Gaseous diffusion

Gaseous diffusion is a technology used to produce enriched uranium by forcing gaseous uranium hexafluoride (UF₆) through semipermeable membranes. This produces a slight separation between the molecules containing uranium-235 (²³⁵U) and uranium-238 (²³⁸U). By use of a large cascade of many stages, high separations can be achieved. It was the first process to be developed that was capable of producing enriched uranium in industrially useful quantities.

Gaseous diffusion was devised by Francis Simon and Nicholas Kurti at the Clarendon Laboratory in 1940, tasked by the MAUD Committee with finding a method for separating uranium-235 from uranium-238 in order to produce a bomb for the British Tube Alloys project. The prototype gaseous diffusion equipment itself was manufactured by Metropolitan-Vickers (MetroVick) at Trafford Park, Manchester, at a cost of £150,000 for four units. This work was later transferred to the United States when the Tube Alloys project became subsumed by the later Manhattan Project.

Of the 33 known radioactive primordial nuclides, two (²³⁵U and ²³⁸U) are isotopes of uranium. These two isotopes are similar in many ways, except that only ²³⁵U is fissile (capable of sustaining a nuclear chain reaction of nuclear fission with thermal neutrons). In fact, ²³⁵U is the only naturally occurring fissile nucleus. Because natural uranium is only about 0.72% ²³⁵U by weight, it must be enriched to a concentration of 2–5% to be able to support a continuous nuclear chain reaction when normal water is used as the moderator. The product of this enrichment process is called enriched uranium.

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