The Nuclear Fuel Cycle
The IAEA defines the nuclear fuel cycle as “an industrial process involving various activities to produce electricity from uranium in nuclear power reactors. The cycle starts with the mining of uranium and ends with the disposal of spent fuel and other radioactive waste.”¹ The cycle consists of two phases: the front-end, which prepares uranium for use in nuclear reactors; and the back-end, which ensures that used fuel is safely managed and disposed of. Between these two phases, electricity is generated using the uranium fuel by means of nuclear reaction, which is known as the ‘service period.’
In some countries, the spent fuel is reprocessed and partially reused. This is known as a closed nuclear fuel cycle, as opposed to an 'open' or 'once-through' fuel cycle where fuel is not reprocessed and is instead stored or disposed. Additionally, the nuclear fuel cycle encompasses the transportation of fuel materials between processing stages.
Front-end
This consists of mining, milling, conversion, enrichment, and fuel fabrication.
Mining
Uranium is a relatively common, slightly radioactive metal found throughout the world in rocks, soils, rivers, and sea water. It is about as common as tin, and 500 times more abundant than gold. The largest producers are Australia, Canada, and Kazakhstan.³
There are several methods of uranium mining. Open-pit mining involves digging a large pit or quarry to access the ore. Underground mining uses shafts and tunnels to access deeper deposits. In-situ leach mining (ISL) involves a chemical process where an acid or alkaline solution is passed through underground deposits via wells or bores, and the uranium is brought to the surface for processing.
Milling
The process to extract the uranium from the ore, usually done near the extraction site. The ore is crushed finely and through chemical processes, often involving sulfuric acid, and the uranium is leached from the ground material. The final result is ‘yellowcake’, a power form of uranium oxide U3O8 and the uranium concentration is raised to about 80%. This is the form that it is sold, packed, and shipped to a conversion facility.
Conversion
The process in which yellowcake is converted into a usable form for either fuel fabrication or enrichment. Because some (but few) nuclear power plants don't require enriched uranium, U3O8 is converted directly into UO2 to be used as fuel. The majority of nuclear power plants require enrichment, however, so, the next step requires the material to be converted into gaseous form - UF6. As a gas, UF6 leaves the production process, is subsequently cooled to become a liquid, and is transferred into 14-ton cylinders designed for storage and transport. Over a period of five days, the UF6 cools further, solidifying within the cylinder. Once in solid form, the UF6 can be transported to an enrichment facility.⁵
Enrichment
Natural uranium consists primarily of two isotopes. 99.3% of natural uranium is the non-fissile isotope, U-238, while 0.7% is the fissile isotope U-235. Because most nuclear power plants require a concentration of 3-5% U-235, the proportion of this isotope must be increased - the process of enrichment.
The main type of enrichment used today is by means of gas centrifuge. This process utilises centripetal force in a series of spinning centrifuges to push heavier isotopes (U-238) to cylinder walls increasing the concentration of lighter isotopes (U-235) toward the centre. Another method that was previously used but is now being phased out is gaseous diffusion. This is where UF6 is pumped through porous membranes allowing lighter isotopes (U-235) to pass through more easily than heavier isotopes (U-238). A method of laser isotope separation is under development but not currently in use.⁶ After enrichment, the UF6 is converted to uranium dioxide (UO2) for fuel fabrication.
Fuel fabrication
All current generation nuclear power plants use uranium dioxide (UO2) for fuel.¹ The uranium dioxide powder is pressed to form small fuel pellets and heated at high temperatures into a ceramic material in a process called sintering. One half-inch uranium fuel pellet can produce as much energy as approximately 3.5 barrels (556 litres) of oil, 17,663,000 BTUs of natural gas, or 1 tonne of coal.⁸ After sintering, the pellets are milled into exact size and shape, and loaded into metal tubes made of zirconium alloys known as fuel rods.¹ Depending on the type of reactor, between 90 and 200 fuel rods are grouped together to form assemblies, which are loaded into the reactor where they remain for several years.⁹
Electricity generation
Much like gas or coal fired power plants, nuclear power plants produce electricity by heating water to produce steam, which at extremely high temperatures and pressures, rotates a turbine, enabling a generator to produce electricity. The source of heat, however, is produced by a nuclear reaction instead of a chemical reaction. When the nucleus of a Uranium-235 atom absorbs a neutron, it can split into two smaller atoms, releasing energy along with additional neutrons. These neutrons can sustain a chain reaction, which can be carefully controlled to produce a desired amount of energy.
