Deuterium (also known as heavy water) is an item that is used for the Fusion Reactor to start the fusion.
It is one of the 2 stable isotopes of hydrogen, the other being normal hydrogen.
Deuterium can be extracted from water by putting a water bucket in a powered Chemical Extractor.
Note that in older versions of Voltz, it was obtained from the Deuterium Extractor.
Fusion Reactor 1.2 Mj a second