Sry, but there are several things i feel a bit confused about. First: If you want to seperate water or heavy water into its atoms, then you need power for it, there is no energy produced in the process. Then, out in space the problem mostly isn't about getting it warm in your craft, more often the problem is to keep it cool, because all the electric stuff inside produces lots of heat, you need a system to get rid of this excess heat. Also, the pure water produced by combining oxygen and hydrogen is toxic to humans because it's missing many minerals (like salt). Lastly, heavy water isn't "2HO2" its "2H2O" wich results in nearly the exact amount of power needed to seperate both parts. What they do on actual space flights is, combining their rocketfuel (LH and LO) to Water, using a fuel cell, which provides power. Combining all of that, I have to say that your idea isn't really good. What can be thought about, are fuel cell's, which use fuel (if our rocket fuel is LH/LO) and generate power.