Nuclear fuel is any substance which is used by nuclear reactors or other nuclear devices as fuel to generate energy. Nuclear fuel contains fissile material, such as uranium, which undergoes nuclear fission in a reactor. The heat energy released by the fission of nuclear fuel can be converted into electricity in a nuclear power station.
Unlike chemical fuels, which release energy via a chemical reaction, nuclear fuels release energy via a nuclear reaction, and have much higher energy density than chemical fuels. The energy released by the fission of one Uranium-235 atom is about 100 million times greater than the energy released by burning one carbon atom in air to produce CO2. A single uranium fuel pellet used in a light-water reactor produces more heat than burning one ton of coal.
Composition Nuclear fuel used in fission reactors requires a fissile material, which can undergo a fission chain reaction driven by neutrons. Uranium-235 is the only naturally-occurring fissile isotope, found mixed with uranium-238 in natural uranium. The only other two fissile materials used as fuel are plutonium-239 and uranium-233, which are obtained through nuclear transmutation from uranium-238 and thorium-232, respectively.
Nuclear fuel can also contain fertile materials, such as or thorium, which can be converted into fissile material when it absorbs a neutron. A breeder reactor is a reactor that uses a fertile material to produce more fissile material than it consumes. Some reactors incorporate a fissile "seed" surrounded by a "blanket" of elements containing only fertile material called blanket fuel. Fuel elements that do contain fissile material, and thus drive the fission reaction, are called driver fuel.
The primary fuel for nuclear reactors is uranium. Pure uranium metal can be used, as well as many different uranium compounds such as oxides, alloys, nitride, or carbide. The most common fuel is uranium dioxide (), typically enriched to 35% uranium-235. Some reactors can also use natural (unenriched) uranium.
Production
Uranium is the only naturally-occurring fissile material on Earth. However, the fissile isotope 235U is present only in small quantity (0.7% of natural uranium). To make nuclear fuel, uranium is mined as ore, converted into uranium hexafluoride, and enriched to increase the amount of 235U present. The enriched uranium is then converted into oxide or metal form and fabricated into fuel elements. These fuel elements are finally packaged in cladding and shipped to a nuclear power plant for use.
Spent nuclear fuel contains residual fissile and fertile material, as well as bred fissile isotopes such as 239Pu or 233U. These potential fuel materials can be reprocessed into new nuclear fuel and used in a power plant, or can be disposed of as radioactive waste. The entire process of production, use, and recycling or disposal is known as the nuclear fuel cycle.
Oxide fuel
Ceramic metal oxide pellets, typically uranium dioxide, are the standard fuel for almost all reactors worldwide. Light water reactors (LWRs) use low-enriched uranium dioxide, while pressurized heavy-water reactors use natural uranium dioxide. Metal oxides have been the most successful power reactor fuels. In 2019, all commercial nuclear power reactors used oxide fuels.
One major advantage of oxide fuels is their high melting temperatures. Oxide fuels have excellent chemical stability, and exist in a single phase over a broad composition and temperature regime. They are also effective at retaining fission products during operation. Oxide fuels can also be doped with other elements to increase their radiation stability, including their fission gas retention. This increases their burnup and fuel efficiency. All three metal oxides of importance, , , and , also all have the same fluorite crystal structure.
Uranium dioxide Uranium dioxide () is a brittle, black, semiconducting solid. As of 2026, fuels more than 90% of nuclear power plants. It has excellent stability and is resists corrosion by water and steam. It also features high tolerance to radiation damage, allowing high fuel burnup. There are over 70 years of operating experience with in commercial reactors, so fabrication techniques are well-established. Uranium dioxide is easy to harness for nuclear power because uranium is naturally fissile. However, it has a complex conversion and fabrication process. also has only one crystal structure below its melting point, and thus has no expansion or contraction associated with phase change.
It is chemically stable at room temperature, and does not react with most elements below 500 °C. However, will oxidize to above 200 °C, and to above 500 °C. It can also self-ignite at small (less than 0.5 μm) particle sizes while at room temperature. melts around 3120 K, which is much higher than the melting point of pure uranium metal (1405 K). One major disadvantage of is its low thermal conductivity, which decreases further with increasing fuel temperature. Low thermal conductivity results in very high temperatures at the center of a fuel pellet.
can be fabricated directly from natural uranium, however most fuel pellets are made from low-enriched uranium. Enriched uranium is produced as uranium hexafluoride (), and must be converted into for fuel fabrication. There are three primary conversion methods: the integrated dry process, and two 'wet' processes using either ammonium diuranate (ADU) and ammonium uranyl carbonate (AUC). The dry process is the most commonly used process, and results in significantly less liquid chemical waste than the wet processes.
In the dry process, gas is hydrolyzed with steam to produce uranyl fluoride, which is then reduced with hydrogen gas in a fluidized bed chemical reactor to produce . The resulting powder is then dried in a rotary kiln.
The dry process results in very little chemical waste, as the HF gas can be easily reclaimed and recycled. It is also easy to automate.
