Two materials were soon determined to be fissile—that is, capable of sustaining a rapid chain reaction: uranium-235 and plutonium-239. Both were difficult to obtain. Plutonium is a man-made element, created by bombarding uranium with neutrons. Uranium-235 exists in nature, but in small amounts. A typical sample of uranium is about 0.07 percent uranium-235, and to get that fissile material the Manhattan Project built a processing facility in Oak Ridge, Tennessee.
