The nuclear fuel cycle represents how the nuclear fuel progresses from creation to disposal. It also describes the series of industrial activities involved in the generation of electricity or energy production from nuclear power, thus resembling the upstream, midstream, and downstream sectors of the oil and gas industry.
The Major Stages of the Nuclear Fuel Cycle
1. Front-End Stage
The front-end stage of the nuclear fuel cycle centers primarily on the preparation of nuclear fuel from a source material. In nuclear fission, source materials include uranium ore and thorium ore. There are more specific steps or activities within the front-end stage. Take note of the following activities involved in uranium preparation:
• Exploration: The cycle starts with activities aimed at locating the sources of uranium ore and the developing of mines for extraction. There are several methods for locating uranium, which include airborne radiometric surveys, sampling of soil and groundwater, and analysis of the underlying geology through explorative drilling. Upon determining the presence of uranium deposits, further analyses are needed to identify economic feasibility based on the quantity of the deposits and corresponding extraction costs.
• Uranium Mining: If there is a substantial amount of uranium deposits in a given area, and if it is cost-effective to extract those, then the next step of the front-end stage of the cycle is mining through either one of the following methods: underground mining, open-pit mining, in-place or in-situ mining, and heap leaching.
• Uranium Milling: Mined uranium ores need to undergo a refining process to extract further the uranium and convert it into a more concentrated form. Milling involves the grinding of ores into a fine powder and the application of chemical solutions to separate uranium from other minerals. The resulting product is a uranium oxide called yellowcake that typically contains 70 to 90 percent triuranium octoxide.
• Conversion: Yellowcake is still a raw product. It is not readily usable as a fuel for a nuclear reactor. To be specific, its triuranium octoxide variant requires further processing to convert it to uranium hexafluoride for use as an input stock in common enrichment facilities.
• Uranium Enrichment: Upon conversion of the yellowcake, the resulting product needs further enrichment to make it a suitable nuclear fuel. The natural uranium hexafluoride contains 99.28 percent U-238 and 0.71 percent U-235 uranium isotopes. An enriched uranium hexafluoride contains a higher concentration of U-235. Note that reactors require higher amounts of U-235 to sustain a nuclear chain reaction. Light water reactors require a uranium fuel with 3.5 percent of U-235 while others require a higher U.235 concentration.
• Fabrication: U-235 enriched uranium hexafluoride is converted further into uranium dioxide powder, which is then processed further into a ceramic pellet form through high-temperature compression. These pellets are stacked into tubes made of corrosion-resistant metal alloy. The pellet-containing tubes are called fuel rods, and their specifications vary depending on the design specifications of a particular nuclear reactor.
2. Service Period
The service period of the nuclear fuel cycle primarily involves the actual usage of the nuclear fuel during the operation of a nuclear reactor. Take note that nuclear fuel is used not only in electricity generation but also in research and naval propulsion. Nonetheless, there are more specific activities within the service period stage.
• Transportation: Moving the fuel assemblies or other materials from one point to another is an integral part of the cycle. Because mining sites, as well as conversion and enrichment facilities, are relatively few when compared to the number of nuclear reactors around the world, specialized transportation activities are needed to maintain supply-demand efficiency and manage the risks associated with radiation exposure and contamination.
• In-Core Management: Once transported into a nuclear reactor facility, the fuel assemblies are stored onsite until they are needed in operation. A reactor needs to be loaded with new fuel rods every 12 to 24 months. However, because each fuel depletes at different rates and different places inside the reactor, not all fuel rods are replaced. Operators optimize the arrangement of fuel assemblies to minimize fuel-cycle costs. Nonetheless, all of these processes collectively comprise the in-core management activity.
3. Back-End Stage
Spent nuclear fuel is the primary byproduct of nuclear reactor operations. Its radioactivity requires proper disposal and if possible, reprocessing. This is the back-end stage of the nuclear fuel cycle.
• Interim Storage: Spent fuel rods are stored in a liquid solution within the reactor facility or a separate facility away from the reactor site. The purpose of interim storage is to cool the fuel and shield the external environment from radiation emissions by immersing it in water or boric acid. Within a few years and once the spent fuel rods cool down, they can be moved to a dry casket storage container.
• Final Disposal: An underground repository is used for the final and permanent disposal of spent fuel and other nuclear wastes collected from different interim storage facilities. The purpose of final disposal is to make these nuclear wastes inaccessible to most people while also containing their radioactivity.
• Reprocessing: Spent uranium fuel contains relevant quantities of fissile U-25 uranium isotopes and Pu-239 plutonium isotope, as well as fertile U-238 uranium isotope. These materials can be chemically separated and recovered from the spent fuel for use as nuclear fuel. One notable product of spent fuel reprocessing is MOX fuel. Composed of reprocessed uranium and plutonium, as well as depleted uranium, MOX fuel behaves similarly to enriched uranium feed and serves as an alternative to low-enriched uranium used in light water reactors.
A Note on the Nuclear Fuel Cycle
The stages and corresponding activities within the nuclear fuel cycle are not entirely sequential. Some of these activities have considerable overlaps. For example, transportation activities occur across the front-end, service period, and back-end stages. Reprocessing activities also involve some aspects of conversion, enrichment, and fabrication.
Moreover, some argue that the term “nuclear fuel chain” is more accurate when describing the processes or activities involved in the production, consumption, and disposal of nuclear fuel. Spent fuel is never fully recycle after all.
Depending on how spent nuclear fuel is managed during the back-end stage, the nuclear fuel cycle can either be an open fuel cycle or a once-through fuel cycle if the used fuels are discarded and disposed, or a closed fueled cycle if they are reprocessed for further use.