A new analysis of Nazi Germany’s B8 nuclear reactor experiment concludes that the project required substantially more uranium and heavy water than German physicist Werner Heisenberg had estimated—and that the materials available to German scientists could not have produced a critical reactor. The findings contrast with earlier research suggesting that Germany possessed enough uranium for the B8 reactor to reach criticality, highlighting how limited information-sharing and shortages of suitable materials constrained the country’s nuclear program.
Reassessing the B8 reactor
Coase and Hiebert previously discovered a box of declassified documents on German uranium in the National Archives, along with about 400 additional uranium cubes from another nuclear-reactor experiment conducted by the Gottow group. They concluded that Germany had enough uranium in its inventory for the B8 reactor to reach criticality. Their interpretation suggested that Germany’s secretive, isolationist approach hindered its nuclear program because the two research groups did not share information or resources.
The latest paper reaches a different conclusion. Koeth, Hiebert, and their coauthors examined historical records, technical reports, correspondence, and other archival material. They also analyzed two surviving uranium cubes to learn more about the uranium, heavy water, and graphite blocks used in the B8 experiment.
More material was needed than expected
The researchers determined that the B8 reactor would have required between 1,292 and 1,775 uranium cubes to reach criticality. It also would have needed twice as much heavy water as the Norsk hydroelectric power plant in Vemork, Norway, produced—the only source of heavy water available at the time throughout the war. “Our calculations suggest that Heisenberg significantly underestimated the materials needed,” the authors write.
Graphite quality posed another obstacle
The authors also cited earlier research identifying important differences between American and German graphite production. American manufacturers used petroleum coke, some of which reached Germany. However, the British naval blockade in January 1940 cut Germany off from vital resources.
German nuclear physicists therefore had to use metallurgical coke from the Ruhr region. It contained twice as many impurities as American coke. US scientists faced a similar problem until they discovered a source of coke in Pennsylvania with unusually low boron content.
Given the materials available to German scientists at the time, the authors conclude that “natural uranium, graphite moderation piles could not reach criticality.”
PNAS Nexus, 2026. DOI: 10.1093/pnasnexus/pgag282.
Source: arstechnica.com
