Nuclear power plants contain a reactor unit that releases heat to turn water into steam which then drives the turbine-generators that generate electricity. There are two types of reactors in the United States: boiling-water reactors and pressurized-water reactors. In a boiling-water reactor, the water heated by the reactor core turns directly into steam in the reactor vessel and is then used to power the turbine-generator. In a pressurized-water reactor, the water heated by the reactor core is kept under pressure and does not turn to steam. The hot, radioactive, liquid water then flows through and heats thousands of tubes in a steam generator. Outside the hot tubes in the steam generator is nonradioactive water which boils and turns to steam. The radioactive water flows back into the reactor core where it is reheated and then returns to the steam generator.