D-D reaction is the 📝fusion of two deuterium nuclei, a reaction that requires higher temperatures than deuterium-tritium fusion and yields less energy per event, but runs on a fuel that seawater supplies without practical limit.
The comparison with its near-neighbor defines the term. At the temperatures magnetic confinement machines can reach, the 📝D-T reaction proceeds hundreds of times faster than D-D and releases 17.6 MeV per reaction against D-D's roughly 4 MeV — which is why D-T is the fuel of first-generation power plants even though it demands 📝tritium, the scarce isotope that must be bred on site. Pure 📝deuterium fuel would sweep that whole breeding problem away, at the price of far harsher 📝plasma conditions; D-D fusion is therefore often described as a second-generation goal rather than an alternative starting point.
The reaction itself has two branches of nearly equal probability: one produces a tritium nucleus and a proton, the other a helium-3 nucleus and a neutron of 2.45 MeV — notably softer than D-T's 14 MeV neutron. That accessible neutron signature, and the absence of radioactive fuel, make deuterium-only operation the standard commissioning regime: experimental campaigns typically run in pure deuterium to shake down a machine before any tritium enters the building.