A stable hydrogen isotope abundant in seawater, deuterium is one of the two fuels in the leading approach to fusion energy.
ReviewedLast reviewed: 9 Aug 2026·Category: Glossary
Deuterium (symbol D or 2H) is a stable isotope of hydrogen whose nucleus contains one proton and one neutron, giving it roughly twice the mass of ordinary hydrogen (protium). It occurs naturally at about 156 parts per million in water, making Earth's oceans a virtually inexhaustible fuel reserve for fusion energy.1
Deuterium in Fusion Reactions
Deuterium participates in several fusion-relevant reactions:
D–T: D + T → 4He (3.5 MeV) + n (14.1 MeV). The highest cross-section at accessible temperatures, and the basis for ITER and most near-term reactor designs.2
D–D: Two branches of roughly equal probability — one producing T + p (4.0 MeV total), the other producing 3He + n (3.3 MeV total).
D–3He: D + 3He → 4He + p (18.3 MeV total). An advanced-fuel reaction prized for its low neutron output.
Key fact: One gallon of seawater contains enough deuterium (about 1/6 of a gram) to produce energy equivalent to roughly 300 gallons of gasoline, if used in D–T fusion.3
Production and Supply
Deuterium is extracted commercially as heavy water (D2O) using the Girdler sulfide process or vacuum distillation. Canada's CANDU reactor program drove large-scale heavy water production, and current global capacity far exceeds projected fusion fuel demand. Unlike tritium, deuterium supply poses no resource constraint for a fusion-powered economy.1
Properties
Heavy water freezes at 3.82 °C and boils at 101.4 °C — slightly higher than ordinary water. Deuterium gas is non-radioactive and chemically almost identical to protium, though the mass difference produces a measurable kinetic isotope effect that is exploited in separation processes.
Scale of the resource: The world's oceans contain approximately 4.6 × 1013 tonnes of deuterium. At projected fusion-era consumption rates, this supply would last billions of years.
Role in Plasma Physics Research
Most magnetic confinement experiments operate with deuterium plasmas (rather than D–T) to avoid the radiological complications of tritium handling and 14.1 MeV neutron activation. D–D campaigns provide valuable confinement data while producing far less activation, making deuterium the workhorse fuel of present-day experimental fusion science.4