Pipes carry water from the steam generator to the reactor, where the water is heated to about 600 degrees. This superheated water is under pressure, and so it does not boil. Instead, it returns to the steam generator and boils a supply of unpressurized water that turns to steam. Nuclear engines operate without air and consume much less fuel than do other engines. About 4 pounds of uranium fuel produces more energy than 10 million gallons of fuel oil.
Some submarines run on diesel engines.
However, the U.S. Navy has not built a diesel-powered submarine since 1959. Diesel engines burn fuel oil and need air for combustion. A submarine can use these engines only when on or near the surface. Electric batteries supply power underwater. The snorkel drew air into the submarine when the craft was near the surface.