One proton, one neutron, and one electron make up a Deuterium
A proton and a neutron make up the nucleus of Deuterium which is an isotope of Hydrogen.
Nuclear fusion:
When two atoms merge to become one, nuclear fusion produces power.
In a fusion reactor, hydrogen atoms combine to generate helium atoms, neutrons, and tremendous amounts of energy.
Two pairs of protons combine to generate two deuterium atoms.
Each deuterium atom joins with a proton to make a helium-3 atom.
Since Deuterium absorbs fewer neutrons than hydrogen, it acts as a moderator, which is crucial since nuclear fission processes need neutrons to trigger their chain reactions.
Because the heavy water is held under pressure, its boiling point rises, allowing it to function at high temperatures without boiling.