Heavy water
The technology is straightforward. Heavy water (D2O) is separated from normal water by electrolysis, because the difference in mass between the two hydrogen isotopes translates into a slight difference in the speed at which the reaction proceeds. To produce pure heavy water by electrolysis requires a large cascade of electrolysis chambers, and consumes large amounts of power. Since excess power was available, heavy water could be purified from the existing electrolyte. Norsk Hydro became the heavy-water supplier for the world's scientific community, as a by-product of fertilizer production for which the ammonia was used. Hans Suess, a German adviser on the production of heavy water, had assessed the Vemork plant as incapable of producing militarily-useful quantities of heavy water in less than five years at its capacity at the time.