The nuclear fuel cycle consists of two stages. front end And that backend. The front-end step prepares the uranium for use in the reactor. Back-end procedures ensure that it is used or SpentHowever, it is still highly radioactive, so nuclear fuel is safely managed, prepared, and disposed of.
In nuclear power plants, a certain type of uranium (U-235) is primarily used for fission because the atoms are easy to split. Although uranium is about 100 times more common than silver, U-235 is relatively rare, making up just over 0.7% of natural uranium. U-235 is produced from uranium ore in uranium plants or separated from slurry in in-situ leaching facilities. uranium concentrate, can be used as fuel. The uranium concentrate is first processed at a conversion and enrichment facility to increase the level of U-235 in the uranium to 3% to 5%, and then processed into reactor fuel pellets and fuel rods at a reactor fuel manufacturing plant. Masu.
Nuclear fuel is loaded into a nuclear reactor and used until the fuel assembly becomes so radioactive that it must be removed for temporary storage or eventual disposal. Spent fuel material can also be processed to recover any remaining uranium that can be fissioned again in new fuel assemblies (spent fuel reprocessing), but this is not allowed in the United States.
Source: Penn State Radiation Science and Engineering Center (Public Domain)
Front end of nuclear fuel cycle
expedition
The nuclear fuel cycle begins with the search for uranium and the development of mines to extract uranium ore. A variety of techniques are used to locate uranium, including airborne radiometric surveys, chemical sampling of groundwater and soil, and test drilling to understand the underlying geology. Once a uranium deposit is found, mine developers typically follow it up at closer intervals. Fill it up, Or, conduct development drilling to determine how much uranium is available and how much it would cost to recover it.
uranium mining
- underground mining
- open pit mining
- In-place solution mining
- heap leaching
Prior to 1980, most uranium in the United States was produced using open pit and underground mining techniques. Currently, most uranium in the United States is produced using a technique commonly referred to as “solution mining.” in situ leaching (ISL) or On-site recovery (ISR) Mining. This process extracts the uranium that covers sand and gravel particles in groundwater reservoirs. Sand and gravel particles are exposed to a solution that uses oxygen, carbon dioxide, or caustic soda to slightly increase the pH. Uranium is dissolved in groundwater, pumped from reservoirs and processed in uranium plants. Another similar process, heap leaching, involves spraying an acidic liquid solution onto a pile of crushed uranium ore. The solution flows downward through the crushed ore, leaching the uranium from the rock and recovering it from beneath the pile. Heap leaching is no longer used in the United States.
Source: U.S. Nuclear Regulatory Commission (Public Domain)
uranium crushing
Uranium ore is extracted from open pit or underground mines and then refined into uranium concentrate in uranium factories. The ore is crushed, ground and ground into a fine powder. When chemicals are added to the fine powder, a reaction occurs that separates the uranium from other minerals. Groundwater from solution mining operations is circulated through a resin bed to extract and concentrate uranium.
Despite the name, enriched uranium products are usually black or brown substances called uranium. yellow cake (U3○8).Mined uranium ore typically yields 1 to 4 pounds of uranium3○8 or 0.05% to 0.20% yellow cake per ton of ore. Solid waste from pit and underground mining operations is factory tailings. The treated water obtained 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 the yellowcake to uranium hexafluoride (UF).6) Conversion facility gas. Three forms (isotopes) of uranium occur naturally: U-234, U-235, and U-238. Current U.S. reactor designs require stronger concentrations (enrichments) of the U-235 isotope to operate efficiently. The uranium hexafluoride gas produced in converter facilities is called natural UF.6 This is because the original concentration of uranium isotopes does not change.
uranium enrichment
UF after conversion6 The gas is sent to an enrichment plant where individual uranium isotopes are separated to produce enriched UF.6containing U-235 at a concentration of 3% to 5%.
Two types of uranium enrichment processes are used in the United States: gas diffusion and gas centrifugation. There is one enrichment plant in operation in the United States, which uses a gas centrifugation process. Reinforced UF6 It is sealed in canisters, allowed to cool and solidify, and then transported by train, truck, or barge to a nuclear reactor fuel assembly plant.
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 diffusion or centrifugation processes and can produce enriched uranium more quickly than other techniques.
Uranium reconversion and nuclear fuel processing
Once uranium is enriched, it is ready to be converted into nuclear fuel. At nuclear fuel production facilities, UF6It is heated in the solid state to become gaseous and then UF is produced.6 The gas is chemically processed to form uranium dioxide (UO).2) powder. The powder is then compressed and formed into small ceramic fuel pellets. The pellets are stacked and enclosed in long metal tubes about 1 centimeter in diameter to form fuel rods. Next, the fuel rods are bundled together to form a fuel assembly. Depending on the reactor type, each fuel assembly has between 179 and 264 fuel rods. A typical reactor core holds between 121 and 193 fuel assemblies.
in a nuclear reactor
Once the fuel assemblies are manufactured, trucks transport them to the reactor site. Fuel assemblies are stored on site. fresh fuel Store it in the storage box until needed by the reactor operator. At this stage, uranium is only mildly radioactive, and essentially all the radiation is contained within the metal tube. Typically, reactor operators replace 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 fuel bundles approximately 12 feet in diameter and 14 feet tall housed in a steel pressure vessel with walls several inches thick. A nuclear reactor core has essentially no moving parts, except for a few control rods that are inserted to control the fission reaction. When the fuel assemblies are placed next to each other and water is added, a nuclear reaction begins.
Backend of nuclear fuel cycle
Interim storage and final disposal in the United States
After being used in a nuclear reactor, the fuel assemblies become highly radioactive and must be removed and submerged in a water pool on the reactor site for several years. Although the fission reaction has stopped, the spent fuel continues to release heat from the decay of radioactive elements created when the uranium atoms split. The water in the spent fuel pool serves to cool the fuel and prevent it from emitting radiation. From 1968 to December 31, 2017, a total of 276,879 fuel assemblies were released and stored at the sites of 119 closed and operating commercial nuclear reactors in the United States.
Within a few years, the spent fuel could be cooled in a pool and transferred to dry cask storage on the power plant site. Many reactor operators store old spent fuel in these special air-conditioned concrete or steel containers.
The final step in the nuclear fuel cycle is the collection of spent fuel assemblies from intermediate storage sites and final disposal in a permanent underground repository. The United States currently has no permanent underground repository for high-level nuclear waste.
Last updated: October 26, 2023.
