Scientists have rebuilt parts of Nazi Germany’s wartime nuclear reactor, and the results settle a long debate. The B8 pile was an experiment built in a cave at Haigerloch in 1945: 664 cubes of uranium suspended in heavy water, an attempt to start a chain reaction. It never worked, and Allied troops dismantled it in April 1945.

Now physicists have reconstructed it and published the findings in PNAS Nexus. Their conclusion: the design would have needed about twice as much uranium and well over twice the heavy water to sustain a reaction. Its multiplication factor reached only about 0.94, short of the 1.0 needed.

The team also confirmed that graphite could not have substituted for heavy water, as the Germans feared boron contamination. In short, Germany’s atomic bomb programme was further from success than some histories suggested. The study turns a wartime what-if into a measured answer.

Haigerloch’s cave laboratory is now a museum devoted to the history of atomic research.