The scientific foundation for nuclear power was established in December 1938 when chemists Otto Hahn and Fritz Strassmann, along with physicists Lise Meitner and Otto Robert Frisch, discovered nuclear fission in uranium. In December 1942, a team led by Enrico Fermi achieved the first human-made, self-sustaining nuclear chain reaction in the Chicago Pile-1 reactor at the University of Chicago as part of the Allied Manhattan Project. Following the Second World War, several nations sought to adapt reactor technology from plutonium production and naval propulsion to civilian electricity generation.¹⁴

Electricity was generated by a nuclear reactor for the first time on December 20, 1951, at the Experimental Breeder Reactor I (EBR-I) near Arco, Idaho, which produced roughly 100 kilowatts, enough to illuminate four light bulbs and later power the facility itself. Following United States President Dwight D. Eisenhower's "Atoms for Peace" address to the United Nations in December 1953, the U.S. Atomic Energy Act of 1954 permitted the declassification of reactor technology for commercial enterprise. On June 26, 1954, the Soviet Union connected the world's first nuclear power plant to an electrical grid at Obninsk, using the 5-megawatt AM-1 graphite-moderated, water-cooled reactor.¹⁵¹⁶ The United Kingdom opened the first industrial-scale commercial nuclear station at Calder Hall in Sellafield in October 1956, employing a 50-megawatt gas-cooled Magnox design that produced both plutonium and grid electricity.¹⁷ In December 1957, the 60-megawatt Shippingport Atomic Power Station in Pennsylvania became the first commercial peacetime reactor in the United States.¹⁸

Global nuclear capacity expanded rapidly from the late 1960s through the 1980s, accelerated by the 1973 oil crisis, which prompted oil-importing countries such as France and Japan to turn to nuclear energy for national energy security. France enacted the Messmer Plan in 1974, constructing 52 standardized pressurized water reactors between 1975 and 1990. By the mid-1990s, nuclear power supplied a peak of roughly 17 percent of global electricity. However, rising regulatory requirements, escalating capital costs, falling fossil fuel prices, and public opposition following the 1979 Three Mile Island accident in the United States and the 1986 Chernobyl disaster in the Soviet Union sharply curtailed new reactor orders in North America and Western Europe.

In the twenty-first century, rising electricity demand in developing economies and international commitments to reduce carbon dioxide emissions prompted renewed interest in reactor construction, particularly in Asia.¹⁹ The March 2011 Fukushima Daiichi accident in Japan triggered temporary or permanent reactor shutdowns in Japan, Germany, Belgium, and Taiwan, causing global nuclear output to dip in 2011 and 2012.²⁰ Growth resumed over the following decade as China, Russia, India, South Korea, and the United Arab Emirates commissioned new units, pushing total global nuclear generation past its 2006 record in 2024 and again in 2025.

A nuclear power plant is a thermal power station in which a nuclear reactor serves as the heat source in place of a fossil-fuel boiler. Inside the reactor core, heavy fissile nuclei such as uranium-235 absorb free neutrons and split into lighter fission products, releasing kinetic energy as heat alongside two or three additional neutrons. For a steady chain reaction to continue, at least one emitted neutron from each fission event must trigger another fission. In thermal-neutron reactors, which make up the vast majority of the commercial fleet, a substance known as a neutron moderator slows fast neutrons down to thermal energies where they are more likely to split uranium-235 nuclei.²¹²² Control rods made of neutron-absorbing materials such as boron, silver, indium, or cadmium are inserted into or withdrawn from the core to regulate the reaction rate or halt it completely. A fluid coolant circulates through the core to remove the heat, which is used directly or via a heat exchanger to boil water into high-pressure steam that drives a turbine generator.

The nuclear fuel cycle encompasses the industrial stages required to extract fuel, generate electricity in a reactor, and manage the resulting spent material, divided into the "front end" and the "back end".²⁴ The front end begins with mining uranium ore through open-pit, underground, or in-situ leaching methods, predominantly in Kazakhstan, Canada, Namibia, and Australia.²⁵²⁶ At a mill, the ore is crushed and chemically leached to produce a concentrated uranium oxide powder called "yellowcake" ($U_3O_8$). Because most reactors require enriched fuel, yellowcake is chemically converted into uranium hexafluoride ($UF_6$) gas, which is spun in high-speed gas centrifuges to separate the lighter, fissile uranium-235 isotope from the heavier uranium-238 isotope. The enriched $UF_6$ is then converted into uranium dioxide ($UO_2$) powder, pressed and sintered into ceramic pellets, and sealed inside corrosion-resistant zirconium alloy tubes to form fuel assemblies.

