Uranium cubes reveal why Germany's wartime nuclear project failed
A new study in PNAS Nexus finds that Nazi Germany's B8 reactor experiment lacked sufficient materials because Werner Heisenberg significantly underestimated the amounts needed. An earlier study by the same researchers had blamed secrecy between rival groups.
Researchers Koeth and Hiebert, examining surviving uranium cubes and historical documents, have concluded that Germany's World War II B8 nuclear reactor experiment could not have reached criticality, the point at which a nuclear chain reaction sustains itself. The findings, published in the journal PNAS Nexus in 2026, offer an explanation for why Nazi Germany's effort to build a working reactor failed.
The earlier study
Koeth and Hiebert previously found a box of declassified documents about German uranium in the US National Archives. They also located about 400 other uranium cubes from a separate reactor experiment run by the Gottow group. At the time, they concluded the combined inventory would have been more than enough for the B8 reactor to go critical. They argued that Germany's secretive, isolationist approach hampered the program because the two groups were not sharing information or resources.
The new findings
In the new paper, the authors analyzed two of the surviving cubes and went through historical records, technical reports and correspondence about the uranium, heavy water and graphite blocks used in the B8 experiment. They calculated that the reactor would have required between 1,292 and 1,775 uranium cubes. It would also have needed twice the heavy water produced over the whole war by the Norsk Hydro plant in Vemork, Norway, which was then the only source. The authors wrote that their calculations suggest Heisenberg significantly underestimated the materials required.
The graphite problem
The authors also cited earlier work on differences between American and German graphite production. US producers used petroleum coke. Some of that supply reached Germany, but the British naval blockade in January 1940 cut off the resource. German physicists had to rely on metallurgical coke from the Ruhr, which contained twice as many impurities as American coke. US scientists had similar problems until they found an unusually low-boron coke in Pennsylvania. The authors therefore conclude that no natural uranium, graphite-moderated pile could have gone critical with the materials available to German scientists at the time.
