Refueling and Overhaul
In the United States of America nuclear Navy, Refueling and Overhaul (ROH) refers to a lengthy process or procedure performed on nuclear-powered Navy warships, 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 a year to two years or longer to perform at a Naval shipyard. Time periods between ROH's on a ship have varied historically from about 5-20 years (for submarines) to up to 25 years (for Nimitz-class carriers). For modern submarines and aircraft carriers, ROH's 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).
Discussion
At a shipyard, a ship to undergo ROH goes into a drydock, which is then closed off from the sea. Then water is evacuated from the drydock with blocks placed under the hull, so the ship's hull will rest on the blocks. At the end of the ROH, the drydock is refilled with water so the ship can float and be let out to sea.
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. During an ROH, all personnel in a maintenance work area are commonly required to wear a hard hat.
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 reactor and putting in a new core with fresh nuclear fuel. Because it is so radioactive, removing a core with expended nuclear fuel from a reactor requires elaborate radiological handling precautions. The internal surfaces inside of a reactor plant that has been critical are considered radioactively contaminated. All water that has come into contact with the inside of such a reactor plant is considered radioactive and requires radiological handling and disposal precautions. In addition to radiological training and qualification required for working in radiation areas or with radioactive materials or contamination, radiation exposure to workers is monitored to ensure maximum exposure limits are not exceeded.
Overhaul
The overhaul commonly includes extensive maintenance and renovation work and checks of various systems and equipment aboard the ship. A major overhaul also typically includes upgrading various systems and equipment to modernize them; for example, old analog electrical equipment may be replaced by new digital electronic equipment. The work for such overhauls is typically planned out by engineers well in advance and new equipment is obtained for any replacements or installations.
An example of renovation work done during refueling and overhauls of submarines is the conversion of a fleet ballistic missile submarine (SSBN) to a guided missile submarine (SSGN). Such a conversion consists of taking the ballistic missiles and their silos out of the missile section in the submarine, and replacing them with more than 100 Tomahawk cruise missiles and special operations force insertion platforms which can carry up to 66 special operations personnel. The first four USS Ohio class submarines have undergone such conversions during their midlife refueling and overhauls. [1]
During an overhaul, an extensive testing program is conducted. Numerous test procedures that have been written are followed, data is recorded as required, and logs of the testing are kept. Among tests that can be conducted include: radiography to test critical welds, testing of fluid systems and other pressure boundaries which includes hydrostatic testing to detect any leaks, and testing of electrical and mechanical setpoints for various types of equipment such as sensor input setpoints for various kinds of automatic trips and safety valve relief pressure setpoints. At the finish of the ROH, the testing data records are bound and retained as a permanent documentation record resulting from the ROH.
When the ship is ready to go towards the end of the ROH, the power plant is warmed or brought back up to the desired operating temperature and pressure and can then be started up when ready.
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