In the United States Navy, Refueling and Overhaul (ROH) refers to a lengthy process or procedure performed on nuclear-powered naval ships, which involves replacement of expended nuclear fuel with new fuel and a general maintenance fix-up, renovation, and often modernization of the entire ship. In theory, such a process could simply involve only refueling or only an overhaul, but nuclear refueling is usually combined with an overhaul. An ROH usually takes one to two years for submarines and up to almost three years for an aircraft carrier, performed at a naval shipyard. Time periods between ROHs on a ship have varied historically from about 5–20 years (for submarines) to up to 25 years (for Nimitz-class aircraft carriers). For modern submarines and aircraft carriers, ROHs are typically carried out about midway through their operating lifespan. There are also shorter maintenance fix-ups called availabilities for ships periodically at shipyards. A particularly lengthy refueling, maintenance, and modernization process for a nuclear aircraft carrier can last up to almost three years and be referred to as a Refueling Complex Overhaul (RCOH).