A modern reconstruction of Nazi Germany’s wartime nuclear reactor has answered a long-standing question: how close were the Germans to an atomic bomb? Physicists rebuilt key parts of the B8 pile, a uranium-and-heavy-water experiment buried in a cave at Haigerloch in 1945. Writing in PNAS Nexus, the team found the design would have needed about twice as much uranium and more than double the heavy water to sustain a chain reaction.

The reactor’s 664 uranium cubes, suspended in heavy water, reached a multiplication factor of about 0.94, short of the 1.0 needed for a self-sustaining reaction. The researchers also confirmed that graphite, which the Germans abandoned over boron contamination fears, could not have worked as a substitute. The B8 pile was dismantled by an Allied mission in April 1945.

The study closes a decades-old debate, showing Germany’s wartime programme was further from success than some accounts suggested. The reconstruction was led by physicist Tim Koeth and colleagues.