Nuclear archaeology reassesses Heisenberg's last reactor experiment

The B8 pile, built in April 1945, was a natural uranium, heavy water reactor experiment and the culmination of Nazi Germany's nuclear program. Its designer, Werner Heisenberg, claimed in a 1947 Nature article that the B8 was "just insufficient" in achieving self-sustaining fission, lacking only a "relatively small amount of uranium." In the following decades, historians have then suggested that the Germans should have pursued graphite reactors like the Americans. Here, we present forensics and document-based archaeology of the B8's uranium, heavy water, and graphites. We create a neutronics model that is validated by in situ 1945 measurements. Our calculations show that the B8, as built, had $k_{eff}=0.94$ . To achieve self-sustaining fission at $k_{eff}=1.0$ , the Germans would have needed about 2.0 times the uranium and about 2.2 times the heavy water than they used, far exceeding Heisenberg's predictions; these inventories were about 1.2 and 1.8 times, respectively, Germany's estimated available wartime inventories. Furthermore, we determine that their graphite, with 3.4 wppm boron impurities and re-used from the prior B7 experiment, would not allow criticality in a hypothetical graphite-natural uranium pile. German graphites were made of metallurgical coke, which naturally carry more boron than petroleum coke-based graphites, which were available in America and uniquely enabled Chicago Pile 1's criticality. Our findings indicate that Germany could not have built either a heavy water nor a graphite reactor in World War II.