How does nuclear power work? Fission, steam and safety

Nuclear power works by splitting atoms of uranium inside a reactor, a process called fission, which releases heat. That heat boils water into steam, the steam spins a turbine, and the turbine drives a generator, just as in a coal or gas plant. What sets it apart is the heat source: a reactor does not burn anything, so it emits almost no carbon dioxide, but it makes radioactive waste and must be kept under tight control.

In short

• A nuclear plant is a steam power plant whose heat comes from splitting uranium nuclei in a controlled chain reaction.
• Control rods and the water around the fuel keep the chain reaction steady; the biggest danger is losing the cooling that carries heat away.
• Over its life cycle it is among the lowest-carbon and safest sources of electricity per unit made, but new plants have been slow and expensive to build.

What happens inside a reactor?

Uranium is a heavy element. One form of it, uranium-235, splits when it absorbs a slow neutron, a particle from the atom's nucleus. The nucleus breaks into two smaller atoms and releases energy, mostly as the speed of the fragments, which becomes heat. Each split also throws out two or three new neutrons. If, on average, exactly one of those causes another split, the reaction keeps itself going at a steady rate. That is a controlled chain reaction.

Natural uranium is only about 0.7% uranium-235, so fuel for most reactors is enriched to a few percent. The U.S. Energy Information Administration (EIA) explains that it is formed into ceramic pellets, stacked in 12-foot metal fuel rods and bundled into assemblies in the reactor core. Each pellet, about the size of a fingertip, produces about as much energy as 150 gallons of oil.

Three things keep the reaction in hand:

• The moderator. Neutrons from fission are too fast to split uranium-235 well. In most reactors, ordinary water slows them down.
• Control rods. Rods of neutron-absorbing material, such as boron, slide in and out of the core. Pushing them in soaks up neutrons and slows the reaction; dropping them fully shuts it down.
• The coolant. Water flows through the core and carries the heat away to make steam.

In a pressurized water reactor, the most common type, the core water is kept under high pressure so it doesn't boil, and it heats a separate loop of water that becomes steam. In a boiling water reactor, the water boils in the core itself. Either way, the steam goes on to the turbine, is cooled back into water, often in the tall cooling towers people associate with nuclear plants, and returns.

How much fuel does a reactor use? A worked example

Take a typical large reactor of 1,000 megawatts that runs 90% of the time, which is normal for nuclear plants.

1. Electricity made in a year: 1,000 megawatts × 8,760 hours × 0.9 ≈ 7.9 terawatt-hours, enough for several hundred thousand homes.
2. Heat needed: a nuclear plant turns about a third of its heat into electricity, so the core must produce about 24 terawatt-hours of heat, or about 86,000 trillion joules.
3. Energy per split: each uranium-235 fission releases about 200 million electronvolts. Over a kilogram of uranium-235, that adds up to about 82 trillion joules.
4. Uranium used up: 86,000 ÷ 82 ≈ 1,050 kilograms. About one tonne of uranium-235 is split each year.
5. Coal for the same heat: at about 24 million joules per kilogram, a coal plant would burn about 3.6 million tonnes.

That tonne is spread through the much larger mass of fuel loaded into the reactor, roughly 20 to 30 tonnes of enriched uranium a year for a plant this size, most of which is not split before it is replaced. Even so, the comparison, one tonne against millions, is why nuclear fuel is cheap to transport and why a reactor's waste is small in volume.

What happens to nuclear waste?

Fuel stays in a reactor for several years. When it comes out, it is intensely radioactive and hot, so it is stored first in steel-lined concrete pools of water, then, after cooling, in sealed steel and concrete dry casks. The U.S. Department of Energy says the country produces about 2,000 metric tons of spent fuel a year, and that all of the roughly 90,000 metric tons made since the 1950s would fit on a single football field less than 10 yards deep. More than 90% of the potential energy is still in it; France recycles some of it, and the United States doesn't.

