There are two main ways which nuclear plants use water for cooling - to push heat from the reactor to the steam turbines, or to remove excess heat from the steam circuit. Therefore it follows that the bigger the temperature difference between the inside (heat source) and the external environment, the more efficient the process of turning a turbine or generator. This is why it is good to have a high internal temperature and a low external temperature. So for wet operations, plants will best be situated near very cold water.
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