 Production of the fissile isotopes uranium-235 and plutonium-239 were enormous undertakings given the technology of the 1940s, and accounted for 80 % of the total costs of the project. Uranium enrichment was carried out at the Clinton Engineer Works near Oak Ridge, Tennessee. Theoretically, enriching uranium was feasible through pre-existing techniques, but it proved difficult to scale to industrial levels and was extremely costly. Only 0.71 percent of natural uranium was uranium-235, and it was estimated that it would take 27,000 years to produce a gram of uranium with mass spectrometers, but kilogram amounts were required.
