This brief summary is intended to explain in layman’s language how natural uranium becomes the fuel for nuclear power stations.
Uranium mining, whether by in-situ leach or by conventional mining, recovers natural uranium, which is usually in the form of U3O8, or tri-uranium oct-oxide. The active ingredient in this natural form is the isotope, U235. Generally this isotope forms about 0.7% of the total isotope content of the uranium, with the balance being mainly U238. A nuclear reactor requires a concentration of about 3-5% of the isotope. The following will explain the steps to get to that concentration, and create fuel for a reactor.
At the mining or recovery operation, the ore is processed to produce “yellowcake.” This is the primary product that producing companies produce and sell. It is the product that the various price indices refer to. In a conventional mining operation, the ore will processed at a mill, by crushing and grounding, followed by a leach circuit, counter-current decantation, thickening and drying. The yellowcake is then packed in drums, similar in size to oil drums, typically weighing about 400 kilograms, and assaying about 87-90% U3O8.
At an ISL operation, the uranium is already in solution when it reaches that plant, and is bonded to resin beads. The uraniferous solution is stripped from the resin by the elution process, and then continues essentially the same way as conventionally mined ore. Because of this similarity in the operations, a conventionally mill can usually be fitted to accept loaded resin beads from an ISL operation, in addition to continuing to process conventional ores, with only minor changes in the flow sheet.
The next steps are refining and conversion. Refining will remove impurities to produce high purity uranium trioxide (UO3). This is followed by the conversion process, and the process used is dependent on the type of reactor. For heavy water reactors, the concentrated uranium is converted into powdered uranium dioxide (UO2); this conversion product is then ready for fabrication into fuel pellets for Candu reactors. For light water reactors, the refined uranium is converted to gaseous uranium hexafluoride (UF6), which will require further processing to create UO2.
The converted uranium is then enriched in the U235 isotope, which is the active ingredient in a reactor. Two methods are currently in use; gaseous diffusion and centrifuge. The material left after enrichment is referred to as “depleted tails”, or simply “tails”. This material has a reduced concentration of the U235 isotope. The enriched product will contain from 3 – 5% U235.
A word about SWU’s. A SWU is an acronym for separative work units, and is the unit by which enrichment services are sold. A SWU is a unit that expresses the energy required to separate the U238 and U235. How uranium is enriched depends on the amount of uranium feed (UF6) at the beginning of the process, the amount of SWU used, and the concentration of U235 atoms left over (tails assay) at the end of the process.
The final step is fuel fabrication, where the converted uranium is manufactured into fuel pellets or rods. The first step is to press powdered UO2 into small cylinders which are then baked at a high temperature to make hard ceramic pellets.
In a light water reactor, the pellets are then packed into tubes, or fuel rods. The rods are then bundled together into a fuel assembly. A typical 1,100 megawatt pressurized water reactor contains 193 fuel assemblies, which are made up of almost 51,000 fuel rods and approximately 18 million pellets.
In a Candu reactor, fuel pellets are loaded into 28 or 37 0.5 meter rods grouped into a cylindrical fuel bundle. Twelve bundles lie end to end in a fuel channel in the reactor core. A typical Candu reactor, such as the Bruce facility in Ontario, could be rated at 790 megawatts. This would require 480 fuel channels, 5,760 fuels bundles, and over 5 million pellets.
