A burst tank at the Ranger uranium mine in Kakadu National Park has released what traditional owners say is up to a million litres of acidic radioactive slurry, in what they describe as one of the biggest nuclear accidents in Australian history.
The site could be closed for up to two months as mine operators seek to contain it, said Justin O'Brien, chief executive of the Gundjeihmi Aboriginal Corporation (GAC), which represents the traditional Mirarr people of the area.
At 1am on Saturday morning a hole was discovered in the side of a leach tank, with staff evacuated before it collapsed.
"This is up to a million litres of radiological material in the form of an acid exploding from a drum, bending a crane, twisting metal all around it, pouring down into stormwater drains, with 20 or so people ordered to evacuate," said Mr O'Brien.
It is the third security breach at the site in just over a month.
Mine operator Energy Resources of Australia (ERA) is seeking to mine at the site for a project called Ranger 3 Deeps but has agreed to do so only with the consent of the traditional owners.
"Day by day, litre by litre, incident by incident, they're losing whatever trust traditional owners have in them," Mr O'Brien told AAP.
He said Ranger 3 Deeps was off the table.
Recent breaches demonstrated that the mine's claims of being the most regulated in the world were incorrect and regulators had been found wanting, he said.
GAC will write to the expert advisory bodies of the World Heritage Committee requesting international help and is calling for a comprehensive external audit of what Mr O'Brien said was an endemically failing site.
ERA in a statement said the material was contained on site and has had no environmental impact and no personnel were harmed.
The capacity of the tank was approximately 1450 cubic metres and there was no explosion. But it was not yet known how much of the material had leaked out, ERA general manager of operations Tim Eckersley said in a statement.
A crane that had been used to assist in blocking the original hole was damaged when the tank gave way.
There were multiple processes in place at the mine designed to contain such a spill, he said.
"Containment systems stopped the flow and this has meant there is no impact to the surrounding environment," Mr Eckersley said.
"ERA is focussing on clean up and recovery and the protection of the environment and the health and safety of our people remains paramount."
Environmental groups are calling for a halt to operations at the mine pending an independent audit of the structural integrity of the plant, along with a review of the impacts of operations at Ranger.
"The time for mining a problematic and polluting mineral in a World Heritage area is over," Australian Conservation Foundation spokesman Dave Sweeney said.