In France, some scientists, including the Curies, realized that, in a homogeneous medium, the speed of neutrons emitted during uranium fission would decrease continuously, thus passing through certain values called resonance, where they are more easily absorbed by other uranium atoms. They wanted to use deuterium (heavy water: hydrogen with one neutron) as a moderator because hydrogen absorbs too many neutrons. The Rjukan plant in Norway is the world’s largest producer of heavy water. The French managed to deceive the Nazi secret services and smuggle 167 liters of heavy water to France. Fermi in the USA instead used very pure graphite as a moderator.
