A pressurized water reactor uses a nuclear reaction to heat water. It then transfers that heat to a steam generator to drive an electric turbine. These reactors use two completely separate water loops to ensure no radioactive water or steam escapes the system. After the steam drives the turbine, it is condensed and flows back into the system, cooling the water in the primary loop, which then cools the reactor.
Originally created for nuclear submarines, pressurized water reactors were designed to provide electricity and propulsion in a sealed-loop system that doesn't expose the crew or the environment to radioactive contamination. Their design proved reliable enough that it became the most common type of nuclear reactor used in the United States, with most electricity-generating reactors following its blueprint.
The use of water as both coolant and nuclear moderator is an important safety feature. Inside the reactor, high-energy neutrons strike hydrogen atoms in the water, converting them to thermal neutrons capable of sustaining a fission reaction. If the temperature of the coolant begins to rise, the water molecules become excited and move apart from each other, reducing the number of neutron strikes. Thus, the reactivity of the nuclear fuel is decreased inside the reactor.