The hard part is time. Some of the waste stays dangerously radioactive for thousands of years. Finland has built the world's first deep underground repository for spent fuel, in bedrock at Onkalo. The United States designated Yucca Mountain in Nevada in 1987 but has never opened it, so its spent fuel sits at power plants.

What went wrong at Three Mile Island, Chernobyl and Fukushima?

Three accidents shape how people think about nuclear power, and each failed in a different way:

	Three Mile Island, U.S.	Chernobyl, Soviet Union	Fukushima Daiichi, Japan
Date	28 March 1979	26 April 1986	11 March 2011
What went wrong	A relief valve stuck open; operators didn't realize coolant was escaping, and the core partly melted	A badly run test on a flawed RBMK reactor caused a power surge, explosions and a graphite fire	A magnitude 9.0 earthquake and a tsunami about 15 meters high knocked out power and cooling; three cores melted
Deaths and illness from radiation	None detected	28 from acute radiation sickness within weeks (two more workers died in the explosion); about 5,000 thyroid cancers in people exposed as children probably caused by radiation, 15 of them fatal by 2005	No radiation sickness; in 2018 Japan's government linked one former worker's lung cancer death to radiation
Evacuated	Voluntary, short-term	About 350,000 people over time	About 160,000 people

The U.S. Nuclear Regulatory Commission found Three Mile Island's releases had no detectable health effects, but the accident changed plant design, operator training and emergency planning across the industry. Chernobyl was far worse because its design could surge in power when it lost water and it had no strong containment building around the core; Western reactors are built differently. At Fukushima, the earthquake and tsunami killed about 19,500 people, while the World Nuclear Association reports 2,313 disaster-related deaths among evacuees from stress, shelter conditions and disrupted medical care. The common lesson is that a reactor must be cooled even after it shuts down, because the radioactive fuel keeps producing heat.

How does nuclear power compare with other sources?

On emissions and safety, nuclear ranks with wind and solar. Over a plant's whole life, from mining to building to decommissioning, coal emits more than 100 times as much greenhouse gas per unit of electricity, according to Our World in Data. Its estimates of deaths per terawatt-hour, counting accidents and air pollution, are:

Source	Deaths per terawatt-hour
Coal	24.6
Oil	18.4
Gas	2.8
Hydropower	1.3 (including the 1975 Banqiao dam failure)
Wind	0.04
Nuclear	0.03
Solar	0.02

Cost and speed are its weak points. Georgia's Vogtle Units 3 and 4, the newest U.S. reactors, began construction in 2009 and entered service in 2023 and 2024, years late; the EIA reported a total cost estimate of more than $30 billion, against an original $14 billion. Worldwide, the World Nuclear Association's reactor table, updated on 2 November 2026, counts 443 operable reactors, which produced about 9% of the world's electricity in 2025. In the United States, nuclear has supplied about a fifth of electricity each year since 1990, according to the EIA.

Still argued

Whether to build more nuclear power is a real debate. Supporters argue that it provides steady, low-carbon power around the clock, uses little land, and has one of the best safety records per unit of energy, which makes it valuable for cutting the emissions behind climate change. Critics argue that new plants cost too much and take too long compared with wind, solar and batteries, that long-term waste disposal remains unsolved in most countries, and that rare accidents can make large areas uninhabitable. Small modular reactors and future fusion are promoted as ways around these problems; neither has yet shown its costs at scale.

Check yourself

• Without looking back: trace the energy from a splitting uranium atom to the electricity in a wire.
• What do control rods and the moderator each do to the chain reaction?
• Why did Fukushima's reactors melt down even though they shut down when the earthquake struck?

Sources

• Nuclear power plants — U.S. Energy Information Administration, 2023
• Plant Vogtle Unit 4 begins commercial operation — U.S. Energy Information Administration, 2024
• 5 fast facts about spent nuclear fuel — U.S. Department of Energy, 2022
• Backgrounder on the Three Mile Island accident — U.S. Nuclear Regulatory Commission
• Chernobyl accident 1986 — World Nuclear Association, 2026
• What are the safest and cleanest sources of energy? — Hannah Ritchie, Our World in Data, 2020