A nuclear-powered ship is constructed with the nuclear power plant inside a section of the ship cded the reactor compartment. The components of the nuclear power plant include a high-strength steel reactor vessel, heat exchanger(s) (steam generator), and associated piping, pumps, and valves. Each reactor plant contains over 100 tons of lead shielding, part of which is made radioactive by contact with radioactive material or by neutron activation of impurities in the lead.
The propulsion plant of a nuclear-powered ship or submarine uses a nuclear reactor to generate heat. The heat comes from the fissioning of nuclear fuel contained within the reactor. Since the fisioning process also produces radiation, shields are placed around the reactor so that the crew is protected.
The nuclear propulsion plant uses a pressurized water reactor design which has two basic systems - a primary system and a secondary system. The primary system circulates ordinary water and consists of the reactor, piping loops, pumps and steam generators. The heat produced in the reactor is transferred to the water under high pressure so it does not boil. This water is pumped through the steam generators and back into the reactor for re-heating.
In the steam generators, the heat from the water in the primary system is transferred to the secondary system to create steam. The secondary system is isolated from the primary system so that the water in the two systems does not intermix.
In the secondary system, the steam flows from the steam generators to drive the turbine generators, which supply the ship with electricity, and to the main propulsion turbines, which drive the propeller. After passing through the turbines, the steam is condensed into water which is fed back to the steam generators by the feed pumps. Thus, both the primary and secondary systems are closed systems where water is recirculated and renewed.
Since there is no step in the generation of this power which requires the presence of air or oxygen, this allows the ship to operate completely independent from the earth’s atmosphere for extended periods of time.
Naval reactors undergo repeated power changes for ship maneuvering, unlike civilian counterparts which operate at steady state. Nuclear safety, radiation, shock, quieting, and operating performance requirements in addition to operation in close proximity to the crew dictate exceptionally high standards for component manufacturing and quality assurance. The internals of a Naval reactor remain inaccessible for inspection or replacement throughout a long core life -- unlike a typical commercial nuclear reactor, which is opened for refueling roughly every eighteen months.
Unlike commercial nuclear power plants, Naval reactors must be rugged and resilient enough to withstand decades of rigorous operations at sea, subject to a ship's pitching and rolling and rapidly-changing demands for power, possibly under battle conditions. These conditions -- combined with the harsh environment within a reactor plant, which subjects components and materials to the long-term effects of irradiation, corrosion, high temperature and pressure -- necessitate an active, thorough and far-sighted technology effort to verify reactor operation and enhance the reliability of operating plants, as well as to ensure Naval nuclear propulsion technology provides the best options for future needs.
With the demise of the commercial nuclear industry in the 1970's, Naval nuclear suppliers have had virtually no other work to help absorb overhead and sustain a solid business base from which to compete for Naval nuclear work. The result has been reduced competition and higher costs. Requirements for naval nuclear propulsion plant components are far more stringent than needed for civilian products. Costly quality control and work production procedures to meet nuclear requirements generally prevent these firms from competing successfully with firms geared for less sophisticated civilian work. There is no civilian demand for quiet, compact, shock-resistent nuclear propulsion systems which would keep skilled designers and production workers current. This is a distinct difference from the aerospace, electronics, and ground vehicle industries from which DOD buys many of its weapon systems.
The Naval Reactors' program has shown the world that nuclear power can be handled safely, with no adverse effects on the public or the environment. While others have stumbled with this challenging technology, the Naval Reactors' program stands out-in the private sector as well as in the public sector-for vision, discipline, and technical excellence.
The nuclear propulsion plants in United States Navy ships, while differing in size and component arrangements, are all rugged, compact, pressurized water reactors designed, constructed, and operated to exacting criteria. The nuclear components of these plants are all housed in a section of the ship called the reactor compartment. The reactor compartments all serve the same purpose but may have different shapes depending on the type of ship. For submarines, the reactor compartment is a horizontal cyhder formed by a section of the ship’s pressure hull, with shielded bulkheads on each end. Cruiser reactor compartments are shielded vertical cylinders or shielded rectangular boxes deep within the ship’s structure.
The propulsion plants of nuclear-powered ships remain a source of radiation even after the vessels are shut down and the nuclear fuel is removed. Defueling removes all fission products since the fuel is designed, built and tested to ensure that fuel will contain the fission products. Over 99.9% of the radioactive material that remains is an integral part of the structural alloys forming the plant components. The radioactivity was created by neutron irradiation of the iron and alloying elements in the metal components during operation of the plant. The remaining 0.1% is radioactive corrosion and wear products that have been circtiated by reactor coolant, having become radioactive from exposure to neutrons in the reactor core, and then deposited on piping system internals.
Modern submarines use either diesel-electric or nuclear power to drive the sub's propeller and to provide internal electric power. Diesel-electric power emerged as the most efficient propulsion system for submarines in the early 20th century, following unsuccessful attempts to use steam or gasoline power. While on the surface, the submarine uses a diesel engine to drive the propeller and generate electricity. When submerged, a battery-driven electrical motor takes over for propulsion and power. The diesel engine recharges the batteries. Using the diesel engine requires air, however, and so a submarine has to surface and thus expose itself to attack. German engineers solved this problem in the design of their World War II submarines, called U-boats. They did so by developing the snorkel, a retractable tube that can be extended above the surface of the water while the submarine operates at periscope depth—about 18 m (60 ft) below the surface. The snorkel provided the air to burn fuel in the diesel engine and vented off the exhaust fumes produced by the engine. Using the snorkel gave a margin of safety for submarines as they recharged their batteries, and allowed submarines to extend their underwater range considerably.
The successful design in the early 1950s of a nuclear reactor small enough to fit inside a submarine hull was the most significant advance in submarine technology since the advent of diesel-electric propulsion a half-century earlier. A U.S. Navy team led by then-Captain Hyman G. Rickover engineered the breakthrough. Their success followed years of scientific speculation that a controlled nuclear fission reaction might be harnessed to power submarines. The theory is as simple as the reactor designs are complex: A controlled nuclear reaction, which takes place within a pressure vessel, produces enormous heat energy. This heat is channeled through a piping system that, in turn, heats water in a second, separate circuit of pipes. The heated water turns to steam, which passes through a turbine to power the submarine's propulsion drive. The steam also provides internal electric power via a turbine-driven generator. Because the nuclear reactor does not need access to fresh air, a modern nuclear submarine can cruise submerged for an unlimited amount of time. During long stretches underwater, the vessel replenishes its supply of breathing oxygen through hydrolysis, a chemical process that extracts oxygen from seawater.