Background
The Naval Nuclear Propulsion Program (NNPP) started in 1948. Since that time, the NNPP has provided safe and effective propulsion systems to power submarines, surface combatants, and aircraft carriers. Today, nuclear propulsion enables virtually undetectable US Navy submarines, including the sea-based leg of the strategic triad, and provides essentially inexhaustible propulsion power independent of forward logistical support to both our submarines and aircraft carriers. Over forty percent of the Navy"s major combatant ships are nuclear-powered, and because of their demonstrated safety and reliability, these ships have access to seaports throughout the world. The NNPP has consistently sought the best way to affordably meet Navy requirements by evaluating, developing, and delivering a variety of reactor types, fuel systems, and structural materials. The Program has investigated many different fuel systems and reactor design features, and has designed, built, and operated over thirty different reactor designs in over twenty plant types to employ the most promising of these developments in practical applications. Improvements in naval reactor design have allowed increased power and energy to keep pace with the operational requirements of the modern nuclear fleet, while maintaining a conservative design approach that ensures reliability and safety to the crew, the public, and the environment. As just one example of the progress that has been made, the earliest reactor core designs in the NAUTILUS required refueling after about two years while modern reactor cores can last the life of a submarine, or over thirty years without refueling. These improvements have been the result of prudent, conservative engineering, backed by analysis, testing, and prototyping. The NNPP was also a pioneer in developing basic technologies and transferring technology to the civilian nuclear electric power industry. For example, the Program demonstrated the feasibility of commercial nuclear power generation in this country by designing, constructing and operating the Shipping port Atomic Power Station in Pennsylvania and showing the feasibility of a thorium-based breeder reactor.
In: Report on Low Enriched Uranium for Naval Reactor Cores. Page 1. Report to Congress, January 2014. Office of Naval Reactors. US Dept. of Energy. DC 2058 http://fissilematerials.org/library/doe14.pdf
According to the text, choose the alternative that presents how long can modern reactor cores stay without refueling.
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