The Manhattan Project was not just a quest to build a nuclear bomb; it was a quest to build a bomb before Nazi Germany completed theirs. One route to a bomb is to build a nuclear reactor to produce fissile material. But it has remained unclear how close Germany was to creating their own reactor. Germany’s final attempted reactor, the B8 pile, used heavy water as a neutron moderator. The only source of heavy water at the time was the Norsk Hydro plant in Vemork, Norway, which was controlled by Germany from April 1940 until its destruction by Allied bombing in November 1944. Uranprojekt leader Werner Heisenberg claimed that the B8 pile was just short of criticality when American forces found and dismantled it in 1945. Timothy Koeth and colleagues assess that claim by analyzing two cubes of natural uranium from the project, as well as historical records, technical reports, correspondence, and other archival materials. Using these sources, the authors conclude that to go critical, the B8 pile would have required twice as much uranium as it had and twice as much heavy water than was produced by Norsk Hydro during the entire war. The graphite available in German-controlled areas would not have been a suitable substitute moderator. According to the authors, given the design the Germans were using, they simply did not have the materials necessary to build a working bomb.
PNAS Nexus
Nuclear archaeology reassesses Heisenberg’s last reactor experiment
29-Sep-2026