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 exploration for uranium and the development of 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 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.
Source: United States Nuclear Regulatory Commission (public domain)
Did you know?
In 2017, about 45.5 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.
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 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. 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 4% 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 currently has one operating enrichment plant, which 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 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. 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 about 179 to 264 fuel rods. A typical reactor core holds 121 to 193 fuel assemblies.
At the reactor
Source: Alternative Energies and Atomic Energy Commission, France (public domain)
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.
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 at the reactor site 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 the 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 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. 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.