Heavy water molecule.

Molecule of water that contains the hydrogen isotope deuterium instead of ordinary hydrogen. The chemical name is deuterium oxide. Deuterium has 1 neutron and 1 proton in its nucleus and is also called heavy hydrogen. Heavy water makes up about 0.02 percent of the water on earth. The presence of deuterium instead of hydrogen gives heavy water slightly different physical properties, like being denser or having the highest melting point.

CANDU type nuclear reactors use heavy water for cooling and as a moderator because of its excellent moderating properties.

In thermonuclear fusion research, heavy water is an interesting source of the fusion fuel deuterium.