The first man-made nuclear reactor was built under the grandstand of Stagg Field, at the University of Chicago.
The first reactor was called a "pile". That may have been wartime jargon to hide its true nature from enemy spies, and it may have been descriptive. In fact, it was a pile of uranium and graphite blocks. It's designation was CP-1, or Chicago Pile #1. CP-1 was part of the Manhattan Project, the US government's secret WW2 program to rush a nuclear weapon into production. Its importance cannot be overstated.
The neutron had been discovered in 1932 by an Englishman named Chadwick. In 1938, Lise Meitner, Otto Hahn, and Fritz Strassman collaborated to recognize and report on the fission of Uranium by neutrons. By 1939 it was understood that excess neutrons from a split atom could maintain a reaction. The next step was inevitable: Make it happen.
Below: The experimental setup that led to the discovery of nuclear fission.
Thus CP-1 was born - an industrial scale-up of what had previously been bench-top experiments in laboratories. The techniques for increasing enrichment of a Uranium isotope with identical chemical properties had not yet been invented. Therefore CP-1 used very pure refined metallic natural (unenriched) Uranium.
With CP-1 fueled by natural uranium, using light water for a moderator was not possible. Light water would absorb too many excess neutrons, and prevent any reactor with such a low percentage of U-235 to maintain a chain reaction. Light water is a good moderator, but it is also a mild poison.
For this reason, graphite was chosen for the moderator, as it absorbs neutrons 100x less often than light water does, and hence would be able to moderate fission neutrons without also creating losses through absorption.
To create CP-1, several other laboratory-level experiments also had to be ramped up to industrial levels. At that point in time, the amount of metallic Uranium in the world was measured in grams. Further complicating issues was the fact that nobody had ever bothered segregating trace amounts of Boron (A very powerful neutron poison) from the Carbon that the graphite moderator was made from. After enough Uranium metal had been obtained, construction began. Small cylinders of Uranium were interspersed between large blocks of graphite.
Enrico Fermi was in charge of the construction of the reactor. Fermi was a brilliant physicist, and was also probably the only man in the world who could have assembled and controlled a nuclear reactor at that time.
Below, a rendering of the CP-1 reactor. The three cylindrical things dangling from a cable are neutron detectors. The man standing is manipulating a control rod, which when removed, stopped absorbing neutrons and allowed a self-sustaining reaction to occur.
However all these issues were overcome. On 2 December 1942, CP-1 was ready for a demonstration. Before a group of dignitaries, George Weil worked the final control rod while physicist Enrico Fermi carefully monitored the neutron activity. The pile "went critical" (reached a self-sustaining reaction) at 15:25. Fermi shut it down 28 minutes later.
Unlike most reactors that have been built since, CP-1 had no radiation shielding and no cooling system of any kind. Fermi had convinced Arthur Compton that his calculations were reliable enough to rule out a runaway chain reaction or an explosion. But, as the official historians of theAtomic Energy Commission noted, the "gamble" remained in conducting "a possibly catastrophic experiment in one of the most densely populated areas of the nation!"
With the proof at hand that a nuclear reaction could be made self-sustaining, the Manhattan Project would rapidly move forward toward a nuclear weapon.