The importance of Heavy Water for the Nazi weapons-development program is overstated. They could simply have used graphite (carbon) as a moderator, as the early Manhattan project breeder reactors did. However, they had made a measurement error on the neutron-absorption cross-section of Carbon, showing it as too high for use as an efficient moderator, hence they went on a wild-goose chase to separate Heavy Water by electrolysis, for use as a moderator using the Norsk Hydro plants at Norway, which were eventually destroyed by Allied sabotage and bombing. All this rigmarole was therefore due to a small laboratory-methods error by some little-known German scientist (probably contamination of his carbon samples with a trace element like Boron or Cadmium). Graphite was readily available in Axis territory - a few more tests would likely have found a purer source without Boron contamination.