The long-running policy debate over the future of nuclear energy is rooted in the technology's inherent characteristics. Initially developed for its unprecedented destructive power during World War II, nuclear energy seemed to hold equal promise after the war as a way of providing limitless energy to all humanity. International diplomacy has focused ever since on finding institutional mechanisms for spreading the perceived benefits of nuclear energy throughout the world while preventing the technology from being used for the proliferation of nuclear weapons. Much of this international effort is focused on key nuclear fuel cycle facilities—plants for enriching uranium in the fissile isotope U-235 and for separating plutonium from irradiated nuclear fuel. Such plants can be used to produce civilian nuclear reactor fuel as well as fissile material for nuclear warheads. Yet even the use of nuclear power solely for peaceful energy production has proven intrinsically controversial. The harnessing of nuclear fission in a reactor creates highly radioactive materials that must be kept from overheating and escaping from the reactor building, as occurred during the accidents at Fukushima, Chernobyl, and, to a lesser extent, Three Mile Island. Spent nuclear fuel that is regularly removed from reactors during refueling must be isolated from the environment for up to 1 million years. Potential technologies to reduce long-lived nuclear waste through recycling usually involve separating plutonium that could be used for nuclear weapons, although technologies designed to reduce proliferation risks are also the subject of worldwide research and development efforts. All nuclear energy technologies, even with recycling, would still leave substantial amounts of radioactive waste to be stored and disposed of. Central storage and disposal sites for nuclear waste have proven difficult to develop throughout the world, as illustrated by long-running controversy over the proposed U.S. waste repository at Yucca Mountain, NV. The March 2011 disaster at Japan's Fukushima Dai-ichi nuclear power plant, which forced the evacuation of areas as far as 30 miles away, has slowed nuclear power expansion plans around the world, particularly in Japan and Western Europe. However, dozens of new reactors are still being planned and built in China, India, Russia, and elsewhere. In these areas, nuclear power's initial promise of generating large amounts of electricity without the need for often-imported fossil fuels, along with the more recent desire to reduce greenhouse gas emissions, remains a compelling motivation. With 98 licensed reactors, the United States has the largest nuclear power industry in the world. But U.S. nuclear power growth has been largely stagnant for the past two decades, as natural gas and renewable energy have captured most of the market for new electric generating capacity. Congress enacted incentives for new nuclear plants in the Energy Policy Act of 2005 ( P.L. 109-58 ), including production tax credits, loan guarantees, and insurance against regulatory delays. Those incentives, combined with rising natural gas prices and concerns about federal restrictions on carbon dioxide emissions, prompted announcements by late 2009 of up to 30 new nuclear power reactors in the United States. However, subsequent declines in natural gas prices and uncertainty about carbon dioxide controls have put most of those projects on hold. Currently, two new reactors in Georgia are under construction. Two identical reactors under construction in South Carolina were cancelled July 31, 2017. An older reactor, Watts Bar 2 in Tennessee, received an NRC operating license on October 22, 2015, after construction had been suspended for two decades. A variety of incentives to renew the growth of nuclear power have been proposed, including a plan by the Trump Administration to provide additional revenue to nuclear and coal power plants in wholesale electricity markets. Existing U.S. nuclear power plants are facing difficult competition from natural gas and renewable energy. Seven U.S. reactors were permanently closed from 2013 through 2018. Three of those units closed because of the need for expensive repairs, and three were operating well but could not compete in their local wholesale electricity markets. The most recent retirement was New Jersey's Oyster Creek plant, which permanently shut down September 17, 2018, to avoid having to construct a cooling tower. All seven units had substantial time remaining on their initial 40-year operating licenses or had received or planned to apply for 20-year license extensions from the Nuclear Regulatory Commission (NRC). The owners of 12 additional reactors have announced that they will permanently shut down by the mid-2020s ( Table 1 ). The actual and planned shutdowns have prompted widespread discussion about the future of other aging U.S. reactors. The extent to which the growth of nuclear power should be encouraged in the United States and around the world will continue to be a major component of the U.S. energy policy debate. Questions for Congress will include the implementation of policies to encourage or discourage nuclear power, post-Fukushima