There's a question that has been bugging me about the plant in Vermonk, Norway that was the focus of the various heavy water operations in WW2. The question is why was the heavy water facility at the plant built when it was ? From the Wikipedia article and other sources I learned that the plant itself made nitrogen for fertilizer production, and the idea for building the facility for heavy water was put forth in 1933, was accepted and it was operational in 1935. The thing I don't understand is why ? Heavy water has pretty much no real uses except as a moderator in some types of reactors. Other uses in chemistry seems to require such minute quantities that an industrial plant of that size seems completely unjustified. The thing is that the very IDEA of nuclear fission was not developed until 1938. In 1934 Fermi was only just trying to see what happened when uranium was hit by neutrons. The first reactor was Pile 1 in Chicago in 1942, seven years later, and the first heavy water reactors was something like fifteen years out. So, why did Norsk Hydro spend the money, time and effort to build the facility to make something on an industrial scale that seemingly had no application at the time ?
I originally asked in ww2, but it was suggested I ask here directly.
The plant in Norway was not a dedicated heavy water plant. It was a nitrogen fixation plant. Heavy water production, chemically, has similarities to the process by which ammonia is produced. So it was not a big change to adapt the plant to produce heavy water — it was a minor byproduct of another product they were already producing in bulk: fertilizer.
As for why it produced it — heavy water was a brand new physical substance that scientists were keenly interested in. There was also speculation (as there often was in the early days of radioactive research) whether there might be interesting medical applications of heavy water. At top capacity, the plant could only produce a dozen tons or so per year, which is not that much (a ton of pure heavy water takes up a little under one cubic meter of volume, and the stuff they produced was not even that pure). They were not producing it as a moderator for nuclear reactors, obviously, but it was still an interesting substance to experiment with. Ernest Lawrence, for example, used deuterium as a source of "deuterons" (protons bound with neutrons) which his particle accelerators would shoot at other atoms like cannonballs, to see what would happen.
But to be more specific in the case of Norsk Hydro, there were several young scientists working at the facility (Odd Hassel and Jomar Brun) who were very interested in heavy water after it was discovered and announced in 1933. The Hydro officials were not extremely convinced of its profitability, but as a major source of hydrogen electrolysis in Europe they were getting requests from British, French, German, and American scientists for samples of high-purity heavy water (which, again, they knew could be produced by any industrial-sized hydrogen electrolysis facility). The initial heavy water production operations were very small-scale (literally they could fit into one laboratory room) but it gradually grew in size. It was sold at a fairly high price initially, about 50 cents a gram (about $10/g today). About 40 kg was sold between 1934 and 1938; not exactly a big market. To put this into context, the first US reactor to run on heavy water (CP-3) used 5,900 kg of very pure heavy water, whereas the total German stockpile of heavy water was 1,760 kg (only 329 kg of it was very pure). The US produced 19,620 kg of heavy water during the Manhattan Project.
Source: Per F. Dahl, Heavy Water and the Wartime Race for Nuclear Energy.