17 lines
4.0 KiB
TeX
17 lines
4.0 KiB
TeX
\section{Introduction}
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Uranium mines generate uranium oxide that is necessary to support the 390 operating nuclear reactors, supplying 10\% of the worlds energy \citep{worldnuclearassociation2024}. Uranium not stored as inventories is shipped to a conversion facility to prepare for enrichment. Uranium recovery facilities establish contracts with nuclear power plants to purchase set quantities of uranium in future years \citep{camecocorporation2024}.
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Most U.S. uranium mines are classified as underground mines, surface mines \footnote{Either open cut or open pit}, or as in situ recovery facilities. The first step to establish a conventional uranium mine is to identify target orebodies. Then a shaft is generally sunk in the vicinity of the deposit and workings are excavated to remove the uranium ore \citep{nuclearregulatorycommission2020}. Blasted ore is brought to the surface and sent to a mill, where it is crushed or ground and processed into uranium concentrate.
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In comparison in situ (in place) mines create minimal ground disturbance, do not produce tailings, and avoid expose of miners to elevated radon levels linked to lung concern \citep{national_academy_of_sciences_health_1999}. In situ mining recovers uranium from groundwater aquifers. A lixiviant\footnote{A lixiviant is any liquid chemical mixture designed to dissolve a ore concentrate \citep{wang2007}} designed to dissociate uranium from the rock is injected into the target formation. For Wyoming in situ mines the lixiviant is a mixture of native groundwater with typical additives such as carbon dioxide, oxygen, and sodium bicarbonate \citep{gregory2015,kehoe2023}, but international mines primarily use acidic lixiviants such as sulfuric acid \citep{worldnuclearassociation2024}. The acid or base dissociates the uranium from a sandstone roll front where a historic oxidation reaction deposited the ore \citep{wilson2015}.
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The lixiviant within a wellfield is pumped from the recovery wells to a plant that contains an ion exchange process. Vessels inside the plant contain ion exchange resin beads that attract uranium ions in the groundwater. Groundwater from the uranium wellfields is passed through the ion exchange beads, which bind the uranium. Once the groundwater leaves the ion exchange vessels, it is refortified with oxygen and carbon dioxide and reinjected into the mining aquifer within the wellfields. The pressure of the injection wells keep the solution within a closed loop in the aquifer. The resin beads, when fully loaded with the uranium, are transferred out of the ion exchange vessel and then stripped of the uranium in a process called elution. Clean resin beads are then transferred back to the ion exchange vessels for re-use.
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This process is repeated, cycling the groundwater between injection and recovery wells until uranium recovery rates becomes subeconomic, and the well grouping is retired. A single recovery facility serves a system of wells. As some wells are retired, others may be added further along the roll front, until all economically recoverable uranium is extracted, and the operation is ended.
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A common well system for in situ mines is referred to as a five spot. A five spot pattern exists when four injection wells are drilled in a rectangle, with a single recovery well in the center. These wells are shallow typically less than 100 ft deep. A piping system, often constructed with PVC pipes, brings the extracted water to processing facility and then back to the injection wells. This piping network is removed after operation are closed, and wells are capped.
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There are multiple environmental advantages of this method of uranium recovery. The chemical injected into the groundwater, are commonly used in household without direct health risk, in Wyoming the most commons lixicant is sodium bicarbonate (baking soda). Rather than removing large volumes of earth only minor holes are created that are capped after completion.
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Wyoming produced uranium entirely with conventional mining methods, until the early 1990s when in situ techniques were adopted \citep{energyinformationadministration2023a}.
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