Nuclear energy FAQ

The reactor core fissions uranium and releases heat; that heat warms pressurized water (or another coolant), which either boils water in a separate loop or drives a steam turbine coupled to a generator directly.

It is the same heat-to-electricity conversion as a coal or gas plant — only the source of the heat differs. A condenser cools the spent steam, using a cooling tower or river or sea water, and returns it to the loop to repeat the cycle.

The exact layout — how many loops separate the reactor from the turbine, what coolant and moderator it uses — is what defines the reactor type: PWR, BWR, PHWR and the rest of the family.

It comes from fission: a slow neutron strikes a heavy nucleus — typically uranium-235 — and splits it in two, releasing heat and two to three new neutrons. If on average one of those neutrons triggers another fission, the reaction sustains itself: that is a critical reactor, and controlling that neutron balance is what the control system does.

One gram of fissioned uranium-235 releases roughly the energy of 2.5 tonnes of burned coal. That heat is what later becomes electricity, through the same turbine-generator cycle as any thermal power plant.

There are 442 operating nuclear power reactors in the world today, across 32 countries, per the record Reactor Atlas builds from the IAEA (PRIS), Global Energy Monitor and national sources.

The figure moves with every connection, shutdown or restart — it updates itself, it isn't written by hand.

By reactor count, the United States leads the world: 94 operating units, 21% of the global total, per the same record.

The order can shift when ranked by installed capacity instead, since not every reactor has the same output — see the full country breakdown.

The family with the most units worldwide is pressurized light water (PWR), followed by boiling water (BWR) and pressurized heavy water (PHWR, the Canadian CANDU line). Russia and Ukraine also operate graphite-moderated light-water reactors (LWGR, known as RBMK, the Chernobyl design), and the United Kingdom runs gas-cooled reactors (GCR/AGR).

Alongside those operating families sit minority designs such as fast breeder reactors (FBR) and a new generation of small modular reactors (SMR) at various stages of construction or licensing.

Chernobyl unit 4 was an RBMK-1000: a Soviet design cooled by boiling light water and moderated by graphite, with 1,661 individual fuel channels and no containment building.

The IAEA's INSAG-7 report (1992) identified design flaws in the reactor — a positive void coefficient under certain conditions, and control rods with graphite tips that could increase reactivity on insertion — as the central cause of the accident, alongside operator errors during a safety test. Russia still operates seven RBMK units, all modified after 1986.

No, by the IPCC's own definition: renewable energy is energy from solar, geophysical or biological sources replenished at a rate equal to or greater than its use, and the panel recognizes six such sources — bioenergy, solar, geothermal, hydropower, ocean and wind.

Nuclear fission runs on mined uranium, a finite resource with no natural replenishment process, so it doesn't belong on that list. It is grouped with renewables as a low-carbon source of electricity, which is a separate category.

Measured in greenhouse-gas emissions per unit of energy across the full life cycle — mining, construction, operation and decommissioning — nuclear has a median of 12 gCO2eq/kWh according to the IPCC: comparable to wind, and well below coal (820 gCO2eq/kWh) and natural gas (490 gCO2eq/kWh).

It emits no sulfur dioxide, nitrogen oxides or particulate matter during operation, unlike fossil-fuel generation. It does produce radioactive waste that conventional electricity generation does not — a separate problem from emissions.

Between 1969 and 2000, the OECD Nuclear Energy Agency recorded zero severe nuclear accidents (five or more immediate deaths) in OECD countries — 0 deaths per GWyr — against 0.157 deaths per GWyr for the coal chain, 0.132 for oil and 0.085 for natural gas, for the same country group and period.

The period's only severe nuclear accident, Chernobyl, happened outside the OECD and caused 31 immediate deaths; studies by the European Commission, the IAEA, the WHO and UNSCEAR cited by the Agency itself estimate between 9,000 and 33,000 long-term cancer deaths, depending on the dose cut-off used.

In favor: it runs with the highest capacity factor of any electricity source — 83.5% on average worldwide between 2022 and 2024, according to the IAEA — and its life-cycle carbon footprint is among the lowest of any technology, comparable to wind.

Against: the capital cost of a new reactor varies widely, between USD 2,157 and USD 6,920 per kWe in OECD countries according to the joint 2020 IEA/NEA cost report, and the first projects of each new reactor generation have racked up delays and cost overruns that the Agency itself documented and attributed to weak supply chains, not an inherent limit of the technology.

On top of that come radioactive-waste management and proliferation risk, each covered with its own data in the next question.

The question starts from a premise. The four real objections behind it are radioactive waste, accident risk, weapons proliferation and cost — here is the data for each:

Waste: spent fuel stays radioactive for thousands of years and is stored today in pools and dry casks at the plants themselves, because no country yet has a deep geological repository in operation. The most advanced project is Onkalo, in Finland: Finnish regulator STUK issued a favorable safety statement in August 2026 — a precondition for the operating licence — and Posiva is aiming to begin disposing of fuel before the end of 2026.

Accidents: between 1969 and 2000, OECD countries recorded zero nuclear accidents with five or more immediate deaths, against 0.157 deaths per GWyr for the coal chain and 0.085 for natural gas, per the OECD Nuclear Energy Agency. The period's only severe accident, Chernobyl, caused 31 immediate deaths and estimates of between 9,000 and 33,000 long-term cancer deaths, per studies by the European Commission, the IAEA, the WHO and UNSCEAR.

Proliferation: enrichment and reprocessing technology is dual-use, civilian and military, which is why an international verification system exists for it: as of 31 December 2025 the IAEA had comprehensive safeguards agreements in force with 183 States and Additional Protocols with 144 States plus Euratom.

Cost: building a new reactor costs between USD 2,157 and USD 6,920 per kWe in OECD countries, and the first projects of each new reactor generation have suffered delays and cost overruns that the OECD Nuclear Energy Agency documented and attributed to weak supply chains and a lack of recent construction experience.

No country yet has a deep geological repository in operation, and those capital costs and construction timelines are real and documented.