Around the world about ten different teams became passionately engaged in the attempt to produce the first artificial nuclear chain reaction, but only Joliot and his team were already in a position to turn this into an industrial or military reality. Joliot’s first problem was to slow down the neutrons emitted by the first fissions, for if these were too fast they would not set off the reaction. The team looked for a moderator that could slow the neutrons without absorbing them or  bouncing them back; thus the ideal moderator would have a set of properties very difficult to reconcile. In their workshop at Ivry, they tried different moderators under different configurations, for example paraffin and graphite. It was Halban who drew their attention to the decisive advantages of deuterium, an isotope of hydrogen, twice as heavy but with the same chemical behavior. It could take the place of hydrogen in water molecules, which then became “heavy.” From earlier work he had done on heavy water, Halban knew that it absorbed very few neutrons. Unfortunately, this ideal moderator had one major drawback: there was only one atom of deuterium in every 6,000 atoms of hydrogen. It cost a fortune to obtain heavy water, and it was produced on an industrial scale at only one plant in the world, which belonged to the Norwegian company Norsk Hydro Elektrisk. (Latour 1999:82-3)
LATOUR, Bruno. 1999. Pandora’s Hope: essays on the reality of science studies. London: Harvard University Press.