McArthur River was carved out of basement rock surrounded by porous Athabasca sandstone. Water constantly flows in and is pumped out of the mine, but a sophisticated refrigeration system is used to freeze the surrounding ground and keep producing areas dry.

In an area where the freezing system was not yet installed, grouting reinforcing concrete retaining walls were being carried out, and rounding ground. Cracks were seen in the retaining walls, and the mine was evacuated. Water began to gush into the tunnel on 6 April.

The mine’s pumps have been running around the clock, and additional pumps have been brought in. Provincial and federal regulators have been at the site, monitoring the flow of water into the mine and surface treatment facilities. Water pumped from the mine contains chemicals such as radium, and must be treated before it is placed into settling ponds.

Cameco president Gerald Grandey said the situation was “critical” and the shutdown would last at least a couple of months. In a later update, he said the company had underestimated the amount of water going into the mine and that this year’s production would be “well below” its annual capacity. He also said that increased pumping capacity was staying ahead of greater-than-expected water flow, but a concrete barrier was taking longer to build than expected. Grandey said the mine could be closed for as long as six months, with Cameco losing up to $5 million in profit for every month McArthur River is out of production.

Although flooding can be a problem for any underground mine, it is particularly worrisome at McArthur River, because of production and processing taking place primarily underground.

The main shaft plunges 685m, and Cameco is currently mining uranium-bearing rock between the 530m and 640m levels. Underground processing was seen as a way to minimise the mine’s environmental impact. A grinding mill at the lowest level of the mine has already been flooded. In its latest update, Cameco said the critical area of the mine, 640m below the surface, contains processing equipment and large pumps that would be lost if pumping capacity does not keep up with the water coming in.

A concrete barrier has been poured, and is expected to take several weeks to cure. In the meantime, the company will continue its efforts to ensure that it stays ahead of the water, and protects the deeper, critical part of the mine.

Mine personnel have improved the method of measuring water inflow and determined that the flow rate is higher than previously estimated. However, they have successfully added more pumping capacity. Water treatment capacity, on the surface, has also been substantially increased and is currently capable of matching the pumping capacity.