Deuterium is an isotope of hydrogen which contains one proton and one neutron in its nucleus (called a deuteron), and one electron orbiting the nucleus, whereas the Hydrogen nucleus lacks any neutron particle. Naturally-occurring hydrogen consists of less than 1% of deuterium and deuterium is found in large quantities in water, more than one atom per ten thousand hydrogen atoms has a deuterium nucleus. The isotope is denoted “2H” or “D”, and is normally known as “heavy hydrogen”. In Star Trek, deuterium, along with its antimatter opposite, antideuterium, is the primary fuel used in the matter-antimatter reactions that power a starship's warp engines, and is also used in the fusion reactors that power the sub-light speed impulse engines.

From left to right, the hydrogen atom and the
isotopes deuterium and tritium

In real science, Deuterium is already widely used in a number of conventional nuclear fission reactors in the form of heavy water (D2O) - water that contains a higher proportion than normal of the deuterium isotope. Heavy water acts as a neutron moderator to slow down neutrons so that they can react with the uranium in the reactor. The use of heavy water essentially increases the efficiency of the nuclear reaction and heavy water reactors are more efficient at breeding plutonium (from uranium-238) or uranium-233 (from thorium-232) than a comparable light-water reactor.

Deuterium also has great potential in the development of nuclear fusion reactors, especially in combination with tritium, because of the large reaction rate and high energy yield of the D-T reaction. Deuterium-tritium is the fuel proposed for the first generation of experimental fusion power plants such as the Joint European Torus (JET) and the International Thermonuclear Experimental Reactor (ITER). Research is also being conducted on ultra-dense deuterium - a super-heavy material which is thought to plays a role in the formation of stars and probably present in giant planets such as Jupiter. It is thought that ultra-dense deuterium may be a very efficient fuel in laser driven nuclear fusion. Ultra-dense deuterium is a million times more dense than frozen deuterium, making it relatively easy to create a nuclear fusion reaction using high-power pulses of laser light.