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| Fig. 1: Ranger uranium mine in Kakadu National Park. (Source: Wikimedia Commons |
Nuclear energy accounts for roughly 10% of global energy production. The fuel used in these facilities are primarily from the Uranium isotope U-235, which comprises roughly 1% of all uranium, with U-238, the less fissile isotope comprising the remaining 99%. While each process from mining all the way through fabrication and use, the purpose of this paper is to explore the impact of uranium mining, and in particular, the environmental impacts as a result of common mining practices.
Kazakhstan, Canada, and Australia are the world's largest uranium producers. Fig. 1 displays Uranium mining activities in a common mining pit in Australia. Due to diminishing uranium stockpiles, the rate of mining was set to increase after 2015. [1] In each of these countries, but perhaps most salient in Canada and Australia, are ISO 14001 certifications. This is the world's most recognized Environmental Management System framework, which allows mining operations to demonstrate their active steps towards best environmental practices. However, environmental impacts as a result of mining, processing, and radioactive waste are measurable and worth noting.
There are three main types of uranium deposits: sedimentary rock deposits, igneous or hydrothermal deposits, and breccia deposits. The techniques for extraction for this hard rock mining are proportionate as follows: In-situ leach (44.9%), underground mining (26.2%), open pit (19.9%) and heap leaching (1.7%), with the remaining 7.3% arising as a result of byproduct from other mining operations. Uranium ore is generally processed through grinding the ore materials to a uniform particle size, which is then treating through chemical leaching to extract the uranium. This results in a dry powder form of the natural uranium commonly referred to as "yellow cake." [2]
The waste products of this process are called mill tailings, sandy waste that contains heavy metals, Radium, and other radioactive contaminants. [3] As the Radium decays over time in these piles, they release Radon, a highly radioactive gas. These mill tailings currently pose the strongest environmental risk in the mining process or uranium. Worldwide, more than 938 million cubic meters of mill tailings have been produced. [2] The radioactivity of the tailings largely depends on the grade of the ore mined, which can vary from 1 Bq/g to 100 Bq/g. [2] The main risks associated with these tailings are gamma radiation from the Radium decay, Radon gas, radioactive dust material blown by the wind to neighboring areas, and increased concentrations of toxic heavy metals that contaminate surface and groundwater sources. Because of their high sulfide content, these tailings contribute to the acidification of groundwater, which in turn accelerates the release of radioactive and other dangerous elements. [3]
As the uranium ore is extracted through chemical processes, the sludge of the mill tailings are dumped in special ponds or piles, which are ideally capped and lined. However, improper disposal of these tailings can lead to the hazards aforementioned. In these storage areas, because long-lived radioactive decay elements are not disposed of, the sludge often contains up to 85% of the radioactivity of the original ore. [2] This is due largely in part by the presence of Th-230 and Ra-226. Th-230, which has a half life of 80,000 years, is the parent product of Ra-226, which means there is a continuous production of the isotope. Further, Radium, which has a half life of 1600 years, is the parent product of Rn-222 gas, which has a half life of 3.8 days. These gases are continuously produced, and are susceptible to escape from the interior of the pilings which are then carried easily, and of great distance, by the wind. [2]
The decay products of this Radon gas have irreversible adverse health effects on humans downwind, such as a significant increase in the occurrence of lung cancer. Thus, weathering of these piles leads to Radon-exhalation and subsequent inhalation by humans in the nearby vicinity. Further, dust blowing picks up Ra-226 and Arsenic, which is then deposited nearby. Because of the long half-lives of these elements, extended timeframes of protective measures must be taken into account. As weathering and climatic erosion occurs, these mill tailings become more susceptible to wind-carry and dispersion of radioactive materials to nearby habitats. The EPA has estimated that the additional lifetime risk of lung cancer for those living nearby mill tailings is 2 in one hundred. [2]
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| Fig. 2: Mill tailing seepage into San Miguel River. (Source: Wikimedia Commons) |
While many of the radioactive components can compromise the clay-silt capping of the mill tailings and enter the air, seepage into groundwater systems is also of utmost concern. Chronic or acute failures of containment systems can occur as the result of gradual deterioration or acute shock that compromises dams or capped valley among many others. [4] When this occurs, the radioactive materials are free to seep into the groundwater and surrounding soils. Again, the high acidity environment lends itself to greater mobility of radionuclides emitted from tailings. Radon and arsenic are common contaminants in the ground and surface water, as well as toxic heavy metals, around these sites, such as the San Miguel River seen in Fig. 2. [4] Direct consumption by humans, as well as consumption of fish from these waters leads to poisoning of nearby populations. Many tailings dams were built of faulty construction and are vulnerable to earthquakes and other acute shocks. These dam failures pose a huge problem through repeat occurrences over time.
As uranium mining continues to increase due to the need for lager national stockpiles, more attention has been paid to environmentally safe procedures of dealing with mill tailings. If stored and capped in ways not adherent to international environmental safety protocols, they run the risk of severe environmental radioactive exposure, as well as high toxicity exposure to byproducts and contaminants in the pilings. [1] Incentive measures need to be fortified in order to prevent hazards such as failure of unsound dams, exhalation of Radon gas from tailing piles that are improperly capped, and gradual erosion and weathering. As stated earlier, due to the extremely long half lives of many of these elements, the time-frame of safe storage is massive, and so extremely risk averse measures need to be taken with new mining endeavors. Previously closed mine tailings may also need to be addressed in order to closely monitor and perhaps repair tailings cover and containment protocols. [3]
© Matthew Stevens. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "The Long Term Stabilization of Uranium Mill Tailings," International Atomic Energy Agency, IAEA-TECDOC-1403, August 2004.
[2] A. Abdelouas, "Uranium Mill Tailings: Geochemistry, Mineralogy, and Environmental Impact," Elements 2, 335 (2006).
[3] Uranium Mining in Virginia (National Academies Press, 2012).
[4] K. Morin et. al., "Migration of Acidic Groundwater Seepage From Uranium-Tailings Impoundments, 1. Field Study and Conceptual Hydrogeochemical Model," J. Contam. Hydrol., 2, 271 (1988).
