Nuclear power plants utilize water or heavy water as both a coolant and a neutron moderator, playing essential roles in their operation.

In a nuclear power plant, the primary function of water or heavy water is to serve as a coolant. The nuclear fission process occurring in the reactor core generates an enormous amount of heat. This heat is harnessed to convert water into steam, which then drives turbines to produce electricity. Without an effective coolant, the reactor core risks overheating, potentially leading to a catastrophic meltdown.

Water is considered an excellent coolant due to its high specific heat capacity, which is defined as the amount of heat energy required to raise the temperature of a substance. This property allows water to absorb significant amounts of heat before it begins to evaporate, making it particularly suitable for cooling the reactor core. Additionally, water is abundant and cost-effective, further enhancing its practicality.

On the other hand, heavy water, chemically known as deuterium oxide, is employed in certain reactor designs, such as CANDU reactors. It fulfills a dual role, acting as both a coolant and a neutron moderator. A neutron moderator is crucial as it slows down the fast neutrons produced during fission to velocities conducive to causing additional fission events, thus sustaining the chain reaction. Heavy water is a particularly effective moderator because it absorbs fewer neutrons compared to regular water, allowing a greater number of neutrons to participate in the ongoing chain reaction.

The use of water or heavy water in nuclear power plants is vital to their efficient operation. These substances facilitate the effective harnessing of heat generated from nuclear fission for electricity production while ensuring that the reactor core remains at a safe temperature. Moreover, in specific reactor types, heavy water is instrumental in maintaining the nuclear chain reaction.