Uranium ore, as it is mined from the ground, is not directly usable for power generation. Much processing must be carried out before uranium can be used efficiently to generate electricity. Uranium’s transformation from ore into nuclear fuel and, eventually, the handling of waste products, is described as the nuclear fuel cycle.
There are several steps in the nuclear fuel cycle:
Mining – Depending on the depth and concentration of the uranium source, and the conditions of the surrounding rock, mining companies will extract uranium ore in one of three ways: open pit mining, underground mining or in-situ recovery.
Milling – To extract the uranium, the ore is crushed in a mill and ground to a fine slurry. The slurry is then leached in sulfuric acid, which produces a solution of uranium oxide (U3O8). The concentrate of this solution is called yellowcake.
Refining – A series of chemical processes separate the uranium from impurities, producing high-purity uranium trioxide (UO3).
Conversion – UO3 is converted to uranium dioxide (UO2) for use in heavy water reactors, or to uranium hexafluoride (UF6) for enrichment, before it can be used in light water reactors.
Enrichment – Uranium-235 is the uranium isotope that can be used in fission, but it makes up only 0.7% of naturally occurring uranium, which is not concentrated enough for light water reactors. So, enrichment processes increase the concentration of U-235 to about 3% – 5%. After undergoing enrichment, the UF6 is chemically transformed back into UO2 powder.
Fuel manufacturing – Natural or enriched UO2 powder is pressed into small cylindrical pellets, which are then baked at high temperatures, and finished to precise dimensions.
Electricity generation – Fuel is loaded into a reactor, and nuclear fission heats a fluid (usually water or heavy water), which in turn generates steam. The steam drives turbines, which generates electricity.
Storage – After fuel is consumed, it is removed from the reactor, and placed in cooling pools for a number of years. Once its radioactivity and heat sufficiently subsides, it is placed in concrete and steel containers, which are stored onsite.
Optional chemical reprocessing – After a period of storage, residual uranium or by-product plutonium, both of which are still useful sources of energy, are recovered from the spent fuel elements and reprocessed.
Disposal – Depending on the design of the disposal facility, the nuclear fuel may be recovered if needed again, or remain permanently stored. At some point in the future the spent fuel will be encapsulated in sturdy, leach-resistant containers and permanently placed deep underground where it originated.