Refueling submarines can take more than two years due to the limited space and expensive maintenance of nuclear power plants on board. In the United States Navy, refueling and reconditioning (ROH) is a lengthy process that involves replacing spent nuclear fuel with new fuel and general maintenance repair, refurbishment, and modernization of the entire ship. An ROH usually takes one to two years for submarines and up to almost three years for an aircraft carrier. For modern submarines and aircraft carriers, RoHs are generally carried out about half their useful life.
There are also shorter maintenance arrangements called availabilities for ships periodically in the shipyards. A particularly lengthy refueling, maintenance and modernization process for a nuclear aircraft carrier is known as a refueling complex (RCOH) overhaul. This was the first RCOH for the Nimitz class and only the fourth achieved on a nuclear aircraft carrier. At the end of the Cold War, in 1989, there were more than 400 nuclear-powered submarines in operation or under construction.
Removing a nucleus with spent nuclear fuel from a reactor requires elaborate radiological handling precautions. The dismantling of nuclear-powered submarines has become an important task for both the United States and Russia. Discharges of rare earth elements formed as fission products by thermal neutrons during nuclear power production are not as well studied. Other sources of 137C in the marine environment include the consequences of the Chernobyl accident and routine operations of nuclear power plants.
The Department of Energy discards some types of contaminated reactor parts from nuclear ships at Hanford facility in Washington State. Although most nuclear-powered vessels are submarines, they range in type from icebreakers to aircraft carriers. Nuclear propulsion has proven to be technically and economically feasible in the Soviet Arctic. The nuclear reactors that power some aircraft carriers typically consume their nuclear fuel at about half of their desired 50-year lifespan.
An example of the renovation work done during submarine refueling and overhauls is the conversion of a fleet ballistic missile (SSBN) submarine to a guided missile submarine (SSGN). The power and energy levels needed to break the ice, along with the difficulties of refueling other types of vessels, are important factors. For use in shallow waters, such as estuaries and rivers, Taymyry-class shallow draft icebreakers with a reactor are being built in Finland and equipped with its nuclear steam supply system in Russia. With the increasing attention paid to greenhouse gas emissions from the burning of fossil fuels for international air and sea transport and the excellent safety record of nuclear-powered vessels, it is quite conceivable that renewed attention will be paid to marine nuclear propulsion.