93%) They have long core lives, so that refueling is needed only after 10 or more years, and new cores are designed to last 25 years in carriers and 10–33 years in submarines, The design enables a compact pressure vessel while maintaining safety. Long core life is enabled by high uranium enrichment and by incorporating a "burnable neutron poison", which is progressively depleted as non-burnable poisons like fission products and actinides accumulate. The loss of burnable poison counterbalances the creation of non-burnable poisons and result in stable long term fuel efficiency. Long-term integrity of the compact reactor pressure vessel is maintained by providing an internal neutron shield. (This is in contrast to early Soviet civil PWR designs where embrittlement occurs due to neutron bombardment of a very narrow pressure vessel.) Reactor sizes range up to ~500 MWt (about 165 MWe) in the larger submarines and surface ships. The French s have a 48 MW reactor that needs no refueling for 30 years. The nuclear navies of the United States, the United Kingdom, and the Russian Federation rely on steam turbine propulsion. Those of the French and Chinese use the turbine to generate electricity for propulsion. Most Russian submarines as well as all U.S. surface ships since Enterprise are powered by two or more reactors. U.S., British, French, Chinese and Indian submarines are powered by one. Decommissioning nuclear-powered submarines has become a major task for American and Russian navies. After defuelling, U.S. practice is to cut the reactor section from the vessel for disposal in shallow land burial as low-level waste (see the Ship-Submarine recycling program). See also List of United States Naval reactors Radioisotope thermoelectric generator Nuclear-powered cruisers of the United States Navy Nuclear-powered submarines of the United States Navy Nuclear Power School References External links The Uranium Information Centre provided some of the original material in this article. Nuclear reactors