EIA reported in 2020 that 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.
[The federal NURE program of the mid-1970s provided a national uranium resources evaluation to assist the uranium exploration and mining companies to locate, and then to evaluate by drilling, coring, and sampling to establish uranium ore reserves for possible mining given the economic incentive. This depended on the price of yellow cake, either on the spot market or via contract prices over a set time at a set price. the latter was usually higher than the spot price, but not always in the past.]
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, and
- 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 within 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 involved spraying an acidic liquid solution onto piles of crushed uranium ore underlain by an engineered pad of polypropylene or other materials to protect leakage to the shallow groundwater. The solution drains down through the crushed ore and leaches uranium out of the rock, which is recovered via a system of piping underneath the pile that lead to the processing plant. Heap leaching is no longer used in the United States.
Uranium Processing/Milling
After in-situ leaching, the uranium fluid is piped to the processing plant. Groundwater containing dissolved uranium from solution mining operations is piped to the processing plant and circulated through a resin bed to extract and concentrate the uranium. The processed water from solution mining is returned to the groundwater reservoir where the hydrochemistry is adjusted to allow the in-situ mining process to be repeated. At the end of mining, the groundwater hydrochemistry is adjusted to remove oxygen which immobilizes the uranium from solution back into a solid as it existed prior to mining.
Other types of ore bodies require the uranium ore to be extracted from an open pit or underground mine. It is transported by truck or other conveyance to be refined into uranium concentrate at a uranium mill. There, 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.
Despite the name, the concentrated uranium product by surface mining and underground mining is typically a black or brown substance, but the uranium produced by situ mining operations are generally a bright yellow, otherwise 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. However in recent years major hard-rock uranium discoveries in Canada have produced ore grades in open pit and underground deposits in the range of 10% to 20% U3O8 over significant thicknesses, with an average ore grade of about 5% U3O8, which is equivalent to about 100 pounds of U3O8 per ton of ore.
In surface and underwound mining, there is a solid waste material called mill tailings, which needs to be managed by sound engineered embankments, covered, and re-seeded as remediation in the past. In-situ mining and processing has fluid waste to deal with and these are disposed down deep disposal well a few thousand feet below the drinking water aquifers above.
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 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 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.
Within 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.
Interim Storage 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 within the reactor. The water in the pool serves to both cool the waste fuel pellets and block the release of radiation. From 1968 through June 2013, 241,468 fuel assemblies had been discharged and stored at 118 commercial nuclear power plants 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 because the federal government has not provided a storage facility as was required by law in the 1980s. The Yucca Mountain facility was constructed at great expense in Nevada, but has not been permitted for operation to date because of hydrogeological issues.
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, so most of the reactor waste is being stored on site. However, much of the nuclear waste maybe become useful one day when so-called "breeder reactor" designs are brought into operation, which can consume the high-level radiation (aka heat) of the waste by converting it into useful energy production in the form of electricity, as in standard nuclear power plants.
[Current day issues of uranium exploration and mining, and some history of the nuclear industry was discussed in a recent conference:
Here is the PDF summary (w/hyperlinks): https://i2mconsultants.com/downloads/corp/UCOM-MidYearReport2020.pdf ]
Managed & Built by I2M Consulting, LLC | © 2010-
