Unaware that this was due to impurities, they did not test ultra-pure graphite (which would have been suitable). Instead, they settled on a heavy-water-based reactor design. A heavy-water-moderated nuclear reactor could be used for nuclear-fission research and, ultimately, to breed the plutonium with which a bomb could be made. Heavy-water production In normal water, there is only one deuterium atom for every 6,400 hydrogen atoms; deuterium is more prevalent in the residue of water used as an electrolyte. An analysis of residues from the Vemork hydroelectric plant, a large-scale nitrate production plant using the Haber process, showed a hydrogen-to-deuterium ratio of 48 (most of the deuterium bound in HDO molecules). Norwegian Institute of Technology lecturer Leif Tronstad and Jomar Brun, head of the hydrogen plant, proposed a project in 1933 (the year heavy water was first isolated). It was accepted by Norsk Hydro, and production began in 1935. 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. Operations to limit German access to heavy water Pre-invasion efforts French research considered the production of 239Pu using reactors moderated by heavy water and graphite. Preliminary French research indicated that the graphite then available commercially was not pure enough to serve the purpose, and heavy water would be required. The German research community had reached a similar conclusion, and had procured additional heavy water from Vemork in January 1940. The German firm IG Farben, a partial owner of Norsk Hydro, had ordered per month; Norsk Hydro's maximum production rate was then limited to per month. The Deuxième Bureau (French military intelligence) directed three French agents (Captain Muller and Lieutenants Mossé and Knall-Demars) to remove the world's extant supply, , of heavy water