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 238U 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 (UO2), typically enriched to 3–5% 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, UO2, PuO2, and ThO2, also all have the same fluorite crystal structure.
Uranium dioxide Uranium dioxide (UO2) is a brittle, black, semiconducting solid. As of 2026, UO2 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 UO2 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. UO2 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, UO2 will oxidize to UO3 above 200 °C, and to U3O8 above 500 °C. It can also self-ignite at small (less than 0.5 μm) particle sizes while at room temperature. UO2 melts around 3120 K, which is much higher than the melting point of pure uranium metal (1405 K). One major disadvantage of UO2 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.
UO2 can be fabricated directly from natural uranium, however most UO2 fuel pellets are made from low-enriched uranium. Enriched uranium is produced as uranium hexafluoride (UF6), and must be converted into UO2 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, UF6 gas is hydrolyzed with steam to produce uranyl fluoride, which is then reduced with hydrogen gas in a fluidized bed chemical reactor to produce UO2. The resulting UO2 powder is then dried in a rotary kiln.
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