The secret development of the reactor was the first major technical achievement for the Manhattan Project, the Allied effort to create nuclear weapons during World War II.

Although the project's civilian and military leaders had misgivings about the possibility of a disastrous runaway reaction, they trusted Fermi's safety calculations and decided they could carry out the experiment in a densely populated area.

[4] After a series of attempts, the successful reactor was assembled in November 1942 by a team of about 30 that, in addition to Fermi, included scientists 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.

At Columbia University in New York, Italian physicist Enrico Fermi collaborated with Americans John Dunning, Herbert L. Anderson, Eugene T. Booth, G. Norris Glasoe, and Francis G. Slack to conduct the first nuclear fission experiment in the United States on 25 January 1939.

[19][20] Szilard obtained permission from the head of the Physics Department at Columbia, George B. Pegram, to use a laboratory for three months, and he persuaded Walter Zinn to become his collaborator.

[21] They conducted a simple experiment on the seventh floor of Pupin Hall at Columbia, using a radium-beryllium source to bombard uranium with neutrons.

[22] Fermi and Szilard still believed that enormous quantities of uranium would be required for an atomic bomb, and therefore concentrated on producing a controlled chain reaction.

Because of his work studying the spectroscopy of the carbon arc, MacPherson knew that the major relevant contaminant was boron, both because of its concentration and its affinity for absorbing neutrons,[31] confirming a suspicion of Szilard's.

[32] Over the next two years, MacPherson, Hamister and Lauchlin M. Currie developed thermal purification techniques for the large scale production of low boron content graphite.

With the help of Eugene Wigner and Edward Teller, he approached his old friend and collaborator Albert Einstein in August 1939, and convinced him to sign the letter, lending his prestige to the proposal.

[39] In April 1941, the National Defense Research Committee (NDRC) created a special project headed by Arthur Compton, a Nobel-Prize-winning physics professor at the University of Chicago, to report on the uranium program.

[42] In May 1941, Emilio Segrè and Glenn Seaborg produced 28 μg of plutonium-239 in the 60-inch (150 cm) cyclotron at the University of California, Berkeley and found that it had 1.7 times the thermal neutron capture cross section of uranium-235.

[49] Another grant, this time of $40,000, was obtained from the S-1 Uranium Committee to purchase more materials, and in August 1941 Fermi began to plan the building of a sub-critical assembly to test with a smaller structure whether a larger one would work.

He scouted around the campus and we went with him to dark corridors and under various heating pipes and so on, to visit possible sites for this experiment and eventually a big room was discovered in Schermerhorn Hall.

[53] In contrast, Columbia University was engaged in uranium enrichment efforts under Harold Urey and John Dunning, and was hesitant to add a third secret project.

The entire pile was then canned by soldering sheet metal around it, and the contents heated above the boiling point of water to remove moisture.

[56] The United States Army Corps of Engineers assumed control of the nuclear weapons program in June 1942, and Compton's Metallurgical Laboratory became part of what came to be called the Manhattan Project.

1,025 acres (415 ha) were leased from Cook County in August,[68][69] but by September it was apparent that the proposed facilities would be too extensive for the site, and it was decided to build the pilot plant elsewhere.

[70] The subcritical piles posed little danger, but Groves felt that it would be prudent to locate a critical pile—a fully functional nuclear reactor—at a more remote site.

Szilard had noted that this reaction leaves behind fission products that may also release neutrons, but do so over much longer periods, from microseconds to as long as minutes.

[80] For a work force they hired thirty high school dropouts who were eager to earn a bit of money before being drafted into the military.

[95][96] While Leona Woods called out the count from the boron trifluoride detector in a loud voice, George Weil, the only one on the floor, withdrew all but one of the control rods.

He wanted to test the control circuits, but after 28 minutes, the alarm bells went off to notify everyone that the neutron flux had passed the preset safety level, and he ordered Zinn to release the zip.

[101]On 12 December 1942, CP-1's power output was increased to 200 W. Lacking shielding of any kind, it was a radiation hazard for everyone in the vicinity, and further testing was continued at 0.5 W.[102] Operation was terminated on 28 February 1943,[103] and the pile was dismantled and moved to Site A in the Argonne Forest, now known as Red Gate Woods.

[110] An accident involving radium and beryllium powder caused a dangerous drop in his white blood cell count that lasted for three years.

As the dangers of things such as inhaling uranium oxide became more apparent, experiments were conducted on the effects of radioactive substances on laboratory test animals.

[69] Though the design was held secret for a decade, Szilard and Fermi jointly patented it, with an initial filing date of 19 December 1944 as the neutronic reactor no. 2,708,656.

Surveys conducted in the 1980s found strontium-90 in the soil at Plot M, trace amounts of tritium in nearby wells, and plutonium, technetium, caesium, and uranium in the area.

There remained concerns about the ability of a graphite-moderated reactor being able to produce plutonium on industrial scale, and for this reason the Manhattan Project continued the development of heavy water production facilities.

[123] On 2 December 2017, the 75th anniversary, the Massachusetts Institute of Technology in restoring a research-graphite pile, similar in design to Chicago Pile-1, ceremonially inserted the final uranium slugs.