MacArthur River uranium mine

The uranium deposit at McArthur River, located in the southeastern portion of the Athabasca Basin in Saskatchewan, Canada, is the world's largest high-grade uranium deposit. The McArthur River mine is the largest in the world.

Cameco CorporationMacArthur River uranium mine. Credit:Cameco Corporation

The principal use of uranium in the civilian sector is as a fuel in nuclear power plants, which currently supply about 18 percent of the world's electricity. The principal military use of uranium in the military sector is in high-density penetrators, such as depleted uranium ammunition. During the World War II and the ensuing Cold War period, uranium was the source of as the fissile explosive material in nuclear weapons, i.e., the "atom bomb."

Ore Quality

The uranium at McArthur River is unique because of its extraordinarily high quality and quantity. Uranium occurs naturally in low concentrations (a few parts per million) in soil, rock, and surface and groundwater.

High-grade ore (~2% U) 200,000 ppm U
Low-grade ore (~0.1% U) 1,000 ppm U
Granite 4 ppm U
Sedimentary rock 2 ppm U
Average in Earth's continental crust 2.8 ppm U
Seawater 0.003 ppm U

ppm = parts per million

With an average ore grade of nearly 21%, the McArthur River uranium is far purer than then the average crustal abundance of uranium, and is more than 100 times higher than the world average for commercial mining operations. Hence, the McArthur River uranium deposits are often referred to as uranium "super deposits."

Reserves and production

The majority owner of the McArthur River uranium is Cameco Corporation, the world's largest publicly traded uranium company, based in Saskatoon, Saskatchewan. The site produced 7,200 metric tons of uranium in 2006, or about 18% of world output, making McArthur River the largest uranium mine in the world. According to Cameco, McArthur River contains 810,000 metric tons of proved and probable uranium reserves, with a recoverable quantity of 367 million pounds of U3O8. The production from McArthur River region and other mines in Saskatchewan make Canada the leading exporter of uranium in the world.

Geology

USGS)Athabasca super deposit diagram: (Source: USGS)

The Athabasca Basin formed within a bowl or depression in the Canadian Shield; in mining terms, referred to as the "basement." Sand began filling this bowl approximately 1.7 billion years ago and over time became sandstone rock. The contact point where the sandstone meets the basement is known as the unconformity. It is near this contact that deposits are formed, hence the term "unconformity deposits." The McArthur River uranium resources are of this unconformity type.

The first common feature of all Basin deposits is a structural trap - an area where mobilized uranium could have "pooled" and deposited uranium minerals. These traps have two notable aspects to them: faulting and graphite. Faults can be created where graphite causes a weakness in the basement rock. They provide channels for mineralized fluids to flow, and can create a "step" where an uplifted block of basement rock forms a trap by blocking the flow of uranium-rich fluids along the unconformity.

The process of uranium deposition results in another common clue, an "alteration halo" surrounding the ore body. Hydrothermal or "hot" fluids passing through rock cause an alteration, adding, or removing minerals, which changes the composition of the rock. In the Basin, alteration of the sandstone enveloping a uranium ore body can form a halo that is tens to hundreds of meters thick.

Mining and Milling

The unique geology of McArthur River uranium, poses several challenges. The high grade of the ore poses radiation health risks to miners. In addition, The presence of significant groundwater, mostly in the sandstone and conglomerate geological units, poses a significant flooding hazard. Cameco uses a unique combination of a non-entry mining technique and ground freezing to address these challenges. Although both techniques are common in mining, neither had been used for uranium production before the McArthur River mine.

All mining methods that would have required workers to enter the mining area were immediately eliminated because the high uranium content of the poses a human health risk. Instead, various remote mining method are used such as the raisebore mining method. From about 530 meters depth, a pilot hole is drilled down through the ore body into the tunnel on the 640 meter level. A 2.4 meter wide reaming head is then attached and rotated as it is pulled up through the hole. The ore falls to the lower level and then is transported to the underground processing circuit where the ore is reduced to the consistency of fine sand in the grinding mill. To avoid flooding, the surrounding water-soaked sandstone is frozen by piping in brine at -30 °Celsius (-22 °F).

The ore is then thickened into a slurry, or mud, in circular tanks that are set in concrete to provide radiation shielding. High pressure pumps push the slurry to the surface where it is placed in storage tanks until it is loaded into specially designed containers for the 80- km truck haul to Key Lake processing plant. At the Key Lake mill it is blended with "special waste rock" to produce 8500 t/yr of U3O8 (7200 tU).

Radon gas released from the ore and groundwater adjacent to the ore meant also poses a health risk. To maintain air quality, high-powered vents blast fresh air through the tunnels at a rate that recycles the underground atmosphere every 7 minutes.

Environmental and Health Issues

On April 6, 2003 a cave-in and flood of radioactive water at closed the McArthur River mine. The mine resumed production on July 2. after the company contained the flood and rebuilt the underground workings. The Canadian Nuclear Safety Commission (CNSC), conducted an evaluation of the radiation exposure estimates made during the water inflow event at McArthur River and concluded that it is unlikely that there will be any negative effects on the health of workers as a result of the doses received during

the water inflow event.. The Canadian Nuclear Workers Council also concluded that there were no adverse consequences to miners from radiation exposure during the inflow incident.

The milling of uranium at the Key Lake mill has several potential impacts, most notably the production of final waste water effluent, and mill tailings- the sludge, mineral residue and waste water (distinct from final effluent) that is left over after the uranium is processed.

As the tailings dry, the remaining fine particles can be transported off-site by wind or water. The mill itself also generates dust. These particles contain radionuclides such as uranium (U), radium (226Ra), lead (210Pb), and polonium (210Po) from the uranium decay series; and the fission product cesium (137Cs). The purpose of tailings management is to isolate and store the waste such that these potentially harmful elements do not enter water supply, the food chain, and are not directly ingested or inhaled. This involves containing the solids and treating the water to quality standards acceptable for release to the environment.

The Key lake facility uses the adjacent mined-out Deilmann open pit as a tailings facility. It was commissioned in January 1996 to store the McArthur River tailings. The tailings facility is constructed in the basement rock of the mined-out pit. Tailings are placed in a highly permeable envelope of crushed rock and sand in the mined-out pit. Residual water on the surface and the tailings is removed during tailings placement and collected for treatment. The consolidated tailings become a low-permeability mass contained in a permeable envelope, the idea being that groundwater will take the path of least resistance, around the tailings, rather than through them.

The biggest health concern is the potential human heal impacts from the consumption of milk or beef produced from animals that are part of a contaminated food chain. A number of studies have documented elevated radionuclides in various food chains in the Key lake area. These observations warrant concern and contained study, but to date no study has demonstrated a clear impact on human health.

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