Chicago, Illinois December 2, 1942

In November 1942 the world’s first artifi­cial nuclear reactor was assem­bled piece­meal below the bleachers of an un­used racquet­ball (squash) court at the Uni­ver­sity of Chicago’s Stagg Field. The impe­tus for building an Amer­i­can nuclear reactor, which con­sisted (mostly) of a huge pile of slip­pery black graph­ite bricks, was the fear that scien­tists in Nazi Germany had a two-year head start in building a nuclear reactor for their coun­try’s atomic wea­pons pro­gram. Metal­lur­gi­cal tech­ni­cians from the Chicago uni­ver­sity spent 17 days of around-the-clock labor arranging the 771,000 lb of bricks into an egg-shaped pile two stories tall and 25 ft wide. The pile was named Chicago Pile-1 (CP‑1). CP‑1’s outer­most bricks were solid hunks of graph­ite, a crys­tal­line form of the non­metallic chem­i­cal ele­ment carbon. Most bricks toward the center had holes drilled into them into which 5‑lb slugs of unen­riched natural ura­nium had been inserted—18,000 slugs capa­ble of firing off neu­trons every which way. (Dis­covered in 1932, a neu­tron, in the par­lance of the day, was seen as an “ele­men­tary par­ticle” of all atoms except those of hydro­gen.) A third com­po­nent of the pile was cad­mium, a kind of neu­tron sponge that was in place to pre­vent the pile from pre­ma­turely going criti­cal, meaning creating as many new neutrons as are lost.

Assembling Chicago Pile-1 was supervised by its designer, the bril­liant Italian-born phys­i­cist and 1938 Nobel Prize lau­re­ate Enrico Fermi. Fermi (1901–1954) had hit upon using graph­ite bricks as a neu­tron “mod­er­ator” in CP-1. Mod­er­ating, or reducing, the speed of neu­trons when they entered the sur­rounding graph­ite would allow a nuclear chain reaction to be sus­tained when another slug of ura­nium absorbed the neu­trons a frac­tion of a second later. If the chain reaction—the process by which the nuclei of atoms under­go fission (split) and inter­act—was not con­trolled (not mod­er­ated), the result would be the release of a huge amount of kinetic energy in the form of heat—in other words, a nuclear meltdown.

On this date, December 2, 1942, nuclear fission theory and nuclear fis­sion tech­nol­ogy came together for Fermi’s team and invited guests—42 peo­ple in all. One by one Fermi ordered the cad­mium rods removed from CP‑1 until the pile went criti­cal. At that moment Fermi achieved the first man-made, self-sus­taining nuclear chain reaction. As Fermi ordered the cad­mium rods rein­serted into the pile, the world’s first nuclear reactor cooled down.

Fermi’s successful demonstration of control­ling a nuclear chain reaction shifted the U.S.-led Man­hat­tan Project into over­drive. With a working nuclear reactor in front of them, the men and women of the Man­hat­tan Atomic Bomb Project—they would eventually number 600,000—could double down on building the indus­trial facil­i­ties required to enrich ura­nium and plu­to­nium (another chem­i­cal ele­ment) to make them highly fission­able. Facil­i­ties were set up in remote U.S. loca­tions in New Mexico, Tennes­see, and Wash­ing­ton state, as well as in Canada, for researching the best ways to wea­ponize nuclear energy and then following that up with related atomic tests. “Little Boy” and “Fat Man,” the ura­nium-based bomb dropped on Hiroshima and the plu­to­nium-based bomb dropped on Naga­saki, respec­tively, were just over two years away from being put to use to end the war with Japan and finally bring World War II to a close.

Enrico Fermi Demonstrates First Self-Sustaining Nuclear Chain Reaction, Chicago, Illinois, December 2, 1942

Sketch of Chicago Pile-1, November 1942Chicago Pile-1 team, December 2, 1946

Left: Erected in November 1942 at the University of Chicago, Chicago Pile‑1, as shown in this sketch, was the first suc­cess­ful U.S. nuclear reactor. CP‑1 was one of at least 29 exper­i­mental piles that were con­structed in 1942 at Stagg Field. On Decem­ber 2, 1942, a group of scientists used CP‑1 to achieve the first self-sus­taining chain reaction and there­by ini­ti­ated the con­trolled release of nuclear energy. CP‑1 contained 45,000 ultra-pure graph­ite bricks weighing 771,000 lb that acted as a “neu­tron mod­er­ator” whose pur­pose was to slow the speed of neu­trons, thus allowing a nuclear chain reaction to be sus­tained. The reactor was fueled by 12,400 lb of ura­nium metal and 80,590 lb of ura­nium oxide in 18,000 slugs weighing 5 lb each. Unlike most subse­quent nuclear reactors, CP‑1 had no radi­a­tion shielding or cooling system as it operated at very low power—about one-half watt.

Right: On December 2, 1946, the fourth anni­versary of their ground­breaking success, mem­bers of the CP‑1 team gathered at the Uni­ver­sity of Chicago to pose for this photo­graph. Team leader Enrico Fermi is in the front row on the left. Fermi is often hailed as the “archi­tect of the nuclear age” and the “archi­tect of the atomic bomb.” His peers called him “the Pope.” In the second row on the right in a light-colored trench coat is Leo Szilard. A Hun­garian-Amer­i­can phys­i­cist and inven­tor, Szilard (1898–1964) con­ceived the nuclear chain reaction in 1933, patented the idea of a nuclear fis­sion reactor in 1934, and wrote the August 2, 1939, letter for Albert Ein­stein’s signa­ture that prompted Presi­dent Frank­lin D. Roose­velt (1882–1945) to launch the $2 bil­lion ($28.5 bil­lion in 2020) Man­hat­tan Pro­ject that built the three atomic bombs that were deto­nated during World War II: “Gadget” at the Trinity test site in the New Mexico desert on July 16, 1945, and the two atomic bombs that laid waste to the Japa­nese cities of Hiroshima and Nagasaki on August 6 and 9, respectively.

Enrico Fermi: Godfather of the Atomic Bomb