The ADU method is the most commonly used process in China. It involves hydrolyzing into uranyl fluoride, followed by reaction with ammonia to produce ammonium diuranate (ADU). The ADU is washed, dried, and calcined to form , which is then reduced using hydrogen or ammonia to form . The ADU method is cumbersome, involves many steps, and produces significant quantities of radioactive liquid waste. The AUC process, first developed in Germany in the 1960s, is similar but involves ammonium uranyl carbonate instead of ADU. The AUC process produces high-quality particles which do not need to be granulated before pressing, but still produces large quantities of liquid waste. Natural uranium dioxide fuel is typically produced from natural via the ADU process.
Following conversion, the resulting powder is granulated and mixed with binding and lubricating agents, then pelleted and pressed into green ceramic pellets. The pellets are then sintered at high temperature in a furnace, before being precision ground to a tight dimensional tolerance.
The thermal conductivity of uranium dioxide is very low compared with that of zirconium metal, and it goes down as the temperature goes up. Corrosion of uranium dioxide in water is controlled by similar electrochemical processes to the galvanic corrosion of a metal surface.
While exposed to the neutron flux during normal operation in the core environment, a small percentage of the in the fuel absorbs excess neutrons and is transmuted into . rapidly decays into which in turn rapidly decays into . The small percentage of has a higher neutron cross section than . As the accumulates the chain reaction shifts from pure at initiation of the fuel use to a ratio of about 70% and 30% at the end of the 18 to 24 month fuel exposure period.
MOX
Mixed oxide, or MOX fuel, is a blend of plutonium and natural or depleted uranium which behaves similarly (though not identically) to the enriched uranium feed for which most nuclear reactors were designed. MOX fuel is an alternative to low enriched uranium (LEU) fuel used in the light water reactors which predominate nuclear power generation.
Adding plutonium to oxide fuel increases fissile concentration without requiring further enrichment, while allowing the use of plutonium recovered from spent nuclear fuel. MOX fuels have been used in European and Japanese LWRs, and represent approximately 2% of newly manufactured fuel as of 2026. Mixed-oxide fuels can also utilize thorium when combined with either enriched uranium or plutonium.
Some concern has been expressed that used MOX cores will introduce new disposal challenges, though MOX is a means to dispose of surplus plutonium by transmutation. Reprocessing of commercial nuclear fuel to make MOX was done in the Sellafield MOX Plant (England). As of 2015, MOX fuel is made in France at the Marcoule Nuclear Site, and to a lesser extent in Russia at the Mining and Chemical Combine, India and Japan. China plans to develop fast breeder reactors and reprocessing.
The Global Nuclear Energy Partnership was a U.S. proposal in the George W. Bush administration to form an international partnership to see spent nuclear fuel reprocessed in a way that renders the plutonium in it usable for nuclear fuel but not for nuclear weapons. Reprocessing of spent commercial-reactor nuclear fuel has not been permitted in the United States due to nonproliferation considerations. All other reprocessing nations have long had nuclear weapons from military-focused research reactor fuels except for Japan. Normally, with the fuel being changed every three years or so, about half of the is 'burned' in the reactor, providing about one third of the total energy. It behaves like and its fission releases a similar amount of energy. The higher the burnup, the more plutonium is present in the spent fuel, but the available fissile plutonium is lower. Typically about one percent of the used fuel discharged from a reactor is plutonium, and some two thirds of this is fissile (c. 50% , 15% ).
Metal fuel
Metal fuels are easy to fabricate, and have high thermal conductivity and fissile atom density. Metal fuels have a long history of use, stretching from the X-10 Graphite Reactor in 1943 to many test and research reactors. Pure uranium or plutonium metal can be used, however metal fuels are typically alloys. Metal fuels can also incorporate minor actinides as part of a closed nuclear fuel cycle. As with oxide fuel, metal fuel is typically contained within a cladding material, both to provide structural support and prevent fission product release into the reactor coolant.
Compared to oxides, metal fuels are easier to manufacture and reprocess, but suffer from greater swelling under irradiation. They have a lower melting point than ceramic fuels, and thus cannot survive equally high temperatures. Metal fuels swell more under irradiation because they retain fission gases better than oxide fuel. Lanthanide fission products also tend to migrate to the surface of the fuel and can react with ferrous cladding materials.
While metal fuels melt at a lower temperature compared to ceramics, their far higher thermal conductivity results in significantly lower peak fuel temperatures, preventing fuel melt. Metal fuel also has a large coefficient of thermal expansion, which serves as an inherent safety feature. As the fuel heats up, the expansion of the metal will increase neutron leakage and slow the reaction. Metals also have a lower Doppler coefficient, preventing reactivity from spiking during cooldown. Compared to ceramics, metal fuels have lower heat capacity, allowing the fuel to be readily cooled during an accident. Thus, even with lower melting temperature, the margin to failure of metal fuels can be significantly higher than ceramics.