Aircraft carriers are incredibly advanced vessels that represent the technological pinnacle of modern naval achievements. The United States Navy has many different types of aircraft carriers, all of which are nuclear-powered. Of course, it wasn't always like that, as previous flat tops were powered via more traditional means. Most carriers spend decades in service, whether they're in the U.S. Navy or another nation's military. This makes them some of the most important warships in any fleet, which has been the case since the mid-20th century.
While the U.S.'s aircraft carriers are all powered via nuclear energy, many nations continue using conventional fossil fuels to power their ships. There are advantages and disadvantages of each system, and they are a key aspect in the overall life of a carrier. Nuclear aircraft carriers require refueling far less often than a ship that uses diesel or another fuel source. The main tradeoff is time, as it can take years to refuel a nuclear aircraft carrier, whereas a diesel-powered flat top receives fuel on an ongoing basis throughout the life of the ship.
The service lifespan of American carriers revolves around their refueling and complex overhaul (RCOH), which is done only once midway through their more than 50-year service. Conventional carriers, on the other hand, undergo aperiodic refits, overhauls, and modernizations as needed by the service that sails them. These often result in an extension of use, making it possible for diesel aircraft carriers to serve for nearly as long as the nuclear-powered variety.