Nuclear-powered ships are a powerful and efficient way to travel the seas, but they require regular refueling and maintenance to keep them running. Refueling and reconditioning (ROH) is a lengthy process that involves replacing spent nuclear fuel with new fuel and general maintenance repair, refurbishment, and often modernization of the entire ship. In the United States Navy, this process usually takes one to two years for submarines and up to almost three years for an aircraft carrier. The nuclear reactors that power some aircraft carriers typically consume their nuclear fuel at about half of their desired 50-year lifespan.
Refueling a nuclear-powered ship is a complex process that requires elaborate radiological handling precautions. The power plant must be heated up or restarted to the desired operating temperature and pressure so that it can start up when it is ready. Once a reactor core has become critical, meaning that it has been used during the operation of a reactor, highly radioactive nuclear fission products have formed in the core, and the core has become highly radioactive. Complex Replenishment and Overhaul (RCOH) is a process for refueling and upgrading nuclear-powered aircraft carriers in the United States.
The reactors aboard these ships contain enough nuclear fuel to fully power the ship and all its needs on board for approximately 20 years. Refueling an aircraft carrier is a very costly task because it is powered by two A4W nuclear reactors. These two reactors drive four propellers that produce a maximum speed of 30 knots. In theory, refueling or just a review could be done separately, but in practice, nuclear refueling is always combined with a review.
Naval onshore reactor prototype plants have also undergone similar refueling and overhauls, not at a shipyard, but at whatever facility they are in. Time periods between RoHS on a ship have historically varied from approximately 5 to 20 years (for submarines) to 25 years (for Nimitz class aircraft carriers). For modern submarines and aircraft carriers, RoHs are generally carried out about half their useful life. There are also shorter maintenance arrangements called availability for ships periodically in shipyards.