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Nuclear explained The nuclear fuel cycle

The nuclear fuel cycle consists of two phases: the front end and the back end. Front-end steps prepare uranium for use in nuclear reactors. Back-end steps ensure that used—or spent—but still highly radioactive, nuclear fuel is safely managed, prepared, and disposed of.

Nuclear power plants primarily use a specific type of uranium (U-235) for nuclear fission because its atoms are easily split apart. Although uranium is about 100 times more common than silver, U-235 is relatively rare, at just over 0.7% of natural uranium. The U-235 is separated from uranium ore at uranium mills or from a slurry at in-situ leaching facilities to produce uranium concentrate, which can be used as a fuel. The uranium concentrate is first processed in conversion and enrichment facilities to increase the level of U-235 in the uranium to 3%–5%, and then in reactor fuel fabrication plants, where it is made into reactor fuel pellets and fuel rods.

Nuclear fuel is loaded into reactors and used until the fuel assemblies become highly radioactive and must be removed for temporary storage and eventual disposal. Spent fuel material could be processed to recover any remaining uranium that could undergo fission again in a new fuel assembly (spent fuel reprocessing), but it is not permitted in the United States.

Nuclear Fuel Cycle

Source: Pennsylvania State University Radiation Science and Engineering Center (public domain)

The front end of the nuclear fuel cycle


The nuclear fuel cycle starts with exploring for uranium and developing mines to extract uranium ore. A variety of techniques are used to locate uranium, such as airborne radiometric surveys, chemical sampling of groundwater and soils, and exploratory drilling to understand the underlying geology. Once uranium ore deposits are located, the mine developer usually follows up with more closely spaced in fill, or development drilling, to determine how much uranium is available and what it might cost to recover it.

Uranium mining

When mine developers find ore deposits that are economically feasible to recover, the next step in the fuel cycle is to mine the ore using one of the following techniques:

  • Underground mining
  • Open pit mining
  • In-place (in-situ) solution mining
  • Heap leaching

Before 1980, most U.S. uranium was produced using open pit and underground mining techniques. Today, most U.S. uranium is produced using a solution mining technique commonly called in-situ-leach (ISL) or in-situ-recovery (ISR) mining. This process extracts the uranium that coats the sand and gravel particles of groundwater reservoirs. The sand and gravel particles are exposed to a solution with a pH that has been elevated slightly by using oxygen, carbon dioxide, or caustic soda. The uranium dissolves into the groundwater, which is pumped out of the reservoir and processed at a uranium mill. Another similar process, heap leaching, involves spraying an acidic liquid solution onto piles of crushed uranium ore. The solution drains down through the crushed ore and leaches uranium out of the rock, which is recovered from underneath the pile. Heap leaching is no longer used in the United States.

Diagram of the in situ recovery process

Source: United States Nuclear Regulatory Commission (public domain)

did youknow


In 2022, about 44.4 million pounds of uranium (U3O8 equivalent) were loaded into commercial U.S. nuclear power reactors.

Uranium milling

After the uranium ore is extracted from an open pit or underground mine, it is refined into uranium concentrate at a uranium mill. The ore is crushed, pulverized, and ground into a fine powder. Chemicals are added to the fine powder, which causes a reaction that separates the uranium from the other minerals. Groundwater from solution mining operations is circulated through a resin bed to extract and concentrate the uranium.

Diagram of a Typical Conventional Uranium Mill

Despite the name, the concentrated uranium product is typically a black or brown substance called yellowcake (U3O8). Mined uranium ore typically yields one to four pounds of U3O8 per ton of ore, or 0.05% to 0.20% yellowcake. The solid waste material from pit and underground mining operations is called mill tailings. The processed water from solution mining is returned to the groundwater reservoir where the mining process is repeated.

Uranium conversion

The next step in the nuclear fuel cycle is to convert yellowcake into uranium hexafluoride (UF6) gas at a converter facility. Three forms (isotopes) of uranium occur naturally: U-234, U-235, and U-238. Current U.S. nuclear reactor designs require a stronger concentration (enrichment) of the U-235 isotope to operate efficiently. The uranium hexafluoride gas produced in the converter facility is called natural UF6 because the original concentrations of uranium isotopes are unchanged.

Uranium enrichment

After conversion, the UF6 gas is sent to an enrichment plant where the individual uranium isotopes are separated to produce enriched UF6, which has a 3% to 5% concentration of U-235.

Two types of uranium enrichment processes have been used in the United States: gaseous diffusion and gas centrifuge. The United States has one operating enrichment plant, and it uses a gas centrifuge process. Enriched UF6 is sealed in canisters and allowed to cool and solidify before it is transported to a nuclear reactor fuel assembly plant by train, truck, or barge.

Atomic vapor laser isotope separation (AVLIS) and molecular laser isotope separation (MLIS) are new enrichment technologies under development. These laser-based enrichment processes can achieve higher initial enrichment (isotope separation) factors than the diffusion or centrifuge processes and can produce enriched uranium more quickly than other techniques.

Uranium reconversion and nuclear fuel fabrication

Once the uranium is enriched, it is ready to be converted into nuclear fuel. At a nuclear fuel fabrication facility, the UF6, in solid form, is heated to gaseous form, and then the UF6 gas is chemically processed to form uranium dioxide (UO2) powder. The powder is then compressed and formed into small ceramic fuel pellets. The pellets are stacked and sealed into long metal tubes that are about 1 centimeter in diameter to form fuel rods. The fuel rods are then bundled together to make up a fuel assembly. Depending on the reactor type, each fuel assembly has 179 to 264 fuel rods. A typical reactor core holds 121 to 193 fuel assemblies.

At the reactor

Once the fuel assemblies are fabricated, trucks transport them to the reactor sites. The fuel assemblies are stored onsite in fresh fuel storage bins until the reactor operators need them. At this stage, the uranium is only mildly radioactive, and essentially all radiation is contained within the metal tubes. Typically, reactor operators change out about one-third of the reactor core (40 to 90 fuel assemblies) every 12 to 24 months.

The reactor core is a cylindrical arrangement of the fuel bundles that is about 12 feet in diameter and 14 feet tall and encased in a steel pressure vessel with walls that are several inches thick. The reactor core has essentially no moving parts except for a small number of control rods that are inserted to regulate the nuclear fission reaction. Placing the fuel assemblies next to each other and adding water initiates the nuclear reaction.

A nuclear fuel assembly

A nuclear fuel assembly

Source: Alternative Energies and Atomic Energy Commission, France (public domain)

The back end of the nuclear fuel cycle

Interim storage and final disposal in the United States

After use in the reactor, fuel assemblies become highly radioactive and must be removed and submerged in a pool of water for several years at the reactor site. Although the fission reaction has stopped, the spent fuel continues to give off heat from the decay of the radioactive elements that were created when the uranium atoms were split apart. The water in the spent fuel pool serves to both cool the fuel and block the release of radiation. From 1968 through December 31, 2017, a total of 276,879 fuel assemblies had been discharged and stored at the sites of 119 closed and operating commercial nuclear reactors in the United States.

Within a few years, the spent fuel cools in the pool and may be moved to a dry cask storage container at the power plant site. Many reactor operators store their older, spent fuel in these special air-conditioned concrete or steel containers.

The final step in the nuclear fuel cycle is to collect the spent fuel assemblies from the interim storage sites for final disposition in a permanent underground repository. The United States currently has no permanent underground repository for high-level nuclear waste.

Last updated: October 26, 2023.