Uranium is a naturally-occurring element in the Earth's crust. Traces of it occur almost everywhere, although mining takes place in locations where it is naturally concentrated. To make nuclear fuel from the uranium ore requires first for the uranium to be extracted from the rock in which it is found, then enriched in the uranium-235 isotope, before being made into pellets that are loaded into the nuclear fuel assembly.
Uranium mines operate in some twenty countries, though about half of world production comes from just ten mines in six countries, in Canada, Australia, Niger, Kazakhstan, Russia and Namibia.
At conventional mines, the ore goes through a mill where it is first crushed. It is then ground in water to produce a slurry of fine ore particles suspended in the water. The slurry is leached with sulphuric acid to dissolve the uranium oxides, leaving the remaining rock and other minerals undissolved, as mine tailings.
However, nearly half the world's mines now use a mining method called in situ leaching (ISL). This means that the mining is accomplished without any major ground disturbance. Groundwater with a lot of oxygen injected into it is circulated through the uranium ore, extracting the uranium. The solution with dissolved uranium is pumped to the surface.
Both mining methods produce a liquid with uranium dissolved in it. This is filtered and the uranium then separated by ion exchange, precipitated from the solution, filtered and dried to produce a uranium oxide concentrate (U3O8), which is then sealed in drums. This concentrate may be a bright yellow colour, hence known as 'yellowcake', or if dried at high temperatures it is khaki.