In Naval Nuclear Propulsion plants, fissioning of Uranium atoms in the reactor core produces heat. Since the fission process also produces radiation, shielding is placed around the reactor to protect the crew. US Naval Nuclear Propulsion plants use a Pressurized Water Reactor (PWR) design which 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 a 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 core 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 a relatively low pressure, boils to create 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 turns the ships propeller, and the turbine generators, which supply the ship with electricity. After passing through the turbines, the steam is condensed back into water and feed pumps return it back to the steam generators for reuse. Thus, the primary and secondary systems are separate, closed systems in which constantly circulating water transforms the energy produced by the nuclear reaction into useful work.

               The advantages to nuclear power are quite diverse for the US Navy. The fact that a nuclear powered ship requires less logistic support is a huge advantage. Ships that require petroleum fuels as an energy source are limited in cruising range, and hence strategic effectiveness is always limited by the on board fuel supply. When refueling is required, either a return to port is required or a hazardous at sea refueling is performed.
Nuclear powered ships have virtually unlimited cruising range. The USS Nautilus steamed 62,562 miles on her first nuclear fuel load. The USS Enterprise steamed over 200,000 miles before being refueled.

               The biggest advantage of nuclear power for propulsion is that nuclear fission and all supporting systems do not require oxygen. This combined with a submarines capability to produce oxygen and purified water from seawater, enables the ship to operate completely independent of the earth's atmosphere for extended periods of time. In fact, the length of time a submarine can stay submerged is limited primarily by the amount of food the ship can carry for the crew! I remember many a patrol where we walked on top of food cans in the passageway.