Nuclear power

Nuclear power is electricity generated from the heat released by nuclear fission, the splitting of heavy atomic nuclei, typically uranium-235, inside a reactor. That heat boils water into steam, which drives a turbine connected to a generator, the same basic mechanical principle used in a coal or gas plant, except the heat source is a controlled nuclear chain reaction rather than combustion. The world’s first commercial-scale nuclear power station, Calder Hall in the United Kingdom, began supplying electricity to the grid in 1956, following earlier experimental and small-scale reactors in the United States and Soviet Union in the 1950s.

How a nuclear reactor generates electricity

Inside a reactor core, uranium fuel rods undergo controlled fission: a neutron strikes a uranium-235 nucleus, splitting it into smaller atoms, releasing energy and additional neutrons that go on to split further nuclei in a self-sustaining chain reaction. Control rods made of neutron-absorbing material are raised or lowered into the core to speed up, slow down, or halt the reaction, giving operators precise control over power output. In the most common reactor design worldwide, the pressurized water reactor, water circulating through the core under high pressure carries that heat to a separate loop, where it boils a second supply of water into steam that spins the turbine, keeping the radioactive primary coolant physically separate from the steam system. The large cooling towers visible at many nuclear sites are not part of the reactor itself; they simply release waste heat from the plant’s final cooling water into the atmosphere, the same function cooling towers serve at many non-nuclear power plants.

Nuclear power’s share of global electricity

Nuclear power supplied about 9 percent of the world’s electricity in 2024, even as total nuclear generation reached a record of roughly 2,677 terawatt-hours that year, a sign that nuclear output keeps setting records in absolute terms even as faster-growing wind, solar, and natural gas capacity dilutes its percentage share of a steadily expanding global electricity market. As of mid-2025, 408 nuclear reactors were operating across 31 countries, though that capacity is unevenly distributed: France generates roughly two-thirds of its own electricity from nuclear power, among the highest shares of any country, while the United States operates the largest absolute fleet of reactors and China has been building new nuclear capacity faster than almost any other country in the twenty-first century.

Safety record and major accidents

Three accidents dominate public perception of nuclear safety: the 1979 partial meltdown at Three Mile Island in the United States, which caused no confirmed deaths but triggered lasting public anxiety and tighter regulation; the 1986 Chernobyl disaster in the Soviet Union, the only nuclear accident rated at the maximum severity level on the International Nuclear Event Scale, which killed plant workers and firefighters directly and required a permanent exclusion zone still in place today; and the 2011 Fukushima Daiichi accident in Japan, triggered by an earthquake and tsunami rather than a reactor design flaw, which led to a large evacuation and years of decommissioning work still ongoing. Supporters of nuclear power point to statistical comparisons showing that, measured per unit of electricity generated, nuclear power has historically caused far fewer deaths than coal, oil, or even some renewable technologies, largely because routine fossil fuel air pollution causes far more cumulative harm than the rare but highly visible nuclear accidents. Critics counter that these comparisons understate the specific risks of a severe accident, the difficulty of permanently resettling contaminated land, and the long timescales over which nuclear risk and waste must be managed, so the overall safety debate remains genuinely contested rather than settled by the accident statistics alone.

Nuclear waste and long-term challenges

Used nuclear fuel remains highly radioactive for thousands of years, and no country has yet opened a permanent deep geological repository for high-level waste, though Finland’s Onkalo facility, carved into bedrock roughly 400 meters underground, is under construction and expected to become the first such repository to begin operation. In the meantime, most spent fuel is stored on-site at reactors, first in cooling pools and later in sealed dry casks, an approach regulators consider safe for decades but that leaves the question of truly permanent disposal unresolved in most of the world. Some countries, including France, reprocess spent fuel to recover reusable uranium and plutonium, reducing waste volume but raising separate concerns about the proliferation risk of separated plutonium.

Growth outlook and small modular reactors

The International Atomic Energy Agency’s projections to 2050 put global nuclear capacity anywhere from about 561 gigawatts in a low-growth case to 992 gigawatts, roughly two and a half times the 2024 level, in a high-growth case, up from 377 gigawatts of operating capacity at the end of 2024. Much of that projected growth depends on small modular reactors, factory-built units far smaller than a conventional reactor that proponents argue can be constructed faster and more cheaply; the IAEA’s high-growth scenario has small modular reactors accounting for 24 percent of new capacity added, compared with just 5 percent in the low-growth case. Renewed interest in nuclear power in the 2020s has been driven partly by climate policy, since nuclear generates electricity with very low direct carbon emissions, and partly by a surge in electricity demand from data centers and artificial intelligence computing, which some large technology companies have sought to meet in part through dedicated nuclear supply contracts.

FAQs

Is nuclear power considered a renewable energy source? No. Nuclear power relies on mined uranium, a finite resource, which is why it is usually classified separately from renewable energy sources such as solar, wind, and hydropower, even though it shares with them the characteristic of generating electricity with very low direct carbon emissions.

Why do so few countries still operate nuclear power plants compared with other electricity sources? Building a nuclear plant requires an enormous upfront capital investment, a lengthy construction and regulatory approval timeline often spanning a decade or more, and specialized technical expertise and safety infrastructure that many countries have chosen not to develop, making nuclear power far more concentrated among a smaller group of established operators than more easily scaled sources like solar or natural gas.

Editorially reviewed by Specialty Digest Editorial TeamLast reviewed November 1, 2026Researched and drafted with AI assistanceReport an issue