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).
Consumption of a 3000MWth (~1000MWe) pressurized water reactor (12-months fuel cycle)
It is an illustrative example, following data do not correspond to any reactor design.
- Typical reactor may contain about 100 tonnes of enriched uranium (i.e. about 113 tonnes of uranium dioxide).
- This fuel is loaded within, for example, 157 fuel assemblies composed of over 45,000 fuel rods.
- A common fuel assembly contain energy for approximately 4 years of operation at full power.
- Therefore about one quarter of the core is yearly removed to spent fuel pool (i.e. about 40 fuel assemblies), while the remainder is rearranged to a location in the core better suited to its remaining level of enrichment (see Power Distribution).
- Annual natural uranium consumption of this reactor is about 250 tonnes of natural uranium (to produce of about 25 tonnes of enriched uranium).
See also: Fuel Consumption
Methods and Techniques of 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. There are three main methods of extracting uranium:
- Open pit mining. In open pit mining, overburden is removed by drilling and blasting to expose the ore body, which is then mined by blasting and excavation using loaders and dump trucks. Open pit mines require large holes on the surface, larger than the size of the ore deposit, since the walls of the pit must be sloped to prevent collapse. In 2012, the percentage of the mined uranium produced by open pit was 19.9 percent.For example, the Rössing Uranium Mine in Namibia is the longest-running and one of the largest open pit uranium mines in the world.
- Underground mining. 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.
In-Situ-Leaching (ISL). In-situ leaching, also known as in-situ recovery, is a mining process different from the conventional method in that it uses a chemical process to separate the uranium in the earth’s crust from the surrounding rock. In-situ is a Latin phrase that translates literally to “on site” or “in position.” In-situ leach for uranium has expanded rapidly since the 1990s, and is now the predominant method for mining uranium. In this technology, uranium is leached from the in-place ore through an array of regularly spaced wells and is then recovered from the leach solution at a surface plant. The chemical solution is injected into a drilled hole into the rock at the periphery of the uranium deposit. Solutions used to dissolve uranium ore are either acid (sulfuric acid or less commonly nitric acid) or carbonate (sodium bicarbonate). This liquid loosens the uranium from the rock and binds it; in other words, the uranium is „flushed“ out of the rock. This solution, now supplemented with uranium, is then brought up to the surface through another borehole. The extracted uranium solution is then filtered through resin beads. Through an ion exchange process, the resin beads attract uranium from the solution. Uranium loaded resins are then transported to a processing plant, where U3O8 is separated from the resin beads and yellowcake is produced. The ISL-method can be used without mining enormous quantities of rock. There are no tailings or waste rock generated. It also has less impact on the environment and health than the conventional method and it is cheaper. On the other hand, there are potential hazards of groundwater contamination. Not all of the contaminated liquid is pumped out. The rock reacts to the chemical solution unpredictably. Therefore, the ISL method can only be performed under certain subsurface conditions. In 2012, the percentage of the mined uranium produced by in-situ leach was 44.9 percent. In this technology, uranium is leached from the in-place ore through an array of regularly spaced wells and is then recovered from the leach solution at a surface plant. For example, the Honeymoon Mine was Australia’s second operating in-situ recovery uranium mine, beginning production in 2011.
Another method of uranium extraction is heap leaching. Heap leaching is an extraction process from ore which has been mined and placed in piles on the surface. Heap leaching is based on the use of a series of chemical reactions that absorb specific minerals and re-separate them after their division from other earth materials. In 2012, the percentage of the mined uranium produced by heap leaching was 1.7 percent. The remaining 7.3% was derived as a byproduct of mining for other minerals, and miscellaneous recovery.