In the United States Navy, refueling and overhaul (ROH) refers to a lengthy refitting 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 process could simply involve only refueling or only an overhaul, but in practice, nuclear refueling is always combined with an overhaul. An ROH usually takes one to two years for submarines and up to 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 carrier s). 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 and complex overhaul (RCOH).
Process
At a shipyard, a ship to undergo ROH goes into a drydock, which is then closed off from the sea. Water is evacuated from the drydock with keel blocks pre positioned under the hull, so the ship's keel area will rest on the blocks as the water is pumped out. At the end of the ROH, the drydock is refilled with water so the ship can be re-floated and removed from the dock. To start ROH, operating procedures are used to shut down and cool down the propulsion power plant to bring it to desired temperatures, pressures, and other conditions. During the ROH, ship's Navy crew stand shutdown watches, and civilian shipyard workers do much of the repair, maintenance, and installation work.
Land-based naval reactor prototype plants have also undergone similar refueling and overhauls, not at a shipyard but at whatever facility they are located.
Refueling
In a nuclear-powered ship, the nuclear fuel is essentially a solid inside a reactor core which is inside the ship's nuclear reactor. Once a reactor core has gone critical, meaning it has been used during a reactor operation, highly radioactive nuclear fission products have formed in the core, and the core has become highly radioactive. Refueling involves taking the expended core out of the reactor and putting in a new core with fresh nuclear fuel. Because it is so radioactive, removing a core with spent nuclear fuel from a reactor requires elaborate radiological handling precautions. All materials that came in contact with the critical core, including the internal surfaces and coolant water, are considered radioactively contaminated
Refueling and Complex Overhaul
Refueling and complex verhaul is a process for refueling and upgrading nuclear-powered aircraft carriers in the US Navy. The nuclear reactors that power some aircraft carriers typically use up their nuclear fuel about halfway through their desired 50-year life spans. Because carriers can last so long before being retired, they are refueled and refurbished with an RCOH to extend their usable lifetime. At the same time a ship is refueled, it is given a complex overhaul in which broken, worn or obsolete parts are repaired or replaced and systems are modernized. The modernization typically includes an upgrade of the ship’s combat systems and warfighting capabilities, its internal distribution systems are upgraded, and allowance is made for future upgrades over the ship’s remaining operational service life. Given the size of an aircraft carrier and the number of systems and subsystems it has, an RCOH is extremely complex, costly (several billion dollars), and time-consuming. Each RCOH is planned to take 46 months.[2][3][4]