Because of these difficulties, the gaseous diffusion plant didn’t begin operating until February of 1945. While the plant was originally planned to produce uranium to the roughly 90% U235 enrichment needed to build a bomb, it was discovered that beyond 36.6% enrichment, different types of barrier and different types of pumps being designed would be required. In the latter half of 1943 the plant was thus redesigned to produce 36.6% enriched uranium (using 2892 diffusion stages) that would then be fed into the electromagnetic process. By the end of the war, the gaseous diffusion plant had “contributed substantially to the manufacture of the fissionable material used in the fabrication of atomic weapons”, and would become the primary method of producing enriched uranium in the early post-war years.
In addition to the electromagnetic and gaseous diffusion separation processes, a plant to separate U235 by liquid thermal diffusion was also built. Thermal diffusion had been considered by the Manhattan Project early on, but it appeared insufficiently promising and there were no initial plans to build a thermal diffusion plant. However, work on the process continued by the Navy as a method of producing fissile material for nuclear reactors. By late 1942, it appeared much more promising as a feasible separation method, and Leslie Groves recommended that it continue to be developed by the Navy. Eventually, in June of 1944 it was decided to build a thermal diffusion plant at Oak Ridge to produce partially-enriched uranium as an input to the electromagnetic separation process, as doing so would speed up overall U235 production. The plant came online in late 1944.