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Well, as nuclear reactors are nuclear reactors, nuclear reactors are not used inside nuclear reactors.
The most common coolant used in nuclear reactors is water. There are light water reactors (using "regular" water), and the heavy water kind of reactor.
Heavy water is used in nuclear reactors as moderator for neutrons.
Current nuclear reactors rely on nuclear fission as their nuclear reaction.
We use nuclear fission in nuclear reactors to tap nuclear energy.
No, at least not for power reactors
All current nuclear reactors are fission reactors, tritium has no function in a fission reactor, in standard water moderated reactors deuterium also has no function, in heavy water moderated reactors deuterium is the moderator. If we are ever able to make a fusion reactor, deuterium/tritium mix will be used as fuel.
Primarily, nuclear reactors are used to generate heat, which is used to flash water to steam, which is used to spin turbines, which are used to generate electricity. It is also possible to use reactors for research, as well as for weapons development, such as in the generation of weapons grade Plutonium-239.
The moderator used in nuclear reactors with natural uranium is generally the heavy water (D2O).
There may be Radium, Thorium, Uranium or even Plutonium which can be used in nuclear reactors.
No substitute for nuclear power reactors especially if there is no available fossil fuel.
It depends. Pressurised water is used in pressurized water reactors (PWR), but molten salt (maybe fluoride?) reactors are said to be better.