Nuclear power is the production of electricity using energy released by nuclear reactions. Commercial nuclear power stations use controlled nuclear fission to generate heat, usually producing steam that drives a turbine and generator. They differ from fossil-fuel power stations primarily in their heat source rather than their basic electricity-generating machinery. Nuclear fusion, which combines light nuclei, remains a separate field of energy research rather than an established commercial electricity source. (eia.gov)
Physical principles and electricity generation
Fission occurs when a heavy atomic nucleus, such as uranium-235, splits into smaller nuclei after absorbing a neutron. The process releases heat, radiation, and additional neutrons. Some of these neutrons induce further fissions, sustaining a chain reaction. A nuclear reactor maintains this reaction at a controlled rate; neutron-absorbing control rods help regulate reactor power or shut down the chain reaction. (eia.gov)
Heat passes from the fuel into a circulating coolant. In conventional stations, steam turns a steam turbine connected to an electric generator. The steam then condenses, and the resulting liquid returns to the cycle. Cooling systems transfer unused heat to the surrounding environment. Nuclear stations therefore need a heat sink as well as reactor cooling equipment; cooling towers are one option, not an essential feature of every plant. (nrc.gov)
Reactor designs
Two established designs use ordinary water as their coolant. In a pressurized-water reactor (PWR), high pressure prevents the water circulating through the core from boiling. This primary circuit transfers heat through a steam generator to a separate circuit, where steam is produced for the turbine. In a boiling-water reactor (BWR), water boils inside the reactor vessel, and the resulting steam flows to the turbine after moisture separation. (nrc.gov)
Other reactor technologies use different coolants, fuels, or neutron energies. Gas-cooled, liquid-metal-cooled, and molten-salt concepts appear in advanced reactor development alongside water-cooled designs. These categories describe technical characteristics, whereas reactor size and modular construction describe different aspects of a plant. Consequently, a small reactor is not necessarily based on an entirely new nuclear process. (un.org)
Historical development
On December 20, 1951, Experimental Breeder Reactor I in Idaho produced usable electricity from fission, initially lighting four 200-watt bulbs. This was an experimental demonstration rather than a utility-scale commercial station. In 1954, the Obninsk APS-1 reactor in the Soviet Union supplied five megawatts to an electrical grid. These milestones distinguish the first usable reactor-generated electricity from early grid-connected generation. Nuclear power subsequently developed into a technology supplying electricity and heat over many decades. (energy.gov)
Fuel cycle and radioactive waste
The nuclear fuel cycle covers fuel preparation, reactor use, and management after irradiation. Its front end generally includes uranium extraction, processing, conversion, enrichment where required, and fuel fabrication. Natural uranium contains slightly more than 0.7% uranium-235, an isotope readily used for fission. Fuel for conventional light-water reactors is commonly enriched to approximately 3–5% uranium-235 and fabricated into ceramic uranium-dioxide pellets enclosed in metal tubes. (eia.gov)
After removal from a reactor, spent nuclear fuel remains highly radioactive and continues producing heat through radioactive decay. It is initially stored underwater, where the water provides cooling and radiation shielding. After sufficient cooling, fuel may be transferred to dry storage containers. Storage is an interim management stage, distinct from permanent disposal. (eia.gov)
Reprocessing can recover reusable materials from spent fuel, but radioactive residues still require management. Radioactive waste also arises from mining, fuel processing, and plant operation. Geological disposal places suitable waste deep underground in stable geological formations, using containment and isolation to restrict its interaction with the accessible biosphere. Developing such facilities requires site evaluation, safety assessment, and a staged programme extending over long periods. (infcis.iaea.org)
Safety, security, and safeguards
Nuclear safety addresses accident prevention and mitigation. Defence in depth combines successive protective measures so that safety does not depend entirely on one component or operational practice. Barriers, cooling systems, maintenance, operator training, and regulatory oversight contribute to this approach. Stopping the chain reaction does not eliminate decay heat, so continued heat removal remains important after shutdown. (nrc.gov)
Security concerns protection against theft and sabotage, while international safeguards address a different objective: verifying that nuclear material is not diverted to nuclear weapons or other nuclear explosive devices. The International Atomic Energy Agency implements safeguards under agreements with states. These verification arrangements are distinct from national regulation of plant operation. (nucleus.iaea.org)
Environmental characteristics and economics
Unlike fossil-fuel combustion, reactor operation does not directly emit carbon dioxide. Nevertheless, uranium processing, plant construction, and other supporting activities consume energy and can produce emissions. A life-cycle assessment therefore considers more than electricity generation alone. Nuclear power’s low-emissions characteristics are relevant to energy-security and climate-change policies, alongside waste management and accident-prevention requirements. (eia.gov)
Nuclear plants are capital-intensive, with comparatively low fuel costs. Construction delays, cost overruns, and financing conditions strongly influence their economics. Small modular reactors generally have electrical capacities up to 300 megawatts per unit and use modular construction approaches. Their proposed economic advantages depend on successful manufacturing, deployment, supply-chain development, and financing rather than size alone. The International Energy Agency’s 2025 assessment identifies these factors as important constraints on future nuclear investment. (iea.blob.core.windows.net)