Gaseous diffusion is a technology used to produce enriched uranium by forcing gaseous uranium hexafluoride 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.
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 It has largely been replaced by the newer gas centrifuge technology, which requires far less energy to produce the same separation.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.