Nuclear submarines are powered by a nuclear reactor located in the rear of the submarine. The nuclear reactor works similarly to a steam engine by heating water into steam. The reactor produces energy in the form of gamma radiation and heat by using a neutron to split an atom of uranium.
The water is heated and piped to the turbine that makes energy needed to power the submarine. The heated water is then cooled and recirculated into the steam generator. The propeller of the submarine, powered by the nuclear reactor, propels the submarine forward.
The nuclear reactor is protected by a thick metal casing that weighs 100 tons. Inside the casing, there is a specially designed alloy to protect the radioactive fuel rods. Nuclear reactors require no oxygen, which frees the submarine from needing to surface frequently to get a fresh supply of air from above the surface.
Nuclear reactors produce a large amount of power that allows the submarine to run at high speed for long periods, and the current models of nuclear submarines never need to be refueled during their 25-year life span. However, the high cost of a nuclear reactor and the history of serious nuclear accidents involving nuclear submarines means that relatively few countries have nuclear submarines.