Nuclear reactors use coolants (a main coolant) to keep form overheating, of course. They are also used totransfer the thermal energy (heat) generated by a reactor to the steam generating units.
Nuclear reactors use nuclear fission.
We use nuclear fission in nuclear reactors to tap nuclear energy.
In nuclear fission reactors
The use of nuclear energy
The most common coolant used in nuclear reactors is water, in either liquid or steam form. Water provides effective heat transfer properties and is readily available and cost-effective. Other coolants, such as liquid sodium or gas, are used in specialized reactors but water-cooled reactors are the most prevalent.
Many pressurized water reactors use "regular" water (light water) as a primay coolant. That means that "only heavy water" is not a rule as regards reactor design. Reactor design specifies the coolant to be used.
Nuclear reactors use controlled nuclear fission reactions to generate heat, which is then used to produce steam that drives turbines to generate electricity. The heat is produced in the reactor core where nuclear fuel rods containing uranium or plutonium undergo fission reactions. The reactor's cooling system helps regulate the temperature and prevent overheating.
To produce electricity
Nuclear power plants require large volumes of water to cool reactors and convert heat to electricity. Reactors use normal water, heavy water, and even newer reactors use other forms of coolant.
They all use nuclear fission
Minecraft doesn't have nuclear reactors.
In nuclear reactors used to generate electricity, and in many nuclear warheads.