(Speaker not an expert. Personal interest.)
Heavy Water is Deuterium oxide, D2O. Deuterium is a hydrogen isotope, chemically identical to normal water but different atomic weight owing to the presence of a neutron in the nucleus. Produced for various uses – nuclear power, industrial, medical etc. and, potentially, the development of nuclear weapons.
Heavy Water is best known by the general public from the 1965 film “Heroes of Telemark” about the wartime attacks on the Norwegian production centres at Vemork and Rjukan starring Kirk Douglas and Richard Harris. Considerably fictionalised in places. A more accurate account is “Stop the Nazis. Battle for Heavy Water” (La Bataille de l’eau lourde.) Norway / France. 1948. Many books, TV series etc since. Lots on internet.
The heavy Hydrogen isotope, Deuterium, occurs at concentrations of 1 in 6420 in ordinary water (around 0.015%). Chemically identical but different atomic weight owing to presence of a neutron as well as a proton in the nucleus. There is also a rarer isotope, Tritium, with two neutrons in its nucleus.
Many eminent scientists contributed to the understanding of atomic structure.
Frederick Soddy investigated nuclear transmutation, radioactivity and isotopes, receiving a Nobel Prize in 1921. J J Thomson worked on cathode rays, electrons and isotopes of stable elements. Gilbert Lewis identified covalent bonds and did work on electrolysis and nitrogen fixation. He produced a significant sample of Heavy Water.
Ernest Rutherford, awarded a Nobel Prize in 1908, proposed an atomic structure with a dense positive nucleus surrounded by orbiting negatively charged electrons. Subsequently he proposed the existence of neutrons having the same mass as protons but no charge. Atomic nuclei contained positively charged protons corresponding to the atomic number of the element and neutrons in varying numbers leading to isotopes. Hydrogen, as the simplest element, has one proton and one electron. Deuterium was similar but also had a neutron, making it ‘heavy’. Harold Urey was credited with the discovery of Deuterium, receiving a Nobel Prize in 1934.
In parallel with this work was an increasing concern that there would be insufficient resources of natural nitrogen-based fertilisers, large quantities of which were being imported, mainly from Chile and Peru, estimated at one third of world requirements (12 million tons). Various processes were looked at to plug the gap with compounds obtained by ‘fixing’ atmospheric nitrogen. These generally required huge quantities of electricity as could be made available from the Norsk Hydro works at Vermork. This had been opened in 1906 with a head of 300 metres and a capacity of 92MW, the largest in the world at the time.
One approach to nitrogen fixation was via electric arcs, pursued by Joseph Swan and others. In 1905 Norsk Hydro introduced their Eyde / Birkeland cascading electrolysis process at Vemork with capacity increased in stages to 185 MW at Rjukan by 1915. This produced about 80kg of Heavy Water annually. By 1916 Norway was exporting saltpetre (Potassium Nitrate). This process became uncompetitive by 1920, and was taken over by IG Farben in Germany in 1927.
The Haber- Bosch process to produce ammonia had been developed at BASF in Germany. This process involved nitrogen and hydrogen being compressed to 250 atmospheres. It coincided with shipments of Chilean nitrate being cut off owing to the war in 1914. Over 500 people were killed in an explosion at Oppau, Germany, caused by dangerous storage of unstable Ammonium Nitrate.
In 1940 during WW2 Norsk Hydro was asked by Germany to supply their stocks of Heavy Water. Norsk Hydro refused this, without any explanation. They subsequently sent it to the French.
Scientific work had continued with Fermi from 1934 experimenting with the bombardment of all elements with neutrons and incidentally discovering paraffin could be used as a moderator in nuclear reactions. Hahn showed that atoms of Barium 56, a stable element, could be split. Szilard observed chain reactions and attempted to patent them! By 1939 such experiments were being led by two groups – Fermi/Szilard and Pierre and Marie Curie.
By 1941 there were several efforts to disable or destroy the Norsk Hydro works, titled - Grouse: Freshman : Swallow : Gunnerside and US bombing. All of these were abandonned following the sinking of the ferry ‘Hydro’ on Lake Tinn by Norwegian saboteurs as it was transporting supplies of Heavy Water.
Subsequent WW2 and postwar work compared heavy water with graphite as a moderator in developing nuclear weapons and nuclear power plants. Graphite showed considerable advantages in controlling chain reactions and became by far the most widely used.
In postwar Germany production of nuclear materials was established at their Haigerloch reactor. Only small volumes were produced and it was evident that they had been a long way from producing any viable weapons based on nuclear technology.