We can thank the heroes of Tel mark for risking their lives to stop the Germans from transporting the heavy water. I always wondered how factual that movie was.
The German research program suffered from Hitler’s perception of it as “Jewish Physics” and so was funded for research, not for production. Heavy water was to be used as a moderator for a nuclear pile-a reactor. Of course we use regular water for many research reactors, the heavy water would have permitted higher activity for less fissile material.
The great barrier was a lack of fissile material. The US reactors that made fissile material initially used carbon as moderator, including the first reactor located in Chicago!!!. Imagine what would have happened if they had a problem like unto Chernobyl, which might have occurred since it was in fact the very first. Dr. Fermi ran a tight ship, comparing predictions to results with a low level of power, acutely aware of the risk.