Steam generators act as heat exchangers in this system. Heat in the reactor core comes from a chain-reaction of exploding uranium atoms. In the core, primary water is heated above 600 degrees Fahrenheit. The primary water returns to the core for re-heating. Primary water does not boil because it's kept at a very high pressure.
Prairie Island has four steam generators, two for each reactor.
Each is about 25 feet tall, with over 3,000 tubes. Water from the core is inside the tubes. Secondary water outside the tubes boils and the resulting steam spins turbines to generate electricity. Less than one third of the energy produced in the core emerges as electricity. The remaining energy is removed as waste heat in the condenser and sent to the river and atmosphere.