Deuterium

Deuterium is a fluid added by NuclearCraft, GregTech 4, and Mekanism.

NuclearCraft
Deuterium is the product of Fusion Reactions and can be synthesized in an Electrolyser.

Mekanism
Deuterium is the product of separating Heavy Water in an Electrolytic Separator. It's used in a Fusion Reactor with Tritium, and optionally, some D-T Fuel, to produce large amounts of energy, and if water-cooled, very large amounts of steam.