The German research program suffered from Hitler’s perception of it as “Jewish Physics” and so was funded for research, not for production. Heavy water was to be used as a moderator for a nuclear pile-a reactor. Of course we use regular water for many research reactors, the heavy water would have permitted higher activity for less fissile material.
The great barrier was a lack of fissile material. The US reactors that made fissile material initially used carbon as moderator, including the first reactor located in Chicago!!!. Imagine what would have happened if they had a problem like unto Chernobyl, which might have occurred since it was in fact the very first. Dr. Fermi ran a tight ship, comparing predictions to results with a low level of power, acutely aware of the risk.
Richard Rhodes tells the story of that December morning in splendid detail.
Fermi, of course, was running the show.
He'd have his workers pull out the damping rods (long wood poles with cadmium IIRC strips, marked in small units of length) one tiny bit at a time. He'd let the neutron detector readings stabilize at their new level, and did a little fiddling with his slide rule; this would tell him how far to withdraw the rods on the next step.
His aim was to withdraw the rods just exactly far enough for the shape of the neutron flux curve to change from an inverse exponential, where the reaction rate would slowly increase to a new stable level, to a straight-line increase without apparent limit.
Going further would turn the exponent positive, which would lead to an ever-accelerating reaction; if unchecked, this would lead to a runaway and meltdown, like Chernobyl (also a carbon-pile reactor, by the way). But of course all he needed to do was to get to the straight-line case, which would demonstrate the sought-for criticality.
Several times during the runup, the instruments would go off-scale, resulting in Fermi's associates stepping them to the next lesser sensitivity. At the beginning, you could hear and see individual clicks and flashes from the Geiger-Müller counters. Eventually, they became a continuous rush, which undoubtely added to the tension of the observers. Except, of course, for Fermi, who knew exactly what he was doing.
When he got the reactor to criticality, demonstrated without any doubt, he ordered it shut down, and that was that. I'm not sure they ever operated that reactor again.
All in a morning's work.
By the way, being the careful experimenter he was, he had assistants standing by with axes at the ropes suspending shutdown rods, and other assistants standing atop the pile (Fermi coined this usage of the word) with buckets of IIRC barium solution to dump into the assembly in case things went wrong with both the reactor and the automatic shutdown safety devices. If they had to do the latter, the pile would be permanently ruined.
I don't know what power level they let the reactor run up to, but I think it was under a watt. If they had let the reaction continue to increase, it would have gone to many megawatts before igniting the carbon blocks and creating the aforementioned Chernobyl, before its time. If they had "pulled out all the stops," it would have gone exponential and likely have created an explosion. Not a true fission bomb, but a sort of dirty bomb--in the middle of Chicago's Hyde Park; and the former site of the World's Columbian Exposition would have become the site of the "World's Columbian Explosion".
After the war, the government founded the Argonne National Lab as their principal nuclear reactor research facility, southwest of Chicago, in a rather less populated area.
Fermi was the rarest of physicists: A recognized genius in both theory and experiment. Like Johnny von Neumann, his would be a life cut short by cancer.