Final answer:
Power plants, especially nuclear reactors, require large amounts of water to cool the reactor cores, where water serves as a coolant that transports heat produced by fission to generate electricity and is subsequently cooled for reuse.
Step-by-step explanation:
Power plants require a large amount of water primarily because water is used for cooling reactor cores. In nuclear power plants, a nuclear reactor coolant, which is often water, carries the heat produced by the fission reaction to an external boiler and turbine to transform it into electricity. This process involves two overlapping coolant loops to help prevent the transfer of radioactivity to the primary coolant loop. Furthermore, the heated water from within the reactor passes through tubes inside a steam generator, where it transfers heat to another water source, causing it to boil and generate steam. The steam then drives turbines connected to generators, producing electricity. The steam is condensed and the water is cooled in a large cooling tower or back into a water body before it is recirculated through the reactor core.