tl;dr: New analysis of surviving uranium cubes and archival records shows Germany's B8 reactor would have needed 1,292–1,775 cubes and twice the heavy water Norway's Vemork plant produced during the entire war—far more than Heisenberg estimated. Additionally, the British naval blockade forced German scientists to use high-impurity metallurgical coke from the Ruhr instead of cleaner petroleum coke, meaning no natural uranium, graphite-moderated reactor could have gone critical with the materials available to them.