Fission of uranium was discovered—or, more precisely, recognized for what it was—in 1938. Scientists quickly recognized that large amounts of energy are released in fission and that there was now, in principle, a path to a chain reaction. By early 1939, it was verified that neutrons are emitted in fission, and it soon became apparent that enough neutrons were emitted to sustain a chain reaction in a properly arranged “pile” of uranium and graphite. It took several more years to demonstrate the practicality of achieving a chain reaction. This work was led by Fermi, who had left Italy for the United States, and it culminated in the development and demonstration of the first operating nuclear reactor on December 2, 1942 at an improvised facility in Chicago.
The discovery and preliminary understanding of fission came at a time when the prospect of war was much on people’s minds. The start of World War II in Europe in August 1939 ensured that military, rather than civilian, applications of atomic energy would take primacy, and the early work was heavily focused on the military side, in both thinking and accomplishments. A major goal of the nuclear program was the production of plutonium-239 (239Pu), which was recognized to be an effective material for a fission bomb. The 239Pu was to be produced in a reactor, by neutron capture on uranium-238 (238U) and subsequent radioactive decays.
The first reactor in Chicago was very small, running with a total power output of 200 W. However, even before the successful demonstration of a chain reaction in this reactor, plans had started for the construction of the much larger reactors required to produce the desired amounts of plutonium. A pilot plant, designed to produce 1 MW, was completed and put into operation at Oak Ridge, Tennessee in November 1943. A full-size 200-MW reactor began operating at the Hanford Reservation in Washington state in September 1944—a millionfold increase in power output in less than 2 years.