The US Navy has a long history of using nuclear powered warships (NPWs) to protect its interests around the world. But are these ships safe? In this article, we'll take a comprehensive look at the safety of US Navy nuclear powered warships and the measures taken to ensure their safety. The US Navy uses pressurized water reactors (PWRs) for its NPWs. PWRs have an established safety record, their operational behavior and risks are understood, and they 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. 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. From these two facts alone, it follows that the amount of radioactivity potentially available for release from the core of a US reactor 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 US NPWs, radioactivity is extremely unlikely to ever be released from the reactor core to the environment. 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 primary coolant. The NPW crew is fully trained and fully capable of responding immediately to any emergency on the ship. Naval operating practices and emergency procedures are well-defined and rigorously applied; and 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 them with with best and earliest monitoring of even smallest changes in plant condition.
Operators become familiar with how their plant sounds, smells and feels. In an extremely unlikely event of an on-board problem involving a US NPW visiting other countries, USA Navy would initiate necessary actions to respond and could call other US forces due to its robust design of its reactor plant, multiple safety systems and fully trained crew. Despite this highly unlikely scenario being very unrealistic, US NPWs must simulate such situations as part of their defense in depth strategy. This strategy ensures that an accident affecting either operation or crew would require numerous unrealistic equipment failures as well as operator failures simultaneously occurring. In conclusion, due to its robust design featuring four barriers between its nuclear core and environment as well as its multiple safety systems backed up by its highly trained crew members living close by its reactors; US Navy nuclear powered warships are extremely safe.
