Manhattan Project engineers first set about enriching uranium, using chemical processes to increase the proportion of fissionable uranium-235 up to levels that could produce supercritical chain reactions. Most of this work was done in Oak Ridge, Tennessee, where Fermi was also involved in building another nuclear reactor, to test whether it was really possible to sustain a critical chain reaction that would produce plutonium-239 from the original uranium fuel. Plutonium, it turned out, was an even more efficient fuel for supercritical chain reactions. Both efforts were successes and went on to provide the raw material for the first and only atomic bombs ever used in war—the Hiroshima bomb of uranium-235 enriched to 70 percent, and the Nagasaki bomb, which had a plutonium core, both ignited by implosion.

Bomb development ultimately led to thermonuclear weapons, in which the fusion of hydrogen atoms releases far greater amounts of energy. The first atomic bomb tested in New Mexico yielded the equivalent of 18 kilotons of TNT; thermonuclear hydrogen bombs yield up to 10 megatons. The Cold War drove both the United States and the Soviet Union to develop ever more lethal nuclear weapons, all based on the principles worked out and put into action by the scientists and engineers of the Manhattan Project. Although the consequences of their actions remain highly controversial, the brilliance of their technological achievements is undimmed.