Most nuclear reactors use uranium dioxide (UO2) as a fuel to create electricity. In 2014, 53 million pounds of uranium were used in commercial U.S. nuclear power reactors. These reactors generated 797 billion kilowatthours of electricity, or about 19% of total U.S. electricity in 2014.

The nuclear fuel cycle consists of front end steps that lead to the preparation of uranium for use in nuclear reactors and back end steps to safely manage, prepare, and dispose of highly radioactive spent nuclear fuel. Chemical processing of spent fuel material to recover remaining fissionable products for use in new fuel assemblies is technically feasible, but is not permitted in the United States.

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

The front end of the nuclear fuel cycle

Exploration

The nuclear fuel cycle starts with the exploration for uranium and the development of mines to extract discovered uranium ore. A variety of techniques are used to locate uranium including 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 further characterize the deposit.

Uranium mining

When economically recoverable ore deposits are located, the next step in the fuel cycle is to mine the ore using either underground or open pit mining techniques, or by using in-place (in-situ) solution mining or heap leaching that uses liquid solvents to dissolve and extract the uranium from ore.

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). This process extracts uranium that is found as a coating in the sand and gravel particles of groundwater reservoirs. The uranium is extracted by exposing the particles to a solution with a pH that has been elevated slightly 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. Heap leaching involves spraying an acidic liquid solution on piles of crushed uranium ore, which drains down and leaches uranium out of the rock and is recovered from beneath the pile. Heap leaching is no longer used in the United States.

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 that is then reacted with chemicals to separate the uranium from other minerals. Groundwater from solution mining operations is circulated through a resin bed to extract and concentrate the uranium.

The concentrated uranium product is typically a bright yellow or orange powder called yellowcake (U3O8). 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 involves the conversion of yellowcake into uranium hexafluoride (UF6) gas. This step is required because there are three forms (isotopes) of uranium that occur in nature: 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. To perform this atomic segregation, the uranium in yellowcake is first converted into a gaseous compound (UF6) from which individual uranium atoms can be sorted.

Uranium enrichment

The uranium hexafluoride gas coming from the converter facility is called natural UF6 because the original concentrations of uranium isotopes are unchanged. This gas is sent to an enrichment plant where the isotope separation takes place. The United States currently has two operating enrichment plants. One uses a process called gaseous diffusion to separate uranium isotopes, and the other uses a gas centrifuge process. Because the smaller U-235 atoms travel slightly faster than U-238 atoms, they tend to leak (diffuse) faster through porous membrane walls of a diffuser, where they are collected and concentrated. The final product has about a 4% to 5% concentration of U-235 and is called enriched UF6. 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.

Another enrichment technique is the gas centrifuge process, where UF6 gas is spun at high speed in a series of cylinders to separate 235UF6 and 238UF6 atoms based on their different atomic masses. Atomic vapor laser isotope separation (AVLIS) and molecular laser isotope separation (MLIS) are new enrichment technologies currently being developed. These laser-based enrichment processes can achieve higher initial enrichment (isotope separation) factors than the diffusion or centrifuge processes and are capable of operating at high material throughput rates.

Uranium reconversion and nuclear fuel fabrication

The next step in the production of nuclear fuel takes place at one of the five U.S. nuclear reactor fuel fabrication facilities, where the enriched UF6 gas is reacted to form a black uranium dioxide 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, there are about 179 to 264 fuel rods in each fuel assembly. A typical reactor core holds 121 to 193 fuel assemblies.

At the reactor

A nuclear fuel assembly
A nuclear fuel assembly

Source: Commissariat √† l'√Čnergie Atomique (public domain)

Following fabrication, the fuel assemblies are transported by truck to the reactor sites where they are stored onsite in fresh fuel storage bins until they are needed by the reactor operators. At this stage, the uranium is only mildly radioactive, and essentially all radiation is contained within the metal tubes. Typically, about one third of the reactor core (40 to 90 fuel assemblies) is changed out every 12 to 24 months.

The reactor core itself is a cylindrical arrangement of the fuel bundles, about 12 feet in diameter and 14 feet high. It is 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 is sufficient to initiate the nuclear reaction.

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 stored under water in a spent fuel pool at the reactor for several years. Even though the fission reaction has stopped, the spent fuel continues to give off heat from the decay of radioactive elements that were created when the uranium atoms were split apart. The water in the pool serves to both cool the fuel and block the release of radiation. As of 2002, there were more than 165,000 spent fuel assemblies stored in about 70 interim storage pools throughout the United States.

The spent fuel cools in a few years in the pool and then may be moved to a dry cask storage container for storage at the power plant site. An increasing number of reactor operators now store their older spent fuel in these special outdoor concrete or steel containers with air cooling.

The final step in the nuclear fuel cycle is the collection of 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.