scale. The most fundamental problem was selecting an appropriate barrier: robust enough to withstand the corrosive uranium hexafluoride gas yet sensitive enough to discriminate between the two uranium isotopes. The barrier—filter might have been a better name—required billions of holes with a diameter less than one-tenth the mean free path of a molecule, about one ten-thousandth of a millimeter. A material so delicate at the same time had to be strong enough to withstand a considerable pressure differential and the mechanical strains of assembly. The presence of uranium hexafluoride also would make it difficult to prevent deterioration of the equipment, contamination of the gas, and plugging of the barriers. No leakage of air into the system could be tolerated, for the water vapor would react with the gas to form uranium oxyfluoride, which would clog the barriers and halt operations. Gaseous diffusion nonetheless seemed fundamentally sound. A plant producing one kilogram of uranium-235 a day would require acres of barrier area and thousands of stages, but the process would be continuous, not batch. Gaseous diffusion offered less likelihood of mechanical difficulty than did other methods such as the centrifuge.

The construction of the gaseous diffusion plant at Oak Ridge, known as K-25, began in June 1943, though no suitable barrier had yet been identified. In 1944, Manhattan Project physicists settled on a nickel barrier as the most promising option and began building working diffusion cascades even as they tested the chosen barrier. Completed at a cost of $500 million and employing 12,000 workers, the K-25 plant had 2,892 cascade stages and used 130,000 instruments and 500,000 specialized valves. The U-shaped K-25 building measured half a mile by 1,000 feet and was larger than the Pentagon.

K-25 production came online in stages, beginning in February 1945, as the plant was completed. The initial step in the process involved vaporization of the uranium hexafluoride feed material by subjecting it to a series of hot baths. The UF6, a gas at this point, then was introduced into the process stream at any of a number of intake points and flowed through the cascade stages. Emerging from the stages, the UF6 went through a stripping section that carried depleted gas from the higher enrichment stages back to the lower part of the cascade for recirculation. By April 1945, K-25 product reached an enrichment level of 1.1 percent uranium-235, which was sent to the Y-12 electromagnetic plant for final enrichment. At the same time, the S-50 thermal diffusion plant began sending its product to K-25 for further enrichment. By June, K-25 was producing nearly 7 percent uranium-235, and when the full plant cascade went on stream in August began producing at the 23 percent level. During fall 1945, K-25 production was far higher than designers had predicted. As a result, gaseous diffusion became the sole uranium enrichment process used during the Cold War, though today newer technologies are replacing this method.

• Centrifuges
• Electromagnetic Separation
• Gaseous Diffusion
• Thermal Diffusion
• Separation Hazards and Wastes

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