Koeth and Hiebert discovered a box of declassified documents about German uranium in the National Archives and found about 400 additional uranium cubes from a separate Gottow Group reactor experiment. They concluded that the combined inventory would have been more than enough to reach criticality in the B8 reactor, suggesting that Germany’s secretive, isolationist approach was hampering its nuclear program because the two groups did not share information or resources.
This latest paper builds on that previous work but reaches a different conclusion. Koeth, Hiebert and their co-authors analyzed two of the preserved uranium cubes and combed through historical records, technical reports, correspondence and similar archival sources to learn more about the uranium, heavy water and graphite blocks used in the B8 reactor experiment.
Among their findings: The B8 reactor would have required between 1,292 and 1,775 uranium cubes to reach criticality and twice as much heavy water as the Norsk Hydro power plant in Vemork, Norway – the only source at the time – produced over the course of the entire war. In short: “Our calculations indicate that Heisenberg significantly underestimated the material requirements,” the authors wrote.
The authors also cited previous work noting significant differences in American and German graphite production. The former used petroleum coke. While some of this supply reached Germany, the British naval blockade in January 1940 deprived the Germans of this vital resource. The German nuclear physicists had to use metallurgical coke from the Ruhr area, which had twice as many impurities as American coke. (U.S. scientists had similar problems until they discovered a coke with unusually low boron content in Pennsylvania.) With the materials available to German scientists at the time, “no natural uranium-graphite-moderated pile could have become critical.”
PNAS Nexus, 2026. DOI: 10.1093/pnasnexus/pgag282 (About DOIs).
