The fissile Uranium-235 isotope makes up only 0.7 percent of pure uranium. Because it is chemically identical to the most common isotope, uranium-238, and has nearly the identical mass, separating the two proved difficult. Three methods were employed for uranium enrichment: electromagnetic, gaseous and thermal. This work was carried out on the Clinton Engineer Works at Oak Ridge, Tennessee. In parallel was an effort to supply plutonium, which was theorised to also be fissile, and could possibly be produced by the nuclear transmutation of uranium in a nuclear reactor. The feasibility of a nuclear reactor was demonstrated in 1942 at