During operation, a fuel assembly typically remains in a light-water reactor core for three to five years, with roughly one-third or one-quarter of the core replaced during refueling outages every 12 to 24 months. As fission progresses, uranium-235 is depleted and fission products accumulate, eventually absorbing too many neutrons to sustain an efficient chain reaction. When removed from the reactor, spent nuclear fuel is intensely radioactive and continues to generate significant decay heat.

The back end of the fuel cycle follows one of two strategies: an open (once-through) cycle or a closed cycle. In both pathways, newly discharged fuel assemblies are first transferred to water-filled spent fuel pools at the plant site, where several meters of water provide radiation shielding and active cooling for at least five to ten years. Once decay heat has subsided sufficiently, the assemblies are often moved to interim dry cask storage—heavy steel and concrete cylinders cooled by natural air convection.

In a closed fuel cycle, practiced commercially in countries such as France and Russia, spent fuel is chemically reprocessed to separate the remaining uranium and newly created plutonium—which together still constitute roughly 95 percent of the mass of spent fuel—from the fission products and minor actinides. The recovered plutonium and uranium can be blended into mixed oxide (MOX) fuel for reuse in thermal or fast reactors, while the separated high-level liquid waste is vitrified (immobilized in borosilicate glass) inside stainless steel canisters. In an open fuel cycle, practiced in the United States, Canada, Sweden, and Finland, spent fuel is treated directly as waste without reprocessing.²⁷ For final disposition of high-level waste, international scientific consensus favors deep geological repositories excavated hundreds of meters below ground in stable rock formations.²⁸ Finland's Onkalo spent nuclear fuel repository, constructed 400 to 450 meters deep in granite bedrock near the Olkiluoto Nuclear Power Plant using the Swedish KBS-3 copper-canister and bentonite-clay method, is the world's first licensed deep geological repository for commercial spent fuel, entering its operational phase in the mid-2020s.

Nuclear power plants are engineered around the principle of "defense in depth," employing multiple overlapping physical barriers and redundant safety systems to prevent radioactive materials from escaping into the environment.²⁹ The primary barriers are the ceramic fuel pellet itself, which retains most solid and gaseous fission products; the sealed metal fuel rod cladding; the thick steel reactor pressure vessel and primary coolant piping; and a reinforced concrete and steel containment building designed to withstand internal steam pressure and external impacts.³⁰

Despite these safeguards, three severe accidents at civilian nuclear power plants have shaped regulation and public perception over more than 20,000 cumulative reactor-years of commercial operation. On March 28, 1979, a stuck pressure-relief valve and operator misinterpretation of instruments at Unit 2 of the Three Mile Island Nuclear Generating Station in Pennsylvania caused a partial core meltdown. Because the containment building held, the release of radiation was minimal and caused no detectable health effects, though the cleanup was costly and led to sweeping reforms in operator training and regulatory oversight. On April 26, 1986, during a poorly designed safety test at Unit 4 of the Chernobyl Nuclear Power Plant in Soviet Ukraine, an RBMK reactor lacking a western-style containment structure suffered an uncontrolled power excursion, steam explosions, and an open-air graphite fire. The disaster killed 31 plant workers and first responders from acute trauma and radiation syndrome, forced the permanent evacuation of over 100,000 people, and spread radioactive fallout across Europe, resulting in thousands of subsequent thyroid cancer cases among those exposed as children. On March 11, 2011, the Great East Japan Earthquake and a subsequent 14-meter tsunami flooded backup diesel generators at the Fukushima Daiichi Nuclear Power Plant, causing a station blackout, core meltdowns in three BWR units, and hydrogen explosions that contaminated surrounding land and forced the evacuation of over 150,000 residents, though no deaths resulted directly from radiation exposure.

