FIRST NUCLEAR REACTOR: UNIV. OF CHICAGO

From https://en.wikipedia.org/wiki/Chicago_Pile-1

First Nuclear Reactor - U.Chicago 1942

Chicago Pile-1 (CP-1) was the world's first nuclear reactor. On 2 December 1942, the first human-made self-sustaining nuclear chain reaction was initiated in CP-1, during an experiment led by Enrico Fermi. The secret development of the reactor was the first major technical achievement of the Manhattan Project, the Allied effort to create atomic bombs during World War II. Although the project's civilian and military leaders had misgivings about the possibility of a disastrous runaway reaction, they nevertheless decided due to time pressure to carry out the experiment in a densely populated area. It was built by the Metallurgical Laboratory at the University of Chicago, under the west viewing stands of the original Stagg Field. Fermi described the apparatus as "a crude pile of black bricks and wooden timbers".

The reactor was assembled in November 1942, by a team that included Fermi, Leo Szilard (who had previously formulated an idea for non-fission chain reaction), Leona Woods, Herbert L. Anderson, Walter Zinn, Martin D. Whitaker, and George Weil. Because the enrichment of uranium had not yet begun at the Oak Ridge site, the reactor used natural uranium rather than uranium enriched in isotope 235. This required a very large amount of material in order to reach criticality, along with graphite used as a neutron moderator. The reactor contained 45,000 ultra-pure graphite blocks weighing 360 tons, and was fueled by 5.4 tons of uranium metal and 45 tons of uranium oxide. Unlike most subsequent nuclear reactors, it had no radiation shielding or cooling system as it operated at very low power – about one-half watt.

The success of the reactor provided the first vivid demonstration of the feasibility of the military use of nuclear energy by the Allies, and the reality of the danger that Nazi Germany would succeed in producing nuclear weapons and win the war with them. Previously, estimates of critical masses had been crude calculations, leading to order-of-magnitude uncertainties about the size of a hypothetical bomb. The successful use of graphite as a moderator paved the way for progress in the Allied effort, whereas the German program languished partly because of the belief that scarce and expensive heavy water would have to be used for that purpose.

In 1943, CP-1 was moved to Red Gate Woods, and reconfigured to become Chicago Pile-2 (CP-2). There, it was operated until 1954, when it was dismantled and buried. The stands at Stagg Field were demolished in August 1957; the site is now a National Historic Landmark and a Chicago Landmark.