Gaseous diffusion is a technology that was used to produce enriched uranium by forcing gaseous uranium hexafluoride (UF6) through microporous membranes. This produces a slight separation (enrichment factor 1.0043) 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of producing enriched uranium in industrially useful quantities, but is nowadays considered obsolete, having been superseded by the more-efficient gas centrifuge process. 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, for the M. S. Factory, Valley. This work was later transferred to the United States when the Tube Alloys project became subsumed by the later Manhattan Project. Background 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 mass, 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. Technology Scientific basis Gaseous diffusion is based on Graham's law, which states that the rate of effusion of a gas is inversely proportional to the square root of its molecular mass. For example, in a box with a microporous membrane containing a mixture of two gases, the lighter molecules will pass out of the container more rapidly than the heavier molecules, if the pore diameter is smaller than the mean free path length (molecular flow). The gas leaving the container is somewhat enriched in the lighter molecules, while the residual gas is somewhat depleted. A single container wherein the enrichment process takes place through gaseous diffusion is called a diffuser. Uranium hexafluoride UF6 is the only compound of uranium sufficiently volatile to be