Back-end
This consists of spent fuel storage, reprocessing, and managing radioactive waste disposal.
Spent fuel storage
Nuclear fuel is typically used in a nuclear reactor for three to six years, and on an annual basis, 25-30% of the fuel is unloaded and replaced with new fuel.¹ As fission fragments and heavy elements accumulate over time, the fuel becomes impractical for continued use. Once removed from the reactor, it emits radiation and heat, requiring interim underwater storage to safely contain the radiation during cooling. After this period, the fuel can either be reprocessed to recycle usable portions or transferred to another pool for wet storage. Alternatively, it may be stored long-term in air-cooled, shielded casks within specialised facilities for dry storage.¹⁰ ¹
Reprocessing
The United States does not reprocess nuclear waste but other countries allow reprocessing because it offers a sustainable way to recycle fuel and significantly reduce waste. Once removed, approximately 96% of this spent fuel still consists of uranium, with less than 1% in its fissionable U-235 form, and around 1% plutonium and 3% high-level radioactive waste products.¹⁰ Reprocessing enables the recycling of uranium and plutonium by dissolving the fuel and chemically separating it into these three components: uranium, plutonium, and high-level waste.¹
The recovered uranium can be converted and re-enriched, allowing it to re-enter the fuel cycle as ‘recovered uranium’ much like newly mined uranium. Meanwhile, the separated plutonium can be combined with uranium to create mixed oxide (MOX) fuel, reducing the need for uranium enrichment and minimising the generation of depleted uranium. The remaining high-level waste is vitrified—encapsulated in glass—for long-term disposal in a high-level waste facility. Currently, about one-third of discharged nuclear fuel undergoes reprocessing.1
Disposal
Managing nuclear waste is a critical concern for the nuclear industry, requiring careful classification by radioactivity level and appropriate disposal methods. Low-level waste, such as contaminated clothing and tools, is disposed of in shallow land burial sites, while intermediate-level waste, including resins and sludges, requires shielding and deep geological storage.
High-level waste, comprising highly radioactive spent fuel and reprocessing byproducts, demands long-term solutions, typically in deep geological repositories that isolate the waste for thousands of years.
Footnotes
International Atomic Energy Agency, Nuclear Fuel Cycle, accessed November 2024, https://www.iaea.org/sites/default/files/18/10/nuclearfuelcycle.pdf.
U.S. Energy Information Administration, "The Nuclear Fuel Cycle," accessed November 2024, https://www.eia.gov/energyexplained/nuclear/the-nuclear-fuel-cycle.php.
World Nuclear Association, "Fast Neutron Reactors," accessed November 2024, https://world-nuclear.org/information-library/current-and-future-generation/fast-neutron-reactors.
International Atomic Energy Agency, "IAEA Facebook Photo," accessed November 2024, https://www.facebook.com/iaeaorg/photos/a.131323982061/10155643351257062/?type=3&theater.
U.S. Nuclear Regulatory Commission, "Uranium Conversion," accessed November 2024, https://www.nrc.gov/materials/fuel-cycle-fac/ur-conversion.html.
U.S. Nuclear Regulatory Commission, "Uranium Enrichment," accessed November 2024, https://www.nrc.gov/materials/fuel-cycle-fac/ur-enrichment.html.
Alex Wellerstein, "Elusive Centrifuges," Restricted Data: The Nuclear Secrecy Blog, June 1, 2012, accessed November 2024, https://blog.nuclearsecrecy.com/2012/06/01/friday-images-elusive-centrifuges/.
Nuclear Energy Institute, "Nuclear Fuel," accessed November 2024, https://www.nei.org/fundamentals/nuclear-fuel#:~:text=Uranium%20is%20an%20abundant%20metal,processed%20to%20create%20nuclear%20fuel.
World Nuclear Association, "How Is Uranium Made Into Nuclear Fuel?" accessed November 2024, https://world-nuclear.org/nuclear-essentials/how-is-uranium-made-into-nuclear-fuel#:~:text=The%20making%20of%20nuclear%20fuel,make%20a%20hard%20ceramic%20material.
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