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Nuclear Propulsion

A nuclear-powered ship is constructed with the nuclear power plant inside a section of the ship called 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.

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.

The fuel in a reactor contains uranium atoms sealed within metal cladding. Uranium is one of the few materials capable of producing heat in a self-sustaining chain reaction. When a neutron causes a uranium atom to fission, the uranium nucleus is split into parts producing atoms of lower atomic number cded fission products. When formed, the fission products initially move apart at very high speeds, but they do not travel very far, ody a few thousandths of an inch, before they are stopped within the fuel cladding. Most of the heat produced in the fission process comes from stopping these fission products within the fuel and converting their kinetic energy into heat.

Radioactivity is created during fission because some of these fission products are highly radioactive when they are formed. Most of the radioactivity produced by nuclear fuel is in the fission products. The uranium fuel in naval nuclear proptision reactor cores uses highly corrosion-resistant and highly radiation-resistant fuel and cladtig. As a resdt, the fuel is very strong and has very high integrity. The fuel is designed, built, and tested to ensure that the fuel construction will contain and hold the radioactive fission products. Naval fuel totally contains fission products with the fuel - there is no fission product release from the fuel in normal operation.

Fissioning of uranium also produces neutrons while the nuclear power plant is operating. Most of the neutrons produced are absorbed by the atoms within the fuel and continue the chain reaction. However, some of the neutrons travel away from the fuel, go outside the fuel, and are absorbed in the metal structure which supports the fuel or in the walls of the reactor pressure vessel. Trace amounts of corrosion and wear products are carried by reactor coolant from reactor plant metal surfaces. Some of these become radioactive born exposure to neutrons.

Reactor coolant carries some of these radioactive products through the piping systems where a portion of the radioactivity is removed by a purification system. Most of the remaining radionuclides transported from the reactor core deposit in the piping systems. These neutrons, when absorbed in the nucleus of a nonradioactive atom like iron, can produce a radioactive atom. For example, iron-54 contains a total of 54 particles. Adding an additiond neutron produces an atom containing 55 particles, called iron-55. This atom is radioactive. At some later time, it changes into a nonradioactive manganese-55 atom by releasing energy in the form of radiation. This is called radioactive decay.

Due to the need for sallors to live on the ships during operation, reactor compartments are designed to attenuate radiation levels outside of the reactor compartment to extremely low levels. The external surface radiation levels for the normal conditions of transportation of the cruisers and LOS ANGELES Class and 0HI0 Class submarines are expected to be a fraction of the 200 mrem per hour on contact under 49CFR173.




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