Key Takeaways
- Nuclear energy is the only commercially available power source that harnesses the massive amount of energy stored in the atomic nucleus, generating large amounts of electricity without carbon emissions or pollution.
- A nuclear reactor core mediates a neutron chain reaction to produce a steady amount of heat. Nuclear power plants contain many built-in and redundant fail safes to safely mediate and control this reaction.
- Coolant is used to transfer the heat from the core to generate steam and spin a turbine, generating electricity.
- Future reactors may use alternative materials to achieve advantages such as lower pressures, more durable fuels, and waste recycling capabilities.
Nuclear reactors use neutrons to create chain reactions that split atomic nuclei, releasing their energy. This releases heat, producing steam which is then used to generate carbon-free electricity.
Nuclear reactors safely produce around 20% of electricity and more than 50% of the carbon-free electricity in the U.S. every year.
This article explains the basics of how nuclear reactors work, what they do, and popular types of reactors — both now and in the future.
In This Article
What Is a Nuclear Reactor?
A nuclear reactor releases the energy stored in atomic nuclei, generating large amounts of heat which can be converted into electricity.
Atoms are made up of protons, neutrons, and electrons. Traditional energy sources, like fossil fuels, release energy using chemical reactions involving electrons. Nuclear power, on the other hand, releases energy from the atomic nucleus — the protons and neutrons — which contain significantly greater amounts of energy.
How Do Nuclear Reactors Work?
Today, all nuclear power is created by fission, a process that splits the atomic nucleus using a neutron chain reaction.
Some atomic nuclei are more stable than others. A uranium-235 nucleus has 92 protons and 143 neutrons, and it can be forced apart with a slight “push” from a low-energy neutron. This is the reaction that occurs in most nuclear reactors today.
When a low-energy neutron fissions a uranium-235 nucleus, it releases large amounts of energy as heat, along with 2-3 high-energy neutrons. The split remnants of the nucleus are called fission products, which are highly radioactive for relatively short periods.
The high-energy neutrons produced in fission collide with other particles to decrease their energy — a process called moderation — lowering them to an energy level that will fission more uranium-235 nuclei. When the new nuclei are fissioned, more neutrons are released, and the chain reaction continues.
The ideal state for a nuclear reactor is called criticality, where one neutron leads to one fission, which creates one neutron that leads to more fissions. At criticality, the chain reaction continues at a stable state, without growing or shrinking in energy.
Nuclear reactors continuously fission uranium-235 nuclei to create a steady supply of heat, which is transferred into a coolant. The coolant can then transport the energy through different mechanisms to eventually reach a steam generator, producing steam that spins a turbine and generates electricity.
What are the Parts of a Nuclear Reactor?
Most (but not all) of today’s reactors use solid fuel and liquid coolant. There are other varieties too, which we’ll discuss later.
Uranium fuel is arranged in long tubes, called fuel rods, which are around 1 cm in diameter and 3-4 m long. Fuel assemblies are used to gather over 100 fuel rods, which makes them easier to handle. A reactor may contain 150-250 assemblies or more than 20,000 individual fuel rods.
Fuel rods are encased in a sealed metal cladding, which prevents fuel and fission products from escaping. The cladding is made of alloys containing zirconium, aluminum, or stainless steel, and has a high melting temperature of nearly 2,000 degrees Celsius.
The fuel assemblies are put together to form the reactor core, which is housed inside a pressure vessel. Retractable control rods are inserted between assemblies at varying heights to ensure the stable criticality of the reactor, or to shut it down if necessary.
A moderator mediates the reaction inside the core to help sustain stable, continuous criticality. The coolant absorbs the heat produced by the fuel and transports it out of the core to a heat exchanger, where in some designs, it is used to heat a secondary coolant loop that generates steam and spins a turbine.
In most of today’s reactors, liquid water serves as both the moderator and the coolant simultaneously.
The fuel cladding and pressure vessel are both examples of containment since they are designed to prevent radioactivity from escaping. The third level of containment is bigger — a large building, commonly made of thick, reinforced concrete which houses the reactor and adds a barrier to prevent any release of radioactivity from the plant.
Several radiation detectors continuously monitor for any potential contamination both inside the containment building and on the land surrounding the nuclear plant. If radiation levels are detected above operating limits, the reactor is shut down and inspected before it can return to service.
Types of Nuclear Reactors
Nuclear reactors come in all shapes and sizes, but they share the same central fission process, splitting atomic nuclei to release energy. Different systems also are used to collect and transfer energy to generate electricity.
Current Nuclear Reactor Designs
Here are the most commonly used reactors today and a few defining features of each:
Light Water Reactors
Most of the nuclear reactors used in the world today use liquid water as both the coolant and the moderator, and solid low-enriched uranium (3-5% uranium-235) as fuel.
A pressurized water reactor (PWR) has two coolant loops. The primary coolant loop is kept under high pressure to prevent water from boiling in the core. The water exits the core and passes to a heat exchanger, where it heats a secondary loop that generates steam and spins a turbine.
A boiling water reactor (BWR) is like a PWR but only contains one coolant loop. As water heats in the reactor core, it rises, and at the top of the pressure vessel, it boils. The steam is used to spin a turbine, then is cooled and cycled back into the bottom of the reactor.
Pressurized Heavy Water Reactor
A pressurized heavy water reactor (PHWR) uses natural uranium as fuel with an enriched moderator of heavy water, D2O. Using deuterium (another name for the hydrogen-2 isotope) in the water gives it better moderating efficiency (sometimes referred to as neutron economy), allowing a lower concentration of uranium-235 to sustain the chain reaction.
