Water is necessary. — heavy water I think it’s called after use.
No. Water is used for two purposes, first is the heat of fission turns water, in a closed loop system, into high pressure steam which drives turbines which in turn drive electric generators, that steam passes through piping surrounded by other water which is never mixed with the closed loop high pressure water, to cool it down. . . This water can be ocean water, river water, or reservoirs of water which in turn is cooled in cooling towers before being returned to its source. None of this is necessarily "heavy" water.
All water is a molecule of two Hydrogen Atoms and one Oxygen Atoms. H2O. However Hydrogen is found in three isotopes. I.e. the atoms of Hydrogen differ in the number of neutrons they contain. Normal Hydrogen, H1 Has one Proton and one Neutron. The second isotope of Hydrogen, H2, Deuterium, has one Proton and two Neutrons. Another isotope of Hydrogen, H3, Tritium, has one Proton and three Neutrons. Either of these last two isotopes with extra neutrons when found in the water compound make up the "heavy" water . . . Because it literally weighs MORE than an equal volume of regular water.
The deuterium and tritium is important because it’s used to make H bombs. . . Because both are unstable, radioactive, releasing their extra neutrons easily, in a cascading fashion. . . and when they do lots of energy is released. You DON’T want that Heavy Water near a fission reactor, because you use a fission bomb to ignite an H bomb! It won’t, but naked neutrons flying around are not a good thing!