How is heavy water made?

Heavy water ($D_{2}O$) is composed of two atoms of deuterium—a hydrogen isotope with double the mass of ordinary hydrogen—and one atom of oxygen.1

Because deuterium occurs naturally in ordinary water at a ratio of approximately one atom for every 6,760 ordinary hydrogen atoms, it must be concentrated through specific industrial processes. Methods for producing heavy water include:1

• Electrolysis: Historically the primary large-scale method, this involves the continued electrolysis of large volumes of water until only a small amount of concentrated deuterium oxide remains.1
• Girdler Sulfide Process: A more cost-effective method where deuterium is exchanged between hydrogen sulfide and water.1
• Fractional Distillation: This process relies on the fact that $D_{2}O$ is less volatile than ordinary water ($H_{2}O$), causing it to become concentrated in the liquid residue during distillation.1

These processes are essential for creating the heavy water used as a neutron moderator in nuclear power plants.1