Nuclear reactors are basically heat engines. As uranium fissions, the breaking apart of atoms releases energy, much of it in the form of heat, which can then be used to do work. In a nuclear-powered submarine, reactor heat produces steam to drive the turbines that provide the submarine's actual power. The development of compact, safe, and highly reliable pressurized water reactors for naval use in the early 1950s was the major technological achievement that made nuclear-powered submarines possible.

How Reactors Work
Naval pressurized-water reactors include a primary coolant system and a secondary coolant system. The primary system circulates water, which is pressurized to keep it from boiling, in a closed loop. As water passes through the reactor, it is heated. It then goes through the steam generator, where it gives up its heat to generate steam in the secondary system. Finally, it flows back to the reactor to be heated again. Inside the steam generator, heat energy is transferred across a watertight boundary to the secondary system, also a closed loop. The unpressurized water in the secondary system turns to steam when heated. The steam, in turn, flows through the secondary system to the propulsion turbines, which turn the propellers, and to the turbine generators, which supply electricity. As it cools, it condenses to water and is pumped back to the steam generator.

Click diagram to enlarge.

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