The Nuclear Fuel Cycle
Once an orebody is discovered and defined by exploration, there are three common ways to mine uranium, depending on the depth of the orebody and the deposit’s geological characteristics:
- Open pit mining is used if the ore is near the surface. The ore is usually mined using drilling and blasting.
- Underground mining is used if the ore is too deep to make open pit mining economical. Tunnels and shafts provide access to the ore.
- In situ recovery (ISR) does not require large scale excavation. Instead, holes are drilled into the ore and a solution is used to dissolve the uranium. The solution is pumped to the surface where the uranium is recovered.
Ore from open pit and underground mines is processed to extract the uranium and package it as a powder typically referred to as uranium concentrates (U3O8) or yellowcake. The leftover processed rock and other solid waste (tailings) is placed in an engineered tailings facility.
Refining removes the impurities from the uranium concentrate and changes its chemical form to uranium trioxide (UO3).
For light water reactors, the UO3 is converted to uranium hexafluoride (UF6) gas to prepare it for enrichment. For heavy water reactors like the CANDU reactor, the UO3 is converted into powdered uranium dioxide (UO2).
Uranium is made up of two main isotopes: U-238 and U-235. Only U-235 atoms, which make up 0.7% of natural uranium, are involved in the nuclear reaction (fission). Most of the world’s commercial nuclear reactors require uranium that has an enriched level of U-235 atoms.
The enrichment process increases the concentration of U-235 to between 3% and 5% by separating U-235 atoms from the U-238. Enriched UF6 gas is then converted to powdered UO2.
5 Fuel manufacturing
Natural or enriched UO2 is pressed into pellets, which are baked at a high temperature. These are packed into zircaloy or stainless steel tubes, sealed and then assembled into fuel bundles.
Nuclear reactors are used to generate electricity.
U-235 atoms in the reactor fuel fission, creating heat that generates steam to drive turbines. The fuel bundles in the reactor need to be replaced as the U-235 atoms are depleted, typically after one or two years depending upon the reactor type. The used – or spent – fuel is stored or reprocessed.
Spent fuel management
The majority of spent fuel is safely stored at the reactor site. A small amount of spent fuel is reprocessed. The reprocessed fuel is used in some European and Japanese reactors.