Schematic diagram of deuterium atom.

An isotope of hydrogen containing one proton and one neutron in the nucleus. Deuterium is a naturally occurring stable isotope of hydrogen. It accounts for approximately 0.0156 percent of hydrogen atoms — in another words, one atom from 6,420 hydrogen atoms is deuterium. In combination with oxygen, it forms so-called heavy water.

In pressurized heavy water-cooled reactors the heavy water is used as a coolant and a moderator because it absorbs almost no neutrons.

Deuterium is also a valuable fuel of the future. The fusion of deuterium with tritium is assumed to take place in the reactor of the first fusion power plant and produce energy. The deuterium-tritium reaction has been successfully achieved repeatedly in various thermonuclear fusion research facilities. Deuterium reserves for these purposes are essentially inexhaustible, with an estimated 4.6 × 10¹³ metric tons of deuterium in seawater.