What is Uranium ore Deposits?
¿donde van a parar los residuos de uranio?
Uranium ore are mined and are economically recoverable concentrations within the earth crust. This material is extracted from the earth.
What is pegmatite's?
The fact that only a few of the numerous uranium minerals qualify as uranium ore minerals and form uranium ore deposits, whereas uranium in small amounts is widely spread throughout the rocks of the earth's crust, adds greatly to the problem of uranium exploration.
Just how readily available are uranium resources, and do their distribution and cost impose restrictions on nuclear power generation?
Compared to a coal-fired power station a nuclear power station requires far less fuel in terms of mass. You have seen that a 1 GW burner reactor requires 5000 t of natural uranium over 30 years, whereas a comparable modern coal-fired power station needs 10, 000 t of coal every day. However, uranium does not occur naturally in metallic form, nor in the concentrations required for reactor fuel.
The average abundance of uranium is only about 3 parts per million (ppm) by mass in the continental crust - equivalent to just 0.4 cm 3 of pure uranium dispersed evenly through a whole cubic metre of granitic rock.
Uranium occurs at concentrations up to 100-10,000 ppm in minor minerals in granites, such as zircon (ZrSiO4), apatite (Ca5(PO4)3(OH)) and titanite (CaTiSiO5). For mining and extraction of uranium to be economic, it must occur at much higher abundances than this.
A typical ore grade for uranium is about 0.3% or 3000 ppm in rock. By how much must uranium be concentrated above its average abundance in continental crust to form an ore of this grade?
The amount of an ore containing 0.3% uranium that must be mined to produce fuel for the 30-year lifetime of a 1 GW burner reactor is 1.7 million tonnes (i.e. 5000 t/0.003 = 1.7 ×106 t). Nevertheless, even that huge quantity is only half the mass of a year's supply of coal to a modern power station.
Some of the world's richest uranium occurrences were formed by hydrothermal processes. Many occur at or below unconformities that are cut by faults, as in the Athabasca Basin of Saskatchewan, Canada, where numerous unconformity related uranium deposits have been discovered.
These include the MacArthur River (~24% uranium) and Cigar Lake (~19% uranium) deposits, which are probably the richest in the world.
Whether the source of the uranium in such settings is fractured metamorphic basement rocks beneath the unconformity or as a result of ‘leaking’ from the overlying sediments down faults is uncertain. Whichever, the uranium deposition was clearly associated with a change in oxidation state of the fluid, when large quantities of oxidising fluids containing soluble uranium ions became reduced and precipitated uraninite.
Similar oxidation-reduction reactions are responsible for uranium deposits in quite different geological settings in the Wyoming, South Dakota and Colorado Plateau areas of the western US. These are sandstone-hosted uranium deposits.
Some important uranium deposits consist of sedimentary grains of uranium-bearing minerals in quartz-pebble conglomerates, such as those of the Witwatersrand Basin in South Africa and Blind River, Elliott Lake in southern Canada. These conglomerates are unusual.
Not only do they contain rounded grains of uranium-bearing minerals, principally uraninite, that were deposited in river gravels in exactly the same form as when they were originally eroded, but also sedimentary grains of gold and pyrite.