Wartime Uranium Cubes Show the Nazi Reactor Was Twice Short of Working

Old uranium left over from World War Two can be examined in the lab, and that turns out to be enough to work out how close the Nazis really came to building a working nuclear reactor. Two surviving cubes from the German wartime project tell a different story from the one the project's leader told after the war.

Heisenberg's own account

Werner Heisenberg led the development of the reactor for Germany's "Nazi bomb" project, which ended with a machine known as the B8 nuclear reactor. In 1947, after the war, he wrote that the reactor was almost finished. It was "just insufficient", he said, to create a reaction that keeps itself going, and it needed only a "relatively small amount of uranium" more to work.

What the two cubes showed

The new study was published Sept. 29 in the journal PNAS Nexus. Its authors used historical documents and forensic investigations of two of the project's surviving uranium cubes to push back against Heisenberg's claim. Physical leftovers from the wartime experiment give a direct check on what the project actually had on hand.

The study concludes that the B8 reactor had less than half the heavy water, and about half the uranium, that it would have needed for a continuous chain reaction. Heavy water is a special form of water that a reactor of this kind relies on if the reaction is to keep going.

"My results argue that Heisenberg was off by roughly a factor of two," said first author Patrick Park, a doctoral student at Princeton University who co-wrote the study while at Columbia University.

Why the wartime sums were rough

Park said the mistake was understandable. Heisenberg's calculations were only approximate. The project had been moved to a rock cellar in the town of Haigerloch, Germany, after Berlin was bombed.

"People were dying or starving and so these reactor calculations were not his first priority," Park told Live Science, adding that many details of the physics behind the reaction were unknown at that time. "Given these circumstances, I think he did the best he could," he said.

What a working reactor would have meant

If it had worked, the B8 reactor would have confirmed that a chain reaction was possible, marking a key step toward German development of an atomic bomb. It could also have spurred the development of specialized reactors used to create weapons-grade plutonium from uranium.

Instead, the atomic bomb was developed first by the United States and was used to force the surrender of Japan in August 1945. Germany had already surrendered in May 1945, and the German reactor project ended without a working reactor.