safety standards, development of new nuclear power and fuel cycle technologies, and nuclear waste management strategies. The 98 licensed nuclear power reactors at 59 sites in the United States generate about 20% of the nation's electricity. The oldest of today's operating reactors were licensed in 1969, and the most recent was Watts Bar 2 in 2015. The most recently licensed reactor before that was its twin unit, Watts Bar 1, in 1996. All U.S. reactors were initially licensed to operate for 40 years, but nearly all of them have received or applied for 20-year license renewals by NRC. Under the current mixture of 40- and 60-year licenses, 28 of today's operating reactors would have to shut down by 2030 and the rest by 2049, except for the newly licensed Watts Bar 2. The owners of four reactors have applied to NRC for "subsequent license renewals," which would allow operation for up to 80 years. Another four subsequent license renewal applications have been announced. As noted above, 12 reactors are currently scheduled to retire before their operating licenses or renewals expire. Whether new reactors will be constructed to replace the existing fleet or even to expand nuclear power's market share will depend largely on costs. The cost of building and operating a new nuclear power plant in the United States is generally estimated to be significantly higher than natural gas combined-cycle plants (which use both combustion and steam turbines to generate electricity) and above wind and solar as well. For example, the Energy Information Administration (EIA) estimates that, for plants coming on line in 2022, the average cost of electricity generation from a nuclear power plant would be 9.3 cents per kilowatt-hour (kwh), while advanced combined-cycle gas-fired generation would cost 4.9 cents/kwh and a coal plant with 90% carbon capture and storage would cost 11.9 cents/kwh. EIA estimates that, including tax credits, electricity from onshore wind would cost 4.8 cents/kwh, solar photovoltaics 5.0 cents/kwh, and geothermal 4.2 cents/kwh. Such estimates depend on a wide range of variables, such as future fuel costs, regional solar and wind availability, current and future tax incentives, and environmental regulations. The two new U.S. reactors under construction at the Vogtle nuclear plant site in Georgia, after considerable construction delays and cost overruns, are now scheduled to begin operating in November 2021 and November 2022. As noted above, construction of two new units in South Carolina has been terminated. Licenses to build and operate 10 additional reactors have been issued by NRC. However, applications for 14 other new reactors have been withdrawn or suspended. Aside from the 2 new Vogtle units, the 10 other planned reactors with issued licenses do not have specific schedules for moving toward construction. Throughout the world, 451 reactors are currently in service or operable, and 54 more are under construction. France is the most heavily nuclear-reliant country in the world, with 58 reactors generating 72% of the country's electricity in 2017. Thirty-one countries in 2017 (plus Taiwan) generated at least some of their electricity from nuclear power. After the Fukushima accident, Germany, which had previously generated about 30% of its electricity with nuclear power, closed 8 of the country's 17 power reactors and decided to shut the remainder by 2022. Japan, which had also generated about 30% of its electricity with nuclear power and had planned to raise that level to 50%, now is planning for about 20% by 2030. All Japanese reactors were closed within a year after the tsunami, and only 9 of Japan's 42 operable reactors are currently in commercial service. An additional 17 Japanese reactors have applied for restart, which involves safety upgrades to meet new regulatory requirements. It is not clear how many of Japan's operable reactors will ultimately seek restart approval. France had planned to reduce nuclear power to 50% of the country's total generation by 2025, although that goal has been delayed. Highly radioactive spent nuclear fuel must regularly be removed from operating reactors and stored in adjacent pools of water. After several years of cooling, the spent fuel can be placed in dry casks for storage elsewhere on the plant site. When existing U.S. reactors were built, spent fuel had been expected to be taken away for reprocessing (separation of plutonium and uranium to make new fuel) or permanent disposal. However, reprocessing has not become commercialized in the United States, for economic and nonproliferation reasons, and central waste storage and disposal facilities have proven difficult to site. As a result, the vast majority of U.S. commercial spent fuel remains at the nuclear plants where it was generated—estimated at 79,389 metric tons at the end of 2017 and increasing at the rate of about 2,000 metric tons per year. The Nuclear Waste Policy Act ( P.L. 97-425 , NWPA), as amended in 1987, named Yucca Mountain, NV, as the nation's sole candidate site for a permanent high-level nuclear waste repository. NWPA required the Department of Energy (DOE) to study the site and seek a license from NRC to build a repository there.