U.S. naval nuclear propulsion plants use a pressurized-water reactor design that has two basic systems: the primary system and the secondary system. The primary system circulates ordinary water in an all-welded, closed loop consisting of the reactor vessel, piping, pumps, and steam generators. The heat produced in the reactor core is transferred to the water, which is kept under pressure to prevent boiling. The heated water passes through the steam generators where it gives up its energy. The primary water is then pumped back to the reactor to be heated again.
Inside the steam generators, the heat from the primary system is transferred across a watertight boundary to the water in the secondary system, also a closed loop. The secondary water (which is at relatively low pressure) boils, creating steam. Isolation of the secondary system from the primary system prevents water in the two systems from intermixing, keeping radioactivity out of the secondary water.
In the secondary system, steam flows from the steam generators to drive the main propulsion turbines (which turn the ship's propellers) and the turbine generators (which supply the ship with electricity). After passing through the turbines, the steam condenses back into water, and feed pumps return it to the steam generators for reuse. Thus, the primary and secondary systems are separate, closed systems in which continuously circulating water transforms the heat produced by the nuclear reaction into useful work (such as propulsion or electricity).
No step in this process requires the presence of air or oxygen. This, combined with the ship's ability to produce oxygen and purified water from seawater, allows a submarine to operate completely independent of the Earth's atmosphere for extended periods of time. In fact, the length of a submerged submarine patrol is limited primarily by the amount of food the ship can carry for the crew.