Routine inspections in late 2008 turned up damage to the 750-metre deep chamber ceiling that might cause debris to fall on the radioactive waste in the former mine, the agency said in a statement. While "no immediate emergency measures are necessary," BfS said they were using concrete to stabilise the chamber entrance "quickly" and "preemptively."
Because a roof collapse can't be ruled out, officials will review possible measures to hinder radioactive dust from spreading, the statement said, adding that the first step is strengthening the entrance, which is several decades old.
In the meantime, employee access to the affected area will be limited. Long-term measures for permanent stabilisation include filling the chamber, the statement said.
Asse operators came under fire in the autumn of 2008 following revelations they failed to alert the government to serious violations and worrying conditions at the site.
German Environment Minister Sigmar Gabriel said in September that the facility has "as many holes as Swiss cheese," adding that it remained unclear who was ultimately to blame for the serious safety violations at the site.