The US Navy has a nuclear-powered navy, or nuclear-powered fleet, which consists of naval vessels powered by nuclear marine propulsion. This revolutionary concept was first proposed for naval warfare and has since been adopted by more than 150 ships. The US Navy operates around 100 nuclear vessels, ranging from icebreakers to aircraft carriers. Before nuclear power, submarines were powered by diesel engines and could only be submerged through the use of batteries.
With the introduction of nuclear energy, these submarines became true submersibles and were limited only by the strength of the crew and supplies. Nuclear power is particularly suitable for ships, which need to be at sea for long periods without refueling, or for powerful underwater propulsion. All US Navy nuclear-powered vessels (NPWs) use pressurized water reactors (PWRs). PWRs have an established safety record, their operational behavior and risks are understood, and are the basic design used for approximately 60% of the world's commercial nuclear power plants.
The mission supported by naval reactors is different from the mission of commercial reactors. There are at least four barriers that work to maintain radioactivity inside the ship, even in the highly unlikely event that a problem involving the reactor occurs. These barriers are the fuel itself, the fully welded primary reactor system, including the reactor pressure vessel containing the fuel, the reactor compartment and the ship's hull. Although commercial reactors have similar barriers, barriers in NPWs are much more robust, resilient and conservatively designed than those in civil reactors due to fundamental differences in mission.
The Navy monitors radioactivity levels in reactor cooling water on a daily basis to ensure that any unexpected conditions are detected and resolved promptly. The third barrier is the reactor compartment. This is the specially designed and constructed high-strength compartment inside which the fully welded primary system and nuclear reactor are located. The reactor compartment would delay the release of any liquid or pressure leaks from the primary coolant system in the event of a leak in the primary system.
The fourth barrier is the ship's hull. The hull is a high-integrity structure designed to withstand significant battle damage. The reactor compartments are located within the central and most protected section of the ship. Consequently, reactors normally shut down shortly after mooring and are usually started shortly before departure, since only very low power is required for port propulsion.
NPWs have multiple safety systems to prevent problems from occurring and expanding. The fully welded primary system is designed with a leak-free design criterion that allows reactor operators (NPWs) to quickly determine if there was even a very small primary coolant leak and take immediate corrective action before it could cause additional problems. NPWs have a fail-safe reactor shutdown system, which causes reactor shutdown very quickly, as well as other multi-reactor safety systems and design features, each of which is backed up. Among them is the ability to remove decay heat, which depends only on the physical layout of the reactor plant and the nature of the water itself (natural convection driven by density differences), not on electrical energy, to cool the core. In addition, naval jets have easy access to an unlimited source of seawater that, if necessary, can be brought on board for emergency cooling and protection and would remain on the ship.
NPWs are located in rugged compartments and have multiple ways to add water to cool the reactor. These multiple safety systems ensure that, even in the highly unlikely event of multiple failures, marine reactors do not overheat and the fuel structure is not damaged by heat produced in the reactor core. Therefore, virtually incredible accident conditions, in which these safety systems and their backrests fail, would be required to cause a release of fission products from the reactor core to the environment. Due to these four barriers in place in US Navy NPWs, radioactivity is extremely unlikely to ever be released from the reactor core to the environment. Defense in Depth ensures that any potential problems are detected quickly and resolved promptly.
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