Water in nuclear reactors serves both as a coolant to remove heat and as a moderator to slow down neutrons.

In many nuclear reactors, water circulates around the fuel rods to absorb the heat produced by fission. This heated water transfers energy to a secondary circuit where steam is generated to drive turbines. Additionally, water slows down the neutrons released during fission, making them more likely to induce further fission reactions, thus maintaining the chain reaction.