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Most often, light water (H2O) is used as a moderator in a nuclear reactor. Sometimes, heavy water (D2O), or graphite is used.
Heavy water(PHWR)--Pressurised heavy water reactor
Heavy water
The moderator used in nuclear reactors with natural uranium is generally the heavy water (D2O).
Water (H2), or heavy water (D2O). Sometimes, liquid sodium is used.
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.
This is used in the nuclear reactor that is known as Boiling Water Reactor (BWR) in which heat produced by the nuclear fission in the nuclear fuel allows the light water reactor coolant to boil. Then, the nuclear reactor moisture separator is used to increase the dryness of the produced steam before it goes to the reactor steam turbines.
Most current power plants use water, but heavy water, graphite, hydrocarbons, etc. could be used instead.
The pressurised water reactor (PWR)
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.
carbon and water.
Heavy water (deuterium) functions as a moderator. It slows down fast neutrons released by fission reactions in order to allow the reaction to be sustained. Fast neutrons pass through the reactor before initiating another fission reaction.