(Adds first inspectors re-enter the site)
By Joseph L. Kolb
ALBUQUERQUE, N.M., April 2 (Reuters) - An inspection team ventured into an underground nuclear waste disposal vault in New Mexico on Wednesday to begin an on-site investigation of a radiation leak nearly seven weeks ago that exposed 21 workers and forced a shutdown of the facility.
The mission by experts from the company that manages the site marked the first time since the mishap that workers have been sent deep into the salt caverns of the Waste Isolation Pilot Plant, where drums of plutonium-tainted refuse from nuclear weapons factories and laboratories are buried.
The unexplained leak of radiation, a small amount of which escaped to the surface, ranked as the worst accident and one of the few blemishes on the plant's safety record since it opened in 1999.
Located about 25 miles east of Carlsbad, New Mexico, in the Chihuahuan Desert, the facility is the nation's only permanent repository for the U.S. government's stockpile of nuclear waste, much of it left over from the Cold War era.
The waste, including discarded machinery, clothing and other materials contaminated with plutonium or other radioisotopes heavier than uranium, are sealed in chambers carved into salt formations more than 2,100 feet (640 meters) beneath the desert surface.
The plant has been closed to further deliveries of waste since Feb. 14, when an air-monitoring system detected an unexplained release of radiation underground.
Although an alarm automatically switched the ventilation system to filtration to keep radiation from spreading, trace amounts of manmade isotopes such as americium-241, a byproduct of nuclear weapons manufacturing, were measured at the surface.
Testing of workers at the site, all of whom were above ground at the time, showed that 21 were contaminated, though managers of the plant said the level of exposure was too low to pose any health risks.
SOURCE OF LEAK UNDETERMINED
The U.S. Department of Energy and