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| Fig. 1: White Mesa Mill Leach Tank. (Source: T. Logan) |
Currently, there is only one conventional uranium mill still in operation in the United States. Located in Blanding, Utah, the White Mesa Mill is operated by Energy Fuels and has the capacity to process 2,000 short tons of uranium ore each year. [1] Previously operating uranium mills have been shut down over time as the Nuclear Regulatory Commission (NRC) has experienced increasing pressure from the public to cease all uranium related operations. While it is important to regulate uranium processing for the sake of public safety, the public is largely misinformed about the dangers associated with uranium.
As mentioned, there is only one conventional mill remaining in the United States. Uranium ore is transported to the mill by truck, where it undergoes extensive leaching. The ore, which typically contains only very little uranium by weight is eventually converted into yellowcake, which can be more than 80% uranium by weight [2] Other uranium extraction operations do not require ore transport at all: in situ leaching is a technique in which the leach acids are pumped into the uranium mine such that the extraction can occur downhole. Processing is still required at the surface, but in situ leaching saves time, energy and risk and as a result, is the favored uranium extraction method.
Uranium ore is brought to the mill via covered trucks. The ore is unloaded into a grizzly hopper, where the ore is crushed into small grains. The crushed ore then flows through a series of leach tanks filled with sulfuric acid where the UO3 found in ore is converted to UO2(SO4)34-. [3] The acid acts as an electron donor to the uranium atoms and they become dissolved in the acid solution. Upon leaching, the ore slurry goes through a series of thickeners in which the leached solids become 99% free of uranium before being pumped to a designated tailings pond. The acidic uranium solution is diverted to a series of solvent extraction tanks in which tertiary amines and kerosene pull off two of the sulfate groups. The organic-sulfate-uranium complex then goes through a series of counter-current strippers containing dissolved ammonium, in which both the sulfate and organic portion of the solution are removed, resulting in (NH4)U2O7. [3] This final product is de-watered then roasted in order to form the final product, U3O8. The mill then exports the 55-gallon drums of 95% U3O8 at its natural isotopic abundance to another facility, where the U3O8 is converted to UF6 for enrichment and eventual industrial use.
The economic advantages of in situ leaching has popularized this extraction method to represent the majority of uranium extraction operations in the United States. However, one of the more publicized reasons that conventional ore milling has fallen out of favor is the perceived risk of transportation. While it is true that ore transport does increase exposure risk, this risk is not as great as the media suggests. Ore is transported in covered trucks and is kept wet in order to prevent dust from becoming airborne. [4]
In general, almost anything associated with uranium, radiation or nuclear energy is portrayed as dangerous. It would seem that just being in the same room as a piece of uranium ore (containing only .05% uranium by weight) would cause some sort of adverse health effects due to radiation. While it is true that uranium is radioactive, it is an alpha- emitter. Unlike the more radioactive beta and gamma particles, alpha particles cannot penetrate human skin. Health risks associated with alpha decay come with inhalation, which is burden bared by the mill workers. However, the workers are routinely tested for radiation due to strict government regulations on exposure limits. [5] Barring a catastrophe in the leach or extraction circuits, the ore transport step is the only step in this process in which human workers may be harmed. Uranium will not evaporate from the leach tanks, and contact with the leach slurry would result in chemical burns that are more deadly than uranium's alpha decay.
Conventional mills have a distinct advantage over in situ extraction operations: provided that uranium is in its natural isotopic ratio, and it is per NRC regulation, conventional mills can handle alternate feed. This can range from parts of decommissioned mills to waste sludge to contaminated soil. [6] This feature is advantageous to many stakeholders. Typical radioactive waste disposal is expensive so conventional uranium mills can take advantage of this fact and process the waste material at a lower cost than the NRC regulated disposal. With minimal process modification depending on the type of feed material, the mill can then extract the waste uranium for profit. The radioactivity of the waste material is reduced as uranium is extracted and treated along with the uranium ore in one of the on-site tailings ponds.
While it is true that uranium extraction poses health risks, there are adequate safeguards put in place by mill operators and the NRC to ensure the safety of workers and the general population. Conventional mills are useful in that they allow for processing both ore and alternate feed materials that would otherwise be costly and inefficient to store.
© Thomas Logan. 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] "2013 Domestic Uranium Report," U.S. Energy Information Administration, May 2014.
[2] D. Y. Goswami and F. Kreigh, Eds., Energy Conversion (CRC Press, 2007).
[3] K. E. Haque, J. J. Laliberté and J. Pruneau, "Batch and Counter-Current Acid Leaching of Uranium Ore," Hydrometallurgy 17, 229 (2012).
[4] D. L. Naftz et al., "Assessment of Potential Migration of Radionuclides and Trace Elements from the White Mesa Uranium Mill to the Ute Mountain Ute Reservation and Surrounding Areas, Southeastern Utah," U.S. Geological Survey, Scientific Investigations Report 2011-5231, 2011.
[5] "Part 20 - Standards for Protection Against Radiation," United States Code of Federal Regulations, 10 CFR 1, January 2014, p. 338.
[6] R. A. Meserve et al., "Memorandum and Order in the Matter of Sequoyah Fuels Corporation, Gore, Oklahoma Site Decommissioning," U.S. Nuclear Regulatory Commission, CLI-01-02, 17 Jan 01.