The one im aware of uses deuterium, aka hydrogen2, to generate helium 3. One of the byproducts being tritium, aka hydrogen3. This means there’s potential for 2 deuteriums to mix with an oxygen molecule,this creating ²H2O, aka heavy water.

I’m neither a chemist, nor physicist. So someone could probably prove me wrong at the drop of a hat, but Im calling it close enough.:p