Gaseous Diffusion - History

History

Scientists working on the Manhattan Project in Oak Ridge, Tennessee developed several different methods for the separation of isotopes of uranium. Three of these methods were used sequentially at three different plants in Oak Ridge to produce the 235U for "Little Boy" and other early nuclear weapons. In the first step, the S-50 uranium enrichment facility used the thermal diffusion process to enrich the uranium from 0.7% up to nearly 2% 235U. This product was then fed into the gaseous diffusion process at the K-25 plant, the product of which was around 23% 235U. Finally, this material was fed into calutrons at the Y-12. These machines (a type of particle accelerator or cyclotron) employed electromagnetic isotope separation to boost the final 235U concentration to about 84%.

The preparation of UF₆ feedstock for the K-25 gaseous diffusion plant was the first ever application for commercially produced fluorine, and significant obstacles were encountered in the handling of both fluorine and UF₆. For example, before the K-25 gaseous diffusion plant could be built, it was first necessary to develop non-reactive chemical compounds that could be used as coatings, lubricants and gaskets for the surfaces which would come into contact with the UF₆ gas (a highly reactive and corrosive substance). Scientists of the Manhattan Project recruited William T. Miller, a professor of organic chemistry at Cornell University, to synthesize and develop such materials, because of his expertise in organofluorine chemistry. Miller and his team developed several novel non-reactive chlorofluorocarbon polymers that were used in this application.

Calutrons were expensive to build and operate and inefficient. As soon as the engineering obstacles posed by the gaseous diffusion process had been overcome and the gaseous diffusion cascades began operating at Oak Ridge in 1945, all of the calutrons were shut down. The gaseous diffusion technique then became the preferred technique for producing enriched uranium.

At the time of their construction in the early 1940s, the gaseous diffusion plants were the largest buildings ever constructed. Large gaseous diffusion plants were constructed by the United States, the Soviet Union (including a plant that is now in Kazakhstan), the United Kingdom, France, China, and South Africa. 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 still remains top secret, and some of the materials that were used remain subject to export controls, as a part of the continuing effort to control nuclear proliferation.