Nuclear power is a viable option for ships that need to remain at sea for long periods without refueling, or for powerful underwater propulsion. Today, more than 150 ships are powered by small nuclear reactors. The U. S.
Navy operates around 100 nuclear vessels, ranging from icebreakers to aircraft carriers. Egypt has recently begun local manufacture of MEKO A200 frigates, and the Indian Navy has ordered its first Project 15B destroyer, INS Visakhapatnam. Pressurized water reactors (PWRs) are the most common type of nuclear reactor used in naval propulsion systems (NPWs). PWRs have an established safety record, and their operational behavior and risks are well-understood.
The mission supported by naval reactors is different from the mission of commercial reactors, and the barriers that work to maintain radioactivity inside the ship are much more robust 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 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. While in port, electrical power for service needs comes from shore power sources.
This has been and will continue to be the case for NPWs in other ports where sufficient onshore power is available.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.The amount of radioactivity potentially available for release from the core of a U. NPW moored in a port is less than about one percent of that of a typical commercial reactor.
A large fraction of the fission products that occur during reactor operation, and which are of concern to human health, decompose soon after the reactor shuts down.Defense in Depth Due to Four Barriers in Place in U. NPW, radioactivity is extremely unlikely to ever be released from the reactor core to the environment. These four 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.The crew of an NPW is fully trained and capable of responding immediately to any emergency on board. Naval operating practices and emergency procedures are well-defined and rigorously applied; people are trained to cope with extraordinary situations and are subject to high standards of accountability.
In addition, the fact that the crew lives so close to the reactor provides the best and earliest monitoring of even the smallest change in plant condition.NPWs have multiple ways to add water to cool the reactor if necessary, such as natural convection driven by density differences or access to an unlimited source of seawater that can be brought on board for emergency cooling and protection. 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.In conclusion, nuclear powered ships are a reality today with more than 150 ships powered by small nuclear reactors around the world. Navy operates around 100 nuclear vessels with multiple safety systems in place to ensure that radioactivity is extremely unlikely to ever be released from the reactor core into the environment.
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