Light water reactor, heavy water reactor; two types of nuclear reactors used in electrical energy production. Light water reactors use standard water as a coolant and thermal medium; heavy water reactors use water that contains higher-than-normal concentrations of modified heavy hydrogen atoms as a coolant.

Hydrogen normally contains one proton and one electron. These modified hydrogen atoms contain a neutron as well, which is normally not needed in a hydrogen atom. This type of hydrogen is referred to as deuterium, so named because its nucleus contains two particles instead of the usual one, a fact which also makes the atom heavier than normal. Deuterium atoms occur naturally in water at a rate of one in approximately 6,500 atoms and is non-radioactive. Tritium is a far rarer form of hydrogen, occurring naturally in only one of every billion billion (10 to the 18th power) atoms. It is radioactive, and contains two neutrons for a total of three particles in the nucleus. When deuterium and/or tritium are concentrated in water, the water becomes a much more efficient coolant, making it useful in nuclear reactors which generate enormous amounts of heat.

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