In routine operation, nuclear power has one of the lowest environmental footprints of any major electricity source. Because reactors do not burn fossil fuels, they emit no sulfur dioxide, nitrogen oxides, particulate matter, or direct carbon dioxide during generation.³¹ Assessments by the Intergovernmental Panel on Climate Change (IPCC) place the median lifecycle greenhouse gas emissions of nuclear power—accounting for uranium mining, enrichment, concrete construction, and decommissioning—at roughly 12 grams of $CO_2$-equivalent per kilowatt-hour, comparable to wind power, lower than solar photovoltaics, and far below natural gas (around 490 g/kWh) and coal (around 820 g/kWh).³² According to the World Nuclear Association, global nuclear generation between 1970 and 2025 avoided an estimated 43.6 billion tonnes of $CO_2$ compared with natural gas or 97.2 billion tonnes compared with coal. Public health analyses comparing total fatalities from workplace accidents, air pollution, and severe accidents consistently rank nuclear energy alongside solar and wind as having the lowest mortality rates per terawatt-hour generated.

The cost structure of nuclear power differs fundamentally from that of fossil-fuel power plants: nuclear stations have very high upfront capital and financing costs, long construction periods, and comparatively low, stable fuel and operating costs. Once constructed, nuclear plants operate with high reliability; in 2025, the global reactor fleet achieved an average capacity factor of 83.7 percent, and the U.S. fleet exceeded 90 percent. Extending the operational licenses of existing reactors from 40 to 60 or 80 years is widely assessed by energy economists and the International Energy Agency as one of the lowest-cost sources of low-carbon electricity available.³³

By contrast, building new reactors in North America and Western Europe during the early twenty-first century has frequently suffered from multi-year schedule delays and severe budget overruns, driven by first-of-a-kind engineering challenges, loss of supply chain expertise, and stringent regulatory changes. Projects such as Olkiluoto 3 in Finland, Flamanville 3 in France, Vogtle Units 3 and 4 in the United States, and Hinkley Point C in the United Kingdom experienced substantial cost escalations, raising the levelized cost of electricity (LCOE) for new western nuclear builds above that of utility-scale solar, wind, and combined-cycle natural gas. Conversely, standardized serial construction programs in China, South Korea, and Russia have completed reactors in five to seven years at significantly lower capital costs per kilowatt.

Public and political debate over nuclear energy centers on these economic trade-offs alongside waste management, accident liability, and nuclear proliferation. Because uranium enrichment and spent fuel reprocessing technologies can be adapted to produce weapons-grade highly enriched uranium or plutonium, civilian nuclear facilities are subject to international inspections and safeguards administered by the IAEA under the 1968 Treaty on the Non-Proliferation of Nuclear Weapons.³⁴ Environmental organizations and political parties remain divided: anti-nuclear movements succeeded in phasing out nuclear power in Italy by 1990, Germany in 2023, and Taiwan in 2025, whereas other governments and climate scientists argue that firm, weather-independent nuclear baseload power is indispensable for decarbonizing electrical grids alongside intermittent renewable energy and powering energy-intensive data centers and heavy industry.

According to the IAEA Power Reactor Information System (PRIS) and the World Nuclear Association's 2026 outlook reports, 417 operable civil nuclear power reactors across 31 countries represent a net installed capacity of approximately 379.6 GWe. In 2025, the global fleet produced a record 2,702 TWh of electricity, surpassing the 2,667 TWh generated in 2024 and accounting for roughly 9 percent of global electricity generation. Approximately 78 reactors with a combined capacity exceeding 81 GWe are under construction across 17 countries.

Five nations account for more than two-thirds of global nuclear generating capacity. The United States operates the largest fleet, with 94 reactors generating 785.6 TWh in 2025—about 19 percent of U.S. electricity. China ranks second in output, producing 449.9 TWh in 2025 from roughly 57 to 60 operating reactors, and leads global expansion with nearly 30 additional reactors under construction.³⁵ France ranks third in total generation (375.7 TWh in 2025) from 57 reactors, which supply roughly 65 percent of its domestic electricity, the highest national share in the world. Russia generated 204.9 TWh in 2025 and remains the leading exporter of reactor technology to international markets through its state corporation Rosatom, followed by South Korea, Canada, Japan, and India.³⁶ Since the 2023 United Nations Climate Change Conference (COP28), 38 countries have endorsed a ministerial declaration pledging to work toward tripling global nuclear energy capacity by 2050. Industry analyses indicate that while national targets and project pipelines could raise global capacity to over 1,400 GWe by 2050 if fully realized, achieving that goal would require annual reactor construction rates several times higher than mid-2020s levels.