How does a nuclear reactor generate energy?

Nuclear power comes from nuclear fission

Nuclear power plants heat water to produce steam. The steam is used to spin large turbines that generate electricity. Nuclear power plants use heat produced during nuclear fission to heat water.

In nuclear fission, atoms are split apart to form smaller atoms, releasing energy. Fission takes place inside the reactor of a nuclear power plant. At the center of the reactor is the core, which contains uranium fuel.

The uranium fuel is formed into ceramic pellets. Each ceramic pellet produces about the same amount of energy as 150 gallons of oil. These energy-rich pellets are stacked end-to-end in 12-foot metal fuel rods. A bundle of fuel rods, some with hundreds of rods, is called a fuel assembly. A reactor core contains many fuel assemblies.

The heat produced during nuclear fission in the reactor core is used to boil water into steam, which turns the blades of a steam turbine. As the turbine blades turn, they drive generators that make electricity. Nuclear plants cool the steam back into water in a separate structure at the power plant called a cooling tower, or they use water from ponds, rivers, or the ocean. The cooled water is then reused to produce steam.

Nuclear reactors in the United States may have large concrete domes covering the reactors, which are required to contain accidental releases of radiation. Not all nuclear power plants have cooling towers. Some nuclear power plants use water from lakes, rivers, or the ocean for cooling.

Containment dome of a nuclear reactor

Source: Stock photography (copyrighted)

Nuclear power plants have generated about 20% of U.S. electricity since 1990

As of July 1, 2022, 92 nuclear reactors were operating at 54 nuclear power plants in 28 states. Thirty-two of the plants have two reactors, and three plants have three reactors. Nuclear power plants have supplied about 20% of total annual U.S. electricity since 1990. Learn more about the U.S. nuclear energy industry.

The United States generates more nuclear power than any other country

In 2020, 33 countries had commercial nuclear power plants, and in 17 of the countries, nuclear energy supplied at least 20% of their total annual electricity generation. The United States had the largest nuclear electricity generation capacity and generated more nuclear electricity than any other country. France, with the second-largest nuclear electricity generation capacity and second-highest nuclear electricity generation, had the largest share—about 69%—of total annual electricity generation from nuclear energy.

Top five nuclear electric generation capacity countries, 2020

Last updated: July 12, 2022

Nuclear reactors are, fundamentally, large kettles, which are used to heat water to produce enormous amounts of low-carbon electricity. They come in different sizes and shapes, and can be powered by a variety of different fuels. 

The Ringhals Nuclear Power Plant, home to four reactors capable of generating 20% of Sweden’s electricity demand  (Image: Vattenfall)

A nuclear reactor is driven by the splitting of atoms, a process called fission, where a particle (a ‘neutron’) is fired at an atom, which then fissions into two smaller atoms and some additional neutrons. Some of the neutrons that are released then hit other atoms, causing them to fission too and release more neutrons. This is called a chain reaction.

The fissioning of atoms in the chain reaction also releases a large amount of energy as heat. The generated heat is removed from the reactor by a circulating fluid, typically water. This heat can then be used to generate steam, which drives turbines for electricity production. 

In order to ensure the nuclear reaction takes place at the right speed, reactors have systems that accelerate, slow or shut down the nuclear reaction, and the heat it produces. This is normally done with control rods, which typically are made out of neutron-absorbing materials such as silver and boron.

Two examples of nuclear fissioning of uranium-235, the most commonly used fuel in nuclear reactors.

Nuclear reactors come in many different shapes and sizes – some use water to cool their cores, whilst others use gas or liquid metal. The most common power reactor types use water, with more than 90% of the world’s reactors being water-based. Further information on the many different types of reactor around the world can be found in the Nuclear Power Reactors section of the Information Library.

Nuclear reactors are very reliable at generating electricity, capable of running for 24 hours a day for many months, if not years, without interruption, whatever the weather or season. Additionally, most nuclear reactors can operate for very long periods of time – over 60 years in many cases. In 2019, units 3&4 at the Turkey Point plant in Florida were the first reactors in the world to be licensed for 80 years of operation.  

A reactor being refuelled (Image: Vattenfall)

What fuels a reactor?

A number of different materials can be used to fuel a reactor, but most commonly uranium is used. Uranium is abundant, and can be found in many places around the world, including in the oceans. Other fuels, such as plutonium and thorium, can also be used. 

Most of today’s reactors contain several hundred fuel assemblies, each having thousands of small pellets of uranium fuel. A single pellet contains as much energy as there is in one tonne of coal. A typical reactor requires about 27 tonnes of fresh fuel each year. In contrast, a coal power station of a similar size would require more than two-and-a-half million tonnes of coal to produce as much electricity.

Nuclear fuel pellets are not much larger than a sugar cube (Image: Kazatomprom)

How about the waste?

Like any industry, the nuclear industry generates waste. However, unlike many industries, nuclear power generates very little of it – and fully contains and manages what it does produce. The vast majority of the waste from nuclear power plants is not very radioactive and for many decades has been responsibly managed and disposed of. If nuclear power was used to supply a person’s electricity needs for an entire year, only about 5 grams of highly-radioactive waste would be produced, which is the same weight as a sheet of paper. 

The used fuel which comes out of the reactor can be managed in different ways, including recycling for energy production or direct disposal. As a matter of fact, many countries have been using recycled fuel for decades to partially fuel their reactors.  

Used nuclear fuel awaiting recycling (Image: Rosatom)

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