deuterium is the stable heavy isotope of hydrogen, whose nucleus carries one proton and one neutron — twice the mass of ordinary hydrogen and one half of the fuel for 📝fusion energy.
The name comes from the Greek deuteros, "second." Harold Urey identified the isotope in 1931, an achievement that earned him the 1934 Nobel Prize in Chemistry. Water made with deuterium in place of ordinary hydrogen is "heavy water," historically important as a moderator in fission reactors — a separate role from deuterium's place in fusion, where the isotope itself is the fuel.
Deuterium's appeal as a fuel is its abundance and reach: roughly one in every 6,400 hydrogen atoms in seawater is deuterium, and it is routinely extracted at industrial scale, so the oceans hold an effectively inexhaustible supply available to any nation with a coastline. Its extra neutron is what makes it fusible at achievable conditions — ordinary hydrogen fusion, the sun's dominant reaction, is far too slow for a power plant. Paired with 📝tritium in the 📝D-T reaction, deuterium yields the highest fusion reaction rate at the lowest temperature of any known fuel combination, which is why nearly every first-generation fusion power plant design burns it.