Shipboard reactors are constructed smaller than similar installations ashore, with special attention given to maintenance, protection from collision, and leakage. Of this total, 22 submarines have been decommissioned or are non-operational.
The United States has built and launched an impressive number of nuclear powered shipsa total of 155 attack and missile submarines, 9 guided missile cruisers (18 reactors), and 5 aircraft carriers (with a total of 24 reactors) and has operated 9 prototype reactors on land. Nuclear fission reactors, usually pressurized water reactors with energy conversion based on a steam-turbine cycle, have been used extensively to power ships. It uses passive natural circulation for the removal of heat from the core. It runs at lower temperatures and with larger water inventories than current light water reactors, and has a passive emergency core-cooling system that utilizes gravity. The AP-600 (Advanced Passive 600 MW e), a pressurized water reactor. The APLWR is the most developed of these new reactor types. Examples of these new designs are two types of Advanced Passive Light Water Reactors (APLWRs), the Liquid Metal Reactor (LMR), and the Modular High Temperature Gas-cooled Reactor (MHTGR). 600 MW) and may be constructed in factories where uniformity and quality control can produce reactors and operating procedures less prone to failure. The new types of reactors are smaller (e.g. Typically, 1000 MW reactors have been constructed on site.
Many current problems arise from the huge scale of reactor construction projects. These features fall into two broad categoriesfeatures that are designed to prevent accidents, and those that mitigate the effects of accidents. Passive safety features can be thought of as characteristics of a reactor that, without operator intervention, will tend to shut or cool a reactor down, keep it in a safe configuration, and prevent release of radioactivity. New reactor designs use passive safety to address this problem. In an accident, this external supply could possibly be disrupted leading to a release of radioactivity. Most worrisome is the need for external electrical power to supply the pumps used in emergency core cooling systems in the types of reactors now used in the United States.
The public has become suspicious of nuclear energy partly because of the elaborate methods used to address safety concerns. Due to such concerns and the complexities of construction and operation, it is unlikely that breeder reactors will ever come into general operation within the next several decades, if ever. Research on breeder reactors has essentially stopped in the United States because of concerns over nuclear proliferation since the plutonium bred in the reactor might be used for making weapons. Liquid sodium metal may be used here as a coolant and heat-transfer fluid. The smaller fission cross sections associated with the fast neutrons (as compared with thermal neutrons) leads to higher fuel concentrations in the core and higher power densities, which, in turn, create significant heat transfer problems. In a fast-breeder reactor, water cannot be used as a coolant because it would moderate the neutrons. A breeder reactor needs to be operated with fast neutrons, a so-called "fast breeder" reactor. It is actually possible to generate more 239Pu than is used up in the reactor by surrounding the core with a uranium blanket and generating 239Pu in this blanket.
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