In Pressurized Water Reactors (PWRs), the water (primary coolant) in the reactor core is prevented from boiling by being maintained at a much higher pressure.

Heat is removed using a steam generator. In a steam generator, the primary coolant flows through a series of metal tubes while secondary cooling water flows around the tubes. In this way, heat is transferred from the slightly radioactive primary coolant system to the nonradioactive secondary coolant system.

The secondary coolant is maintained at a much lower pressure than the primary coolant. Thus, as the heat is transferred, the secondary coolant flashes to steam. This steam is then drawn from the steam generator and used to rotate the turbine generating electricity.