Nuclear fuel is material that releases energy through nuclear fission, the process of splitting heavy atoms apart. The two primary fissile materials used in nuclear power are uranium-235 and plutonium-239. When a neutron strikes one of these atoms, the atom splits into two smaller fragments, releasing both energy and additional neutrons that can split more atoms, creating a self-sustaining chain reaction. A single kilogram of uranium-235 can generate roughly 24 million kilowatt-hours of heat, compared to about 8 kWh from a kilogram of coal.
How Uranium Becomes Reactor Fuel
Nuclear fuel starts as uranium ore pulled from the ground. At a processing mill, the ore is refined into a concentrated powder called yellowcake, which is essentially uranium oxide. This yellowcake is then shipped to a conversion facility, where it reacts with fluorine to become uranium hexafluoride, a compound that exits the process as a gas, cools to liquid, and eventually solidifies inside large storage cylinders over about five days.
The reason for this chemical transformation is enrichment. Natural uranium is 99.3% uranium-238, a stable isotope that doesn’t easily split. Only about 0.7% is the fissile uranium-235 that actually powers a reactor. To work in a commercial power plant, that concentration needs to rise to between 3.5% and 5%. The gaseous form of uranium hexafluoride makes it possible to separate the lighter uranium-235 atoms from the heavier uranium-238 through centrifuge spinning. Once enriched, the material is converted into uranium dioxide, a dense ceramic powder that gets pressed and heated into small, hard pellets.
From Pellets to Fuel Assemblies
A finished nuclear fuel pellet is roughly 1 centimeter in diameter and slightly longer than it is wide, about the size of a pencil eraser. Despite that small size, a single pellet contains as much energy as about a ton of coal. These pellets are stacked end to end inside long, thin metal tubes called fuel rods or pins. The cladding on these rods is typically made from a zirconium alloy, chosen because it’s strong, corrosion-resistant, and largely transparent to neutrons, meaning it doesn’t absorb the particles needed to sustain the chain reaction.
Fuel rods are then grouped together into fuel assemblies. The exact arrangement depends on the reactor design. Pressurized water reactors and boiling water reactors use square grid assemblies held together with zirconium alloy frames. Other designs, like the UK’s advanced gas-cooled reactors, bundle rods in circular formations housed inside graphite sleeves. A single reactor core holds hundreds of these assemblies, and they’re periodically rotated and replaced as the fissile material is consumed.
Enrichment Levels and Their Uses
The percentage of uranium-235 determines what the fuel can do. The existing fleet of commercial reactors worldwide runs on low-enriched uranium, enriched up to 5%. Many next-generation reactor designs require what’s called high-assay low-enriched uranium (HALEU), enriched between 5% and just under 20%. These higher concentrations allow engineers to build smaller reactor cores that extract more power per unit of volume. Anything enriched above 20% is classified as highly enriched uranium and is restricted to research reactors and military applications, including naval propulsion and weapons.
One kilogram of natural uranium, after enrichment and use in a light water reactor, yields about 45,000 kWh of electricity. That’s equivalent to burning roughly 14,000 kilograms of coal or 10,000 kilograms of oil. This extraordinary energy density is the central advantage of nuclear power: a relatively small amount of material produces an enormous amount of energy with no direct carbon emissions.
Mixed Oxide Fuel
Not all nuclear fuel starts as freshly mined uranium. Mixed oxide fuel, or MOX, blends plutonium with depleted uranium to create a fuel that works in existing commercial reactors. The plutonium can come from two sources: reprocessed spent fuel from civilian reactors, or surplus military weapons stockpiles. The United States and Russia, for example, agreed to each convert at least 34 metric tons of weapons-grade plutonium into MOX fuel for use in power plants.
Physically, MOX pellets look similar to standard uranium fuel pellets and are made through the same sintering process. The key difference is in the microstructure. Standard uranium fuel is a homogeneous blend of uranium isotopes, while MOX contains tiny plutonium-rich clusters distributed through a matrix of depleted uranium dioxide. Despite this structural difference, MOX can substitute for a portion of the regular fuel load in a conventional reactor without major modifications.
Thorium as an Alternative
Thorium-232 is sometimes called the fuel of the future, though it works quite differently from uranium. Thorium itself isn’t fissile, meaning it can’t sustain a chain reaction on its own. Instead, it’s “fertile”: when thorium-232 absorbs a neutron inside a reactor, it transforms into uranium-233, which is fissile and can then be split to release energy. This is analogous to how uranium-238 absorbs neutrons and becomes plutonium-239 in a conventional reactor.
The thorium fuel cycle has several theoretical advantages. It can achieve higher conversion ratios in thermal reactors than uranium fuel, meaning it stretches the fuel supply further. The waste it produces contains far less plutonium and related long-lived elements, reducing the radioactive toxicity of spent fuel over timescales of 500 to 100,000 years. Thorium is also roughly three to four times more abundant in the Earth’s crust than uranium. However, no country has yet built a commercial-scale thorium power plant, and the infrastructure for processing and fabricating thorium fuel remains undeveloped compared to uranium.
TRISO: A Newer Fuel Design
One of the most advanced fuel forms in development is the TRISO particle, which the U.S. Department of Energy has called “the most robust nuclear fuel on Earth.” Each TRISO particle is tiny, smaller than a poppy seed, with a fuel kernel made of uranium dioxide or uranium oxycarbide at its center. That kernel is then wrapped in four layers of carbon and ceramic coatings that act as a miniature containment system, trapping radioactive byproducts inside the particle itself.
What makes TRISO remarkable is its heat tolerance. In testing, irradiated TRISO particles were exposed to more than 300 hours at temperatures up to 1,800°C (over 3,000°F), well beyond the worst-case accident conditions predicted for high-temperature reactors. The particles showed minimal damage and extremely low release of radioactive material. Because the containment is built into each individual fuel particle rather than relying solely on external structures, TRISO-fueled reactors carry a fundamentally different safety profile than conventional designs.
What Happens to Spent Fuel
After several years in a reactor, fuel assemblies are removed as “spent fuel.” This doesn’t mean the uranium is gone. Spent fuel with a typical burnup still contains about 93.4% uranium, though only around 0.8% of that remains as fissile uranium-235, down from the original 3.5% to 5%. The rest of the composition is roughly 5.2% fission products (the atomic fragments created when uranium atoms split), 1.2% plutonium (created when uranium-238 absorbs neutrons), and 0.2% other heavy elements like neptunium, americium, and curium.
The fission products and heavy elements are what make spent fuel intensely radioactive and hot. Spent assemblies are first placed in water-filled cooling pools at the reactor site, where they remain for several years as the shortest-lived radioactive isotopes decay. After sufficient cooling, the fuel can be transferred to dry storage casks made of steel and concrete. Some countries, notably France, reprocess spent fuel to extract the remaining usable uranium and plutonium for recycling into new MOX fuel. Others, including the United States, currently treat spent fuel as waste destined for long-term geological storage.