Inside Nuclear Fuel: How Reactors Create Electricity From the Power of Atoms

The International Atomic Energy Agency (IAEA) supports countries in managing nuclear fuel safely, securely and exclusively for peaceful purposes.

Nuclear fuel sits at the heart of every nuclear power plant, providing the energy needed to generate electricity for millions of homes and businesses around the world. Unlike coal, oil or natural gas, nuclear fuel does not produce energy through burning. Its power comes from changes occurring inside atoms, releasing enormous amounts of heat that can be converted into electricity without producing greenhouse gas emissions during operation.

The material used as nuclear fuel varies depending on reactor designs and energy requirements, though uranium remains the most widely used option. Scientists and engineers continue developing alternative fuels as countries look for efficient and reliable low-carbon energy sources capable of supporting growing electricity demand.

Understanding how nuclear fuel works requires looking beyond the reactor itself. The process begins with mining and fuel production, continues through years of electricity generation and ends with the careful management of spent fuel and radioactive waste. Together, these stages form what is known as the nuclear fuel cycle.

The science behind nuclear power

The most common nuclear fuel is uranium oxide. Natural uranium contains only about 0.7% uranium-235, the isotope capable of sustaining the chain reactions required for electricity generation. Because this percentage is too low for most power reactors, uranium is often enriched before use.

Low enriched uranium (LEU), containing three to five percent uranium-235, powers most commercial nuclear plants operating today. Highly enriched uranium (HEU), which contains more than 20% uranium-235, is mainly used in research reactors and remains subject to strict international controls because of proliferation concerns.

Another option is mixed oxide, or MOX fuel, which combines uranium with plutonium recovered from previously used nuclear fuel. This approach allows valuable materials to be recycled and reused rather than discarded.

Researchers are also exploring thorium-based fuels. Although thorium itself cannot sustain a chain reaction, it can be transformed inside a reactor into uranium-233, which can serve as a nuclear fuel. Several thorium technologies remain under development but are attracting attention for their future potential.

How Nuclear Fuel Produces Electricity

Inside a nuclear reactor, fuel is arranged into assemblies made up of long metal rods filled with ceramic uranium dioxide pellets. When a uranium-235 atom absorbs a neutron, the atom becomes unstable and splits into smaller atoms in a process known as nuclear fission.

This reaction releases heat and additional neutrons. The newly released neutrons strike other uranium atoms, creating a chain reaction that continues under carefully controlled conditions inside the reactor core.

The heat generated from these reactions is used to convert water into steam. The steam drives turbines connected to generators, producing electricity that can be supplied to homes, businesses and industries. Instead of relying on combustion, nuclear power stations create energy through atomic transformations, avoiding direct greenhouse gas emissions during electricity production.

What Happens When Fuel Is Used Up?

Nuclear fuel does not remain inside a reactor permanently. After roughly three to five years of operation, its ability to efficiently sustain chain reactions decreases. At that stage, the fuel assemblies are removed and classified as spent fuel.

Although no longer suitable for efficient power generation, spent fuel remains radioactive and continues producing heat. It contains radioactive fission products, large quantities of uranium-238 and newly created elements such as plutonium, neptunium, americium and curium.

For safety reasons, spent fuel is initially placed in specially designed cooling pools located at reactor sites. Water removes residual heat and provides shielding from radiation. After several years of cooling, the fuel may be transferred to heavily protected dry storage casks or centralized storage facilities designed for long-term containment.

The Three Stages of the Nuclear Fuel Cycle

The nuclear fuel cycle covers every step in the life of nuclear fuel.

The front end begins with uranium exploration, mining and milling. The processed uranium is converted into uranium hexafluoride gas, enriched and manufactured into ceramic fuel pellets that are loaded into fuel rods and assemblies.

The service period is the operational stage inside the reactor. Fuel assemblies remain in the reactor core for several years, generating heat and electricity through controlled fission reactions.

The back end starts once fuel leaves the reactor. Some countries classify spent fuel as waste destined for long-term disposal in deep geological repositories. Others reprocess spent fuel to recover usable uranium and plutonium for future applications. Any remaining high-level radioactive waste is ultimately prepared for permanent disposal under strict regulatory controls.

The IAEA's Role in Nuclear Fuel Management

The International Atomic Energy Agency (IAEA) supports countries in managing nuclear fuel safely, securely and exclusively for peaceful purposes. Its work includes developing international nuclear safety standards, implementing safeguards that prevent the misuse of nuclear materials, supporting technical cooperation and research activities, organizing training programmes and facilitating knowledge-sharing among experts worldwide.

The agency also maintains specialized networks dedicated to spent fuel management and nuclear fuel cycle issues, helping member states access technical expertise, best practices and the latest developments in the field.

As nuclear energy gains renewed attention as a low-carbon electricity source, effective fuel management remains a critical part of ensuring safety, sustainability and public confidence across every stage of the nuclear fuel cycle.