Key facts
- Principle used in current power plants: controlled nuclear fission.
- Global installed capacity: around 420 GW at the end of 2025.
- Global nuclear generation: up around 1.2% in 2025, reaching a record level.
What is nuclear energy?
Nuclear energy is associated with the forces binding protons and neutrons within an atom's nucleus. Certain transformations of these nuclei release large amounts of energy. Current plants use fission: a heavy nucleus, usually uranium, splits after capturing a neutron and releases heat and new neutrons.
Fusion works differently: very light nuclei combine to form a heavier nucleus. This reaction powers stars, but continuously controlling it on Earth remains a research challenge. It is not currently a commercial electricity source. Operating fission reactors must therefore be distinguished from experimental fusion projects.
How does a plant generate electricity?
In a fission reactor, fuel is placed in the core. The chain reaction is regulated by systems controlling the number of available neutrons. Fission heats a fluid, generally water. The heat produces steam that drives a turbine connected to a generator. This converts mechanical energy into electricity, which is then transmitted through the grid.
The circuit removing heat from the reactor and the circuit supplying the turbine may be separate, depending on technology. After the turbine, steam is cooled and condensed to restart the cycle. A plant therefore needs cooling, control, containment and backup systems. Safety relies on several physical barriers and arrangements designed to prevent accidents and limit their consequences.
Rated power, expressed in megawatts (MW) or gigawatts (GW), indicates maximum generating capacity at a given moment. Energy generated over time is expressed in megawatt-hours (MWh) or terawatt-hours (TWh). A 1 GW plant does not automatically generate 1 GW continuously: maintenance shutdowns, refuelling and unavailability affect annual generation.
What role does it play in the global electricity system?
Nuclear energy supplies electricity that can be generated independently of weather conditions. It can help stabilise a grid as demand varies, although reactors' actual operation and flexibility depend on design, national rules and operating choices.
According to the International Energy Agency, global nuclear capacity stood at around 420 GW at the end of 2025. Annual generation increased by 1.2% to a record, with restarts in Japan, strong performance in France and new reactors commissioned in several countries. The IAEA counted 417 operating reactors in 31 countries in 2025. Capacity and generation figures do not describe the same indicator: the former measures installed power, the latter electricity actually generated.
National trajectories differ. Some countries extend or build reactors to meet demand and reduce fossil fuels' share of electricity. Others shut down facilities, favour other sources or lack the infrastructure and financing required for a nuclear programme. New construction is particularly active in Asia, while projects are also being developed in the Middle East and Europe.
Strengths and constraints
During operation, reactors emit few greenhouse gases per unit of electricity compared with coal or gas plants. They also provide generation concentrated on one site and relatively little dependent on daily fuel imports. These characteristics can support security of supply and reduce electricity-sector emissions.
These advantages must be assessed across the entire life cycle. A plant requires uranium, mines and conversion and enrichment facilities, followed by transport and storage resources. Construction involves large amounts of capital and can take many years. Delays, cost increases and financing uncertainties strongly influence project competitiveness.
Operational safety is a permanent requirement. Three Mile Island, Chernobyl and Fukushima prompted reviews of practices, designs and emergency preparedness. Radioactive wastes have differing hazard levels and durations: some are low-level and managed through suitable routes, while spent fuel and high-level waste require long-term containment. Management involves technical, regulatory and political choices that vary between countries.
The future of nuclear energy
Third-generation reactors, including the EPR, incorporate design developments and strengthened safety systems. Small modular reactors (SMRs) are designed as more compact units, but their costs, schedules, industrial chains and large-scale performance remain to be demonstrated. Advanced reactors are the subject of longer-term research.
Nuclear energy's future role will depend on national choices: capital costs, construction duration, grid capacity, skills availability, waste solutions and comparison with renewables, storage, hydropower and interconnections. There is no single model suitable for every electricity system.
Frequently asked questions
Is nuclear energy renewable?
Not in the usual sense: uranium is a limited mineral resource. However, nuclear electricity is often classified as low-carbon because of low direct emissions during generation.
Does nuclear energy produce no emissions?
Reactors do not emit CO₂ through combustion during operation. However, ore extraction, construction, transport and decommissioning generate emissions across the life cycle.
Does fusion already generate electricity?
No. Scientific experiments study controlled fusion, but no commercial plant currently supplies grid electricity using this technology.