Uranium mining and milling is the starting process for all nuclear fuel cycles. In this process uranium ore is extracted from the Earth’s crust similarly as for mining of copper, zinc, and other metals. Uranium is often found with copper, phosphates, and other minerals; thus, it is often a co-product of other mining operations. The worldwide production of uranium in 2015 amounted to 60496 tonnes. Kazakhstan, Canada, and Australia are the top three producers and together account for 70% of world uranium production.
Uranium is commonly found at low levels (a few ppm – parts per million) in all rocks, soil, water, plants, and animals (including humans). Uranium occurs also in seawater, and can be recovered from the ocean water. But only a few of the uranium ores known contain sufficient uranium (greater than 0.1%) to extract commercially. Significant concentrations of uranium occur in some substances such as uraninite (the most common uranium ore), phosphate rock deposits, and other minerals.
Natural uranium refers to uranium with the same isotopic ratio as found in nature. It consists primarily of isotope 238U (99.28%), therefore the atomic mass of uranium element is close to the atomic mass of 238U isotope (238.03u). Natural uranium also consists of two other isotopes: 235U (0.71%) and 234U (0.0054%). The abundance of isotopes in the nature is caused by difference in the half-lifes. All three naturally-occurring isotopes of uranium (238U, 235U and 234U) are unstable. On the other hand these isotopes (except 234U) belong to primordial nuclides, because their half-life is comparable to the age of the Earth (~4.5×109years for 238U).
Underground Uranium Mining
As with other types of hard rock mining there are several methods of extraction. In case of uranium mining, it strongly depends on the concentration of uranium in the ore. For example, the ore extracted from the Australian Olympic Dam Mine has a concentration of 0.05 %. Most reserves have uranium with a concentration of between 0.1 bis 0.2 %. Noteworthy, in Canadian Saskatchewan ore is mined that contains more than 20 % uranium.
Underground uranium mining is in principle no different from any other hard rock mining and other ores are often mined in association (e.g., copper, gold, silver). The uranium ore is extracted through mechanical means such as blasting, drilling, pneumatic drilling, picks and shovels, and then transported to the surface. Underground mining is usually used when the uranium is too far below the surface for open pit mining. This type of mining exposes underground workers to the highest levels of radon gas.In 2012, the percentage of the mined uranium produced by underground mining was 26.2 percent. For example, the Olympic Dam mine is a large poly-metallic underground mine located in South Australia, 550 km NNW of Adelaide. It is the fourth largest copper deposit and the largest known single deposit of uranium in the world, although copper is the largest contributor to total revenue.