Study says Nazi reactor was far from criticality, contradicting Heisenberg
A new PNAS Nexus study by U.S. researchers concludes the B8 nuclear pile that Werner Heisenberg's team assembled in a cellar beneath a church at Haigerloch in April 1945 was far from criticality. Its neutron multiplication factor was 0.942, against the 1 needed to go critical. Criticality would have required at least 2,994 kg of uranium and 3,411 litres of heavy water; all German facilities together held 2,572 kg of uranium and 1,890 kg of heavy water. Heisenberg wrote in Nature in 1947 that the team came very close.