Uranium ore is not loaded into a power reactor as it is extracted from the mine. Processing the ore must go through a series of stages, including chemical conversion, enrichment for some types of reactors, and fuel production. These stages are called the “front end” of the nuclear fuel cycle.
The fuel production route may vary depending on the reactor type. Light water reactors generally use low-enriched uranium fuel, while some heavy water reactors can run on natural uranium.
1. Exploration, mining and milling
Suitable uranium deposits are determined through geological studies. Ore can be produced by methods such as open pit, underground mining or in situ dissolution under suitable geological conditions.
By processing the mined ore, its uranium content is concentrated. The resulting uranium concentrate is commonly known as "yellowcake". This product is not yet reactor fuel.
2. Transformation
The concentrate is converted into the chemical form needed by the next stage. In fuel cycles where enrichment will be applied, uranium can be converted into uranium hexafluoride, which can be processed into gaseous state.
Conversion alone does not increase the isotope ratio. It prepares the material into the appropriate chemical form for enrichment or fuel production.
3. Enrichment
The majority of natural uranium consists of the uranium-238 isotope, and the smaller portion consists of the uranium-235 isotope. Enrichment is the process of increasing the proportion of uranium-235 to the level required for certain reactor fuels.
The common approach to commercial enrichment is gas centrifuges. At the end of the process, while the uranium-235 ratio of one stream increases, the ratio of the other stream decreases. Low-enriched uranium used for power reactor fuel is not at the same level of enrichment as nuclear weapons material.
4. Fuel pellet production
Enriched uranium is converted into uranium dioxide powder after appropriate processes. The powder is pressed and sintered at high temperature to produce small ceramic pellets. The dimensions, density and chemical properties of the pellets undergo strict quality controls.
5. Fuel rod and fuel harness
Ceramic pellets are placed in long metal tubes and the tubes are hermetically sealed. These structures are called fuel rods. A fuel bundle is formed by bringing together a large number of fuel rods in a certain geometry.
Fuel bundles are loaded into the reactor core. Over the course of use, the composition of the fuel changes and fission products are formed. The storage, reprocessing, or disposal stages after removal from the reactor are the “back end” of the fuel cycle.
Is enrichment necessary for every reactor?
No. The requirement depends on the reactor design, moderator and fuel strategy. Light water reactors use enriched uranium, while some heavy water reactors can run on natural uranium.
Resources
- IAEA, Nuclear Fuel Cycle Simulation System: https://infcis.iaea.org/nfcss/modeling
- IAEA, The Front End of the Uranium Fuel Cycle: https://www.iaea.org/sites/default/files/gc/gc50inf-3-att6_en.pdf
- IAEA, Integrated Nuclear Fuel Cycle Information System: https://infcis.iaea.org/