The Nazis almost built a working nuclear reactor.

[hunter]

5. I was thinking this as well, but now it seems it was more of an organizational problem.

They had enough uranium, they had enough heavy water, but not all in one place. They weren't willing to put all their eggs in one basket.

Boron free nuclear graphite, as used by the U.S.A. and later the Soviet Union might have been beyond Germany's means even if their physicists had recognized the problem. Removing the boron from German coal derived graphite wouldn't have been easy, and the diversion of coal required to accomplish it from the coal powered war machine would have been a very hard sell.

Nevertheless, "writing as late as 1947, Heisenberg still did not understand that the only problem with graphite was the boron impurity."

The U.S.A. and the Soviet Union had abundant sources of energy and boron free carbon.