Most nuclear reactors use uranium dioxide (UO2) as a fuel to create electricity. In 2015, about 47 million pounds of uranium were loaded into commercial U.S. nuclear power reactors. These reactors generated 797 billion kilowatthours of electricity, or about 20% of total U.S. electricity in 2015.
The nuclear fuel cycle consists of front end steps that prepare 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 any remaining product that could undergo fission again in a new fuel assembly is technically feasible, but is not permitted in the United States.
The front end of the nuclear fuel cycle
The nuclear fuel cycle starts with exploration for uranium and the development of mines to extract the 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.
When ore deposits that are economically feasible to recover are located, 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). This process extracts 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. 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.
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.
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.
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 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 separate the three types of uranium isotopes, the UF6 gas is sent to an enrichment plant where the individual uranium isotopes are separated.
The uranium hexafluoride gas produced in the converter facility is called natural UF6 because the original concentrations of uranium isotopes are unchanged. The United States currently has two operating enrichment plants (where isotope separation takes place). One plant uses a process called gaseous diffusion to separate uranium isotopes, and the other plant uses a gas centrifuge process. Because the smaller U-235 isotopes travel slightly faster than U-238 isotopes, they tend to leak (diffuse) faster through the 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 isotopes based on their different atomic masses. Atomic vapor laser isotope separation (AVLIS) and molecular laser isotope separation (MLIS) are new enrichment technologies currently 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. The United States has five 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, each fuel assembly has about 179 to 264 fuel rods. A typical reactor core holds 121 to 193 fuel assemblies.
At the reactor
Once they are fabricated, trucks transport the fuel assemblies 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, about 12 feet in diameter and 14 feet high, that 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 initiates 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 at the reactor under water in a spent fuel pool 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. From 1968 through June 2013, 241,468 fuel assemblies had been discharged and stored at 118 commercial nuclear reactors operating 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 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.