New research shows Nazi nuclear team far from critical, Heisenberg’s claims overstated

A recent PNAS Nexus study shows Werner Heisenberg’s wartime reactor was nowhere near achieving a self‑sustaining chain reaction. The B8 pile, assembled in a Haigerloch cellar in April 1945, held 664 uranium cubes in about 1,400 L of heavy water, surrounded by a 40‑cm graphite shell, aluminium and a 60‑cm water layer. Six years of work produced no criticality; a few injected neutrons multiplied but never sparked a chain reaction. Heisenberg later wrote in a 1947 Nature article that a little more uranium would have made the reactor work, but the new analysis finds the opposite. Density tests on the surviving 14 cubes out of roughly 1,100 show they were porous, with a measured density of 18.535 g/cm³ versus 19.01 g/cm³ for solid uranium, and one cube contained only 0.724 % uranium‑235, indicating natural, not enriched, material. Heavy‑water purity was also lower than assumed. In March 1944 Karl Wirtz reported an accidental dilution to 96.85 % purity; a 1947 post‑war sample from Haigerloch measured 96.8 %. Using records from Norsk Hydro, researchers estimate Germany’s total pure heavy‑water stock at 1,836 kg. Simulations give the B8 pile a neutron multiplication factor (k) of 0.942 – well below the critical value of 1.0. Even perfect‑purity heavy water would raise k only to 0.95. To reach criticality the pile would have needed at least 2,994 kg of uranium and 3,411 L of heavy water, far more than the 1,538 kg uranium and 1,549 kg heavy water actually present. German facilities combined held just 2,572 kg of uranium and 1,890 kg of 96.8 % pure heavy water, giving a best‑case k of 0.959. By contrast, the U.S. Chicago Pile‑1 went critical on 2 December 1942 using natural uranium and graphite with only 1.4 wppm boron, while German graphite contained 3.4 wppm boron, a neutron absorber that made criticality impossible. The decisive gap was industrial capacity: the Manhattan Project created about 130,000 jobs and spent $2.2 billion, resources Germany never matched.

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