In a 2004 interview, Dr. Garwin spoke about Germany and the first U.S. plutonium-production reactors:

This work was all shared between the British and the Americans, ultimately. The British then bombed the Norwegian plants — heavy water plants — to keep the Germans from getting the heavy water. The Germans had initially tried graphite as a moderator, but it absorbed too many neutrons and they gave it up. But Szilard knew that the graphite manufacture — heating oil, wood, whatever to very high temperature — was usually done using boron carbide electrodes or other materials. The boron has a tremendous appetite for neutrons, so tiny traces of boron in the graphite were responsible for this parasitic loss of neutrons.

Szilard then worked with the suppliers, got somebody to provide very pure graphite without boron, and that’s how our first reactors were made: the Fermi Reactor that went critical December 2nd, 1942 in Chicago, and then the production reactors. Fermi’s Reactor was about two watts of thermal power maximum under the west stands, because it didn’t have any shielding and would have exposed people to too much radiation. The next step was the design of these 200-megawatt reactors for producing plutonium. You get about one gram of plutonium per day for a one-megawatt reactor. So, 200-megawatt reactor, two-tenths of a kilogram per day, and the first Nagasaki bomb used six kilograms of plutonium. So you could make — every 30 days you could make a new core for such a bomb.