Shares in Energy Resources of Australia (ERA) have dived more than 12 per cent following a toxic spill at the company’s Ranger uranium mine in the Northern Territory.

ERA shares were 16 cents, or 12.3 per cent, lower at $1.14 at 1148 AEDT.

The fall came after the company said it was still assessing the impact of the weekend spill on full year 2014 production guidance.

ERA, which is owned by Rio Tinto, has not given a time frame on how long the mine will be closed or the nature and extent of the repair works required.

Still, it says the leak has been contained.

“Processing operations have been ceased and clean-up operations are well advanced,” the company said in a statement on Monday.

Traditional owners say up to a million litres of acidic radioactive slurry poured from a burst tank at the mine in heritage-listed Kakadu National Park, making it one of the biggest nuclear accidents in Australian history.

Workers were evacuated from the nearby area after the tank “failed” and a mixture of slurry escaped.

ERA said containment systems had operated as designed and all water monitoring points at the mine and surrounding areas had reported normal readings.

“The slurry moved outside the bundled containment area, but has been captured and contained on site,” ERA said.

“There is no impact on the environment surrounding the Ranger Project Area.”

ERA plans to investigate the incident and co-operate with federal and territory government regulators.

Production guidance at the mine for 2013 remained at 2,800 to 3,200 tonnes, the company said earlier.

ERA recently unveiled a new water processing machine to prolong the mine’s future beyond an originally planned 2021 closure.

In July, ERA chief executive Rob Atkinson resigned after the company reported a $53.5 million half year loss.

The company warned that in the short term, the outlook for the uranium market was negative due to continued weak demand.