How is Uranium Produced?
Uranium is an element which exists naturally in the Earth’s crust; specifically in sedimentary rocks, such as sandstone. In its elementary state uranium is only weakly radioactive due to its unstable isotopes, which vary naturally. Uranium ore must be mined and processed in order for it to be used commercially as fuel and in weapons.
Uranium ore is currently mined in around 20 countries, though more than half of the world’s supply of uranium comes from mines in just three countries: Australia, Canada and Kazakhstan. Other notable uranium producers include China, Namibia, Niger, Russia and the USA.
There are three methods of mining uranium: traditional open-pit mining, underground mining and in-situ leaching. Open-pit mining is only possible if the uranium ore is near the surface of the rock, and involves using machinery to remove soil and rock to reach the uranium ore deposits below. Underground mining is necessary where the uranium ore is too far underground to be reached by open-pit mining. The uranium ore is blasted and the resulting debris transported to the surface.
After both open-pit and underground mining, the uranium ore concentrated in the extracted rocks is weak, usually only around 0.3%. The rocks are crushed to a fine powder and added to water to create a slurry. The slurry is then ‘leached’ with sulphuric acid, or sometimes an alkaline solution if certain mineral rocks are present, to separate the uranium from the base rock.
In-situ leaching, known as in-situ recover mining in the US, has now become a more widely-used mining method, as it is more economical and environmentally friendly than open-pit or underground mining. In-situ leaching simplifies the mining process by essentially creating a slurry while the uranium is still in the ground. Heavily oxygenated water is pumped into a uranium well, dissolving the uranium but leaving the base rock intact. The water based uranium is then pumped back to the surface, where it’s already ‘leached’.
All three mining methods produce uranium suspended in liquid. The liquid is filtered and the uranium extracted from it by ion exchange to form a uranium oxide concentrate. This is a bright yellow powder, known as yellowcake. Yellowcake is only mildly reactive and must go through the enrichment process before uranium is usable in a commercial way.
To enrich the uranium oxide, it is first converted to gas, (uranium hexafluoride) though a process called calcining, essentially heating to a very high temperature. The gas can then be enriched to make it concentrated in uranium isotope 235, the isotope most needed for nuclear power. To do this, the centrifuge process is used, separating the uranium 235 from waste uranium by repeatedly diffusing the gas through a silver-zinc membrane in thousands of fast-spinning vertical tubes.
Once the uranium hexafluoride has been enriched, it is chemically converted to uranium dioxide powder. The powder is shaped into small cylindrical fuel pellets and heated to solidify them. The uranium dioxide pellets are slotted into thin tubes to form rods of fuel, which are grouped together to create fuel assemblies of several metres each.