This is a simplified diagram of your standard PWR. Reactor pressure vessel contains coolant. The fuel rods generate heat via fission, which is absorbed by the surrounding coolant. That coolant is pumped into the steam generator (In a PWR, this water is kept under sufficient pressure to prevent boiling). In the steam generator, cold water runs over the outside of a bank of small pipes, which allows the heat from the hot water to be transferred into the cold water.
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