The moderator/coolant passes through a heat exchanger, where a secondary loop connects to a steam generator and turbine.
This type of reactor is most popular in Canada and is called the CANDU reactor. It can be refueled by replacing fuel bundles without shutting down. CANDU reactors can sustain a fission reaction when fueled with natural uranium, depleted uranium (with uranium-235 concentration below natural levels), and even spent nuclear fuel from other reactors.
Gas-Cooled Reactors
Gas-cooled reactors use gas, rather than liquid, coolants.
In the advanced gas-cooled reactor (AGR), low-enriched uranium (2.5-3.5% uranium-235) oxide fuel pellets are placed in tubes in a solid graphite core. The graphite acts as a neutron moderator, with channels that allow CO2 to pass through as coolant, and space for control rods to be inserted.
The CO2 reaches high temperatures (650 degrees Celsius) compared to other reactor types and heats steam generator loops from inside the pressure vessel, achieving a higher efficiency of up to 41%.
Advanced Nuclear Reactors
Many additional nuclear reactor technologies are currently being developed that operate with different strategies to create, sustain, and transfer the energy from a nuclear reaction.
There are many advanced nuclear reactors in various stages of development.
Fast Reactors
While traditional reactors use low-energy neutrons to sustain the fission chain reaction, fast reactors use high-energy neutrons. This allows a wider range of isotopes to be used as fuel. Almost every isotope heavier than uranium can be used as fuel in a fast reactor. This is particularly advantageous because it means that used nuclear fuel could be recycled as fuel.
Fast reactors don’t use moderators, since neutrons must be high energy. This also means that coolant materials in the core must be neutron-transparent.
A common form of fast reactor design is the Liquid Metal Fast Reactor, such as a Sodium Fast Reactor (SFR) or Lead Fast Reactor (LFR). These reactors use liquid metal sodium or lead as a coolant, to benefit from ambient pressure operation and excellent thermal conductivity to remove heat from their fuel.
Molten Salt Reactor (MSR)
Molten salt reactors can operate at a variety of neutron energies. These reactors use salts, like FLiBe salt, a mixture of lithium fluoride and beryllium fluoride, as coolants. FLiBe is chemically stable, with excellent neutronics and heat transfer properties. Others are designed based on a chloride-salt system, using sodium and/or potassium bonded to the chlorine — very similar to table salt!
With a coolant melting temperature of around 450 degrees Celsius, molten salt reactors can be operated at atmospheric pressure, making them safer than pressurized systems.
Using molten salt as a coolant comes with several safety advantages. FLiBe can dissolve uranium or thorium, creating a liquid-based fuel that is completely meltdown-proof. Freeze plugs can be placed beneath the core so that in the event of an overheating reactor, the plugs melt and drain the core, ending the reaction.
Molten salt reactors can also use other fuel forms, as the salt can function as a simple coolant to remove heat at atmospheric pressure.
High-temperature Gas-cooled Reactor (HTGR)
High-temperature Gas-cooled Reactors operate at temperatures of more than 700 degrees Celsius and use helium as a coolant and graphite as a moderator. The higher temperature increases thermal efficiency and allows the reactor to be easily used for other applications, such as process heat or hydrogen production.
HTGR designs include the pebble bed reactor, which allows online refueling with TRISO fuel formed into billiard ball-sized spheres. TRISO fuel is an extremely stable fuel that takes a poppy seed-sized kernel of uranium and coats it in carbon and ceramics that prevents fission products from escaping, making waste easier to manage. Another common HTGR design is the prismatic reactor, which uses graphite blocks similar to the AGR to contain TRISO fuel formed into pellets.
Nuclear Reactor Safety
One of the most frequently raised concerns about nuclear energy is whether it is safe or not. Though it can be an intimidating topic, there are several reasons why nuclear energy is one of the safest energy sources available.
Nuclear Energy Pollutes Less Than Fossil Fuels
Burning fossil fuels releases emissions and pollution into the air, the latter of which is estimated to cause 8 million premature deaths every year. Nuclear power plants, on the other hand, do not release any carbon emissions or pollution.
Nuclear energy can reach the scales necessary to replace carbon-intensive fossil fuel plants to decarbonize the energy sector and protect the environment and human health.
Nuclear Waste
Nuclear reactors produce highly radioactive waste in the form of used nuclear fuel. However, this waste isn’t considered an insurmountable issue — because there isn’t very much of it.
Used nuclear fuel is currently stored in large pools of water or placed in temporary storage using reinforced dry casks. Certified by the Nuclear Regulatory Commission (NRC) in the U.S. and by other regulators around the world, these casks are capable of surviving earthquakes, severe weather, and even plane or missile impacts without releasing any radioactivity.
Agencies worldwide are looking for a permanent waste storage solution. Deep underground burial is an attractive long-term solution. Alternatively, the next generation of advanced nuclear reactor designs may allow the recycling of nuclear fuel by fissioning it in fast reactors, which would significantly reduce waste hazards and timelines.
Nuclear Reactors are Safer Than Ever
Operational experience is used to continuously improve and upgrade nuclear reactors to the highest standards of safety and efficiency.
Organizations like EPRI create detailed reports on how reactor safety can be improved in response to potential risks, and we’ve helped utilities, regulators, and the general public with greater reactor safety.
The Institute for Nuclear Power Operations (INPO) in the U.S. and the World Association of Nuclear Operators (WANO) globally document lessons learned from nuclear power plant operators, and help everyone implement them as best they can.
Widely shared operational experience helps ensure that all operators are using the most up to date information to best manage their power plants.
This article is one of many developed by EPRI’s Advanced Nuclear Technology program. Stay up-to-date on the newest nuclear technologies by becoming a member today.