Manhattan Project at Chicago

... [T]his trouble with graphite [that boron impurities in it quenches a chain reaction even before it gets started] is one of the reasons America far outpaced Germany in the race to obtain a sustained nuclear chain reaction. The arrogance of German physicists made them suppose they alone had the intellect and talent required to achieve their goal. They didn’t bother to consult chemists and engineers, the very people who made the American project feasible. Tests by German physicists showed that graphite was unsatisfactory as a moderator in a reactor, but the physicists did not realize that the trouble lay in traces of boron. So they opted for heavy water, a material available only at great expense from one Norwegian hydroelectric plant. Sabotage prevented the Germans from ever attaining enough heavy water for a nuclear reactor. Pure graphite can be made in quantity far more cheaply and easily than heavy water.

That's from Work on the Manhattan Project, Subsequent Events, and Little Known Facts Related to its Use, a short article by Lawrence Bartels, University of Michigan chemist who's "one of the few remaining survivors of the war-time project."