CARLSBAD, New Mexico (AP) -- Back-to-back accidents and an above-ground radiation release have closed the U.S. government's only deep underground nuclear waste dump indefinitely, raising questions about a cornerstone of the Department of Energy's $5-billion-a-year program for cleaning up waste scattered across the country from decades of nuclear bomb making.

On Feb. 5, the mine was shut and six workers sent to the hospital for treatment of smoke inhalation after a truck hauling salt caught fire.

Nine days later, a radiation alert activated in the area where newly arrived waste was being stored. Preliminary tests show 13 workers suffered some radiation exposure, and monitors have since detected elevated levels of plutonium and americium in the air. Ground and water samples are being analyzed.

Officials said they're confident the incidents are unrelated.

The Waste Isolation Pilot Plant is the nation's only deep underground geological repository for anything contaminated by more than the lowest levels of radiation. And opponents will certainly use the case to fight against any expansion of WIPP's mission, which is to take only transuranic waste from federal nuclear sites.

The closure highlights a lack of alternatives for disposing of tainted materials like tools, gloves, glasses and protective suits from national labs in Idaho, Illinois, South Carolina and New Mexico.

With operations at the plant on hold, so are all shipments, including the last of nearly 4,000 barrels of toxic waste that Los Alamos National Laboratories has been ordered to remove from its campus by the end of June. That waste is now stored outside with little protection.

Also on hold are tests to see if the dump can expand its mission to take more than so-called lower level transuranic waste from the nation's research facilities, including hopes by DOE that it can ship hotter, liquid waste from leaking tanks at Washington state's Hanford nuclear waste site.