|Fig. 1: Sketch of the Chicago Pile One reactor. (Source: Wikimedia Commons)|
The Chicago Pile One (CP-1), constructed as part of the Manhattan Project, was the first man-made, controlled nuclear chain reaction and thus the first man-made nuclear reactor. [1-3] Motivated by the potential to utilize nuclear fission both as a power source and in weaponry, there existed, in the late 1930s and 1940s, a strong interest in fundamental research into nuclear fission and the ability to control this process. [1-4] To this end, the CP-1 experiment demonstrated that it was possible to establish a self-sustaining nuclear fission reaction, adjust its reactivity through control elements, and shut down the chain reaction when desired.  The CP-1 experiment was carried out at the University of Chicago, by researchers from that institution and from Columbia University, led by Italian physicist Enrico Fermi. [1-4]
By 1939, Fermi and co-workers had proposed two means by which a fission chain reaction could be developed using uranium as a fuel source.  Each proposal sought to mitigate a fundamental hindrance to the desired chain reaction: the parasitic (unproductive) absorption of neutrons in the reactor, especially by the abundant U-238 isotope in the uranium fuel. The first proposal was the use of U-235-enriched uranium; however, this was deemed impractical due to the difficulty of enriching natural uranium. The second proposal was the use of natural uranium in conjunction with a readily-available light element that would slow the neutrons to thermal energies, minimizing the loss by U-238 absorption. It was recognized that hydrogen, in the form of water or paraffin wax, could serve in this capacity; however, its behavior is complicated by its ability to absorb neutrons and thus lower the neutron population, preventing the chain reaction.  Ultimately, graphite was selected as the moderator, both due to its acceptable behavior as a moderator and due to its ready availability in pure form. [3,4] An additional consideration was the exact nature of the fuel source. Although pure uranium metal was considered a viable fuel, it was only available (as of 1941) in small amounts and with variable purity.  However, by 1942, highly pure forms of both uranium oxides and uranium metal were available, and these were employed together as the fuel in the CP-1. [3,4]
Initial experiments indicated that an ideal reactor design consisted not of a uniform distribution of uranium throughout the graphite moderator but, instead, of a lattice consisting of "lumps" of uranium in the graphite. This approach led to minimal parasitic absorption of neutrons by U-238. Furthermore, it was discovered that loss of neutrons by diffusion out of the reactor could be minimized by simply making the reactor very large, thus trapping neutrons within the reactor.  With these preliminary findings in hand, models were constructed at Columbia University in 1941 and thereafter at the University of Chicago. These models led to greater insight into, and mathematical models for, the behavior of neutrons in such a matrix. [1,3,4] Despite the risk of conducting an unprecedented nuclear fission experiment in a densely populated major city, construction of the CP-1 began in October, 1942, on the campus of the University of Chicago.  As its name implies, the CP-1 consisted of a spherical arrangement of "piles" of alternating uranium and graphite layers. Embedded into this matrix were three control rods composed of cadmium, a strong neutron absorber, which were used to control the reactor (see Fig. 1). [1,3,4]
As the size of the pile was increased, researchers carefully monitored the neutron output until the measurements indicated that the pile was close to becoming self-sustaining ("critical"). The late-stage construction was performed with the cadmium rods in place, and on the morning of December 2, 1942, it was calculated that the pile would reach criticality if the rods were removed. [1,4] During the key criticality experiment, all but one of the rods were removed.  One control rod was automatically set to be inserted if the reactivity passed a preset level. Another was suspended by a rope and pulley and could be released manually in an emergency. The final rod was controlled manually by George Weil, who was tasked with carefully removing the rod while Fermi and others measured the increasing neutron output, as the reactor approached criticality. [1,4] With each measured extension of the control rod, the reactor output increased in intensity but soon stabilized at constant output. This indicated that although the fission reaction was occurring, it was not yet self-sustaining. Finally, as Weil withdrew the rod a further distance, Fermi and others observed the neutron count rise at a slow but increasing rate -- evidence that the reactor had become self-sustaining and had reached criticality.  After this event, the researchers re-inserted the control rod, ending the self-sustaining reaction. 
The success of the CP-1 experiment established that a nuclear chain reaction could be achieved using natural uranium as a fuel source. Perhaps more importantly, the experiment demonstrated that such a reaction could be readily controlled. Clearly, this experiment had significant implications for the use of nuclear fission as an energy source and in weaponry.  Fermi later remarked that the success of the CP-1 "meant that release of atomic energy on a large scale would be only a matter of time."  Presumably due to its applications in weaponry, one of the researchers, Leo Szilard, remarked that the event would be regarded as "a black day in the history of mankind."  Indeed, within two years of the CP-1 experiment, reactors based on its principles were being used to produce plutonium.  However, the ability to employ nuclear fission as a power source also presented itself: although the maximum energy output of the CP-1 was only approximately 200 watts , scientists at the time were interested in using the heat output of a fission reactor as a source of electricity.  The outcome of the CP-1 experiment -- the demonstration that an energy-producing, self-sustaining fission reactor is feasible and controllable -- also served to validate this application of the technology. Fermi later commented on such applications with optimism, stating "we all hoped that with the end of the war emphasis would be shifted decidedly from the weapon to the peaceful aspects of atomic energy" and "we hoped that perhaps the building of power plants, production of radioactive elements for science and medicine would become the paramount objectives." 
© James T. Masters. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 E. Fermi, "Fermi's Own Story," in The First Reactor, United States Department of Energy, DOE/NE-0046, December 1982.
 S. Vander Hook, The Manhattan Project (Essential Library, 2011).
 J. Wood, Nuclear Power (Inst. Eng. Technol., 2007).
 E. Fermi, "The Development of the First Chain Reacting Pile," Proc. Am. Phil. Soc. 90, 20 (1946).