Refueling and Overhaul
In the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

A navy is the branch of a nation's armed forces principally designated for naval and amphibious warfare; namely, lake- or ocean-borne combat operations and related functions...

, Refueling and Overhaul (ROH) refers to a lengthy process or procedure performed on nuclear-powered Naval ship
Since the end of the age of sail a ship has been any large buoyant marine vessel. Ships are generally distinguished from boats based on size and cargo or passenger capacity. Ships are used on lakes, seas, and rivers for a variety of activities, such as the transport of people or goods, fishing,...

s, which involves replacement of expended nuclear fuel
Nuclear fuel
Nuclear fuel is a material that can be 'consumed' by fission or fusion to derive nuclear energy. Nuclear fuels are the most dense sources of energy available...

 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
Shipyards and dockyards are places which repair and build ships. These can be yachts, military vessels, cruise liners or other cargo or passenger ships. Dockyards are sometimes more associated with maintenance and basing activities than shipyards, which are sometimes associated more with initial...

. Time periods between ROHs on a ship have varied historically from about 5-20 years (for submarine
A submarine is a watercraft capable of independent operation below the surface of the water. It differs from a submersible, which has more limited underwater capability...

s) to up to 25 years (for 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
Aircraft carrier
An aircraft carrier is a warship designed with a primary mission of deploying and recovering aircraft, acting as a seagoing airbase. Aircraft carriers thus allow a naval force to project air power worldwide without having to depend on local bases for staging aircraft operations...

 can last up to almost three years and be referred to as a Refueling Complex Overhaul (RCOH).


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
Hull (watercraft)
A hull is the watertight body of a ship or boat. Above the hull is the superstructure and/or deckhouse, where present. The line where the hull meets the water surface is called the waterline.The structure of the hull varies depending on the vessel type...

, 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
Hard hat
A hard hat is a type of helmet predominantly used in workplace environments, such as construction sites, to protect the head from injury by falling objects, impact with other objects, debris, bad weather and electric shock. Inside the helmet is a suspension that spreads the helmet's weight over the...


Land-based Naval reactor prototype plants have also undergone similar refueling and overhauls, not at a shipyard but at whatever facility they are located.


In a nuclear-powered ship, the nuclear fuel is essentially a solid inside a reactor core which is inside the ship's nuclear reactor
Nuclear reactor
A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...

. 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. The internal surfaces inside a reactor plant that has been critical are considered radioactively contaminated
Radioactive contamination
Radioactive contamination, also called radiological contamination, is radioactive substances on surfaces, or within solids, liquids or gases , where their presence is unintended or undesirable, or the process giving rise to their presence in such places...

. 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
Radiation exposure
The term radiation exposure commonly has several uses:* Absorption of high-energy ionizing radiation by an object. In living beings a high absorbed dose can lead to radiation poisoning.* Absorption by an object of non-ionizing radiation...

 to workers is monitored to ensure maximum exposure limits are not exceeded.


The overhaul commonly includes extensive maintenance and renovation work and checks of various systems and equipment aboard the ship
Since the end of the age of sail a ship has been any large buoyant marine vessel. Ships are generally distinguished from boats based on size and cargo or passenger capacity. Ships are used on lakes, seas, and rivers for a variety of activities, such as the transport of people or goods, fishing,...

. 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 engineer
An engineer is a professional practitioner of engineering, concerned with applying scientific knowledge, mathematics and ingenuity to develop solutions for technical problems. Engineers design materials, structures, machines and systems while considering the limitations imposed by practicality,...

s 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 missile
Ballistic missile
A ballistic missile is a missile that follows a sub-orbital ballistic flightpath with the objective of delivering one or more warheads to a predetermined target. The missile is only guided during the relatively brief initial powered phase of flight and its course is subsequently governed by the...

s 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 s have undergone such conversions during their midlife refueling and overhauls.
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
Industrial radiography
Industrial Radiography is the use of ionizing radiation to view objects in a way that cannot be seen otherwise. It is not to be confused with the use of ionizing radiation to change or modify objects; radiography's purpose is strictly viewing. Industrial radiography has grown out of engineering,...

 to test critical welds
Welding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material that cools to become a strong joint, with pressure sometimes...

, testing of fluid systems and other pressure boundaries which includes hydrostatic test
Hydrostatic test
A hydrostatic test is a way in which leaks can be found in pressure vessels such as pipelines and plumbing. The test involves placing water, which is often dyed for visibility, in the pipe or vessel at the required pressure to ensure that it will not leak or be damaged. It is the most common method...

ing to detect any leak
A leak is a hole or other opening, usually unintended and therefore undesired, in a container or fluid-containing system, such as a tank or a ship's hull, through which the contents of the container can escape or outside matter can enter the container...

s, 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
Safety valve
A safety valve is a valve mechanism for the automatic release of a substance from a boiler, pressure vessel, or other system when the pressure or temperature exceeds preset limits....

 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
Operating temperature
An operating temperature is the temperature at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the...

 and pressure and can then be started up when ready.

Refueling and Complex Overhaul

Refueling and Complex Overhaul (RCOH) is a process for refueling and upgrading nuclear-powered aircraft carrier
Nimitz class aircraft carrier
The Nimitz-class supercarriers are a class of ten nuclear-powered aircraft carriers in service with the United States Navy. With an overall length of and full-load displacements of over 100,000 long tons, they are the largest capital ships in the world...

s in the US Navy
United States Navy
The United States Navy is the naval warfare service branch of the United States Armed Forces and one of the seven uniformed services of the United States. The U.S. Navy is the largest in the world; its battle fleet tonnage is greater than that of the next 13 largest navies combined. The U.S...

. The nuclear reactors that power some aircraft carriers typically use up their nuclear fuel
Nuclear fuel
Nuclear fuel is a material that can be 'consumed' by fission or fusion to derive nuclear energy. Nuclear fuels are the most dense sources of energy available...

 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 or worn parts are repaired or replaced and systems are modernized. The modernization typically includes an upgrade of 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 almost three years.

External links

  • CVN-68 Nimitz-class Modernization -, launched in 2000, is a public policy organization focusing on the fields of defense, space exploration, intelligence, weapons of mass destruction and homeland security...

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