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Uranium mining
By Ir Richard FUNG

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Uranium is a common fuel for nuclear reactors. It is a silvery grey metal nearly twice as heavy as lead. It is naturally found in rock, soil and even seawater, albeit in a very low concentration. Natural uranium is composed mainly of uranium-238, with 0.72% uranium-235 and a very small fraction of a percent of other uranium nuclides. It only has weak radioactivity.

 

Uranium ore has typically 0.1% concentration and mostly exists as oxides. Commercially, it is extracted from the ground—by open pit mining if the ore is near the surface, by underground excavation if it is deep down, or by in-situ leaching (ISL) if it is located in porous rocks. In ISL, water with chemical additives is injected to dissolve the uranium in a solution. The uranium is then extracted by pumps. Presently, ISL accounts for 56% of uranium mining, whereas open pit and underground excavation account for 38%. For the remaining 6%, uranium is a byproduct of the extraction of other metals, such as copper.

 

The first two mining methods, open pit and underground excavation, produce ore residues known as tailings. Tailings contain the decay products mainly of uranium-238, such as radium and radon, which contribute to about 70% of the original ore’s radioactivity. These residues are disposed of with layers of clay, topsoil and vegetation coverings, which mitigate the egress of the radioactive elements into the environment. In contrast, ISL results in most decay products remaining underground. However, the effluent produced in uranium extraction needs to be evaporated or treated before returning underground for safe containment.

 

Mine rehabilitation is essential upon ore depletion. Any rocks and soil disturbed during mining are restored, any tailings are covered, any ISL wells are sealed, and any processing facilities are dismantled, so that the site can be released for other uses.

 

The workers engaged in uranium mining need to be monitored and protected against radiation hazards, particularly when working underground (for which proper ventilation is needed to limit the concentration of gaseous radon and dust emitted from the ore); or when they are extracting or crushing a particularly rich uranium ore, in which case radiation may be significant at close distance.

 

 

This article is contributed by Ir Richard Fung with the coordination of the Nuclear Division.

 

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