The original plan for the first experimental test pile had been to place it in the Argonne Forest, General Groves explains in Now It Can Be Told: The Story of the Manhattan Project, some fifteen miles out of Chicago, where special facilities were being built to accommodate the pile and its accompanying laboratories:
The already insufficient time available for this construction was cut even further by some labor difficulties which, while not particularly serious, led to delays.
In the meantime, work had begun on a small pile under the West Stands of Stagg Field at the University of Chicago. This pile was to be used to perform exponential experiments to determine the feasibility of the larger test pile. An exponential experiment, as its name indicates, is one from which, using measurements of the results obtained under varying conditions, the results to be expected under vastly different conditions can be calculated. When the supply of pure graphite necessary for the construction of a self-sustaining pile became available somewhat sooner than had been anticipated, Compton raised the question: “Why wait for Argonne?”
There was no reason to wait, except for our uncertainty about whether the planned experiment might not prove hazardous to the surrounding community. If the pile should explode, no one knew just how far the danger would extend. Stagg Field lies in the heart of a populous area, while the Argonne site was well isolated. Because of this, I had serious misgivings about the wisdom of Compton’s suggestion. I went over the situation with him, and told him of my feelings, but I did not interfere with his plans, nor did I display outwardly my concern by being present during the initial test. I learned then that nothing is harder for the man carrying the ultimate responsibility, in this case myself, than to sit back and appear calm and confident while all his hopes can easily be destroyed in a moment by some unexpected event over which he has no direct control.
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Although the committee was in the Chicago laboratories on December 2, 1942, when the Fermi experimental atomic pile was first placed in operation, the only committee member to witness the actual demonstration was Greenewalt. He was invited by Compton on the ground that he was the youngest and would be able to talk about it for the most years.
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“The Italian navigator [Fermi] has just landed in the new world. The natives are friendly.”
The December 2 test proved that a controlled chain reaction could be achieved, but it gave no assurance that it could be used to produce plutonium on a large scale. Neither did it give us any assurance that a bomb using plutonium or U-235 would explode. In the reactor the chain reaction was based on slow neutrons, i.e., ones slowed down by graphite or other means. In the bomb, the neutrons would be fast, for because of technical limitations there could be no moderators. Nevertheless, the committee, basing its opinion on what it had seen and heard during its inspections, reported favorably on the plutonium process.
[…]
In his letter, Compton was quite positive. He stated that the production of plutonium following the procedure then in hand was feasible; that there was a 99 per cent probability that it would be successful; that the probability of a successful bomb was 90 per cent; and that the time schedule, assuming continued full support, would see delivery of the first bomb in 1944 and a production rate of one bomb per month in 1945. This was by far the most optimistic estimate that I ever received prior to the explosion of the first bomb some thirty months later; and it was not at all justified by the existing knowledge.
The take away is that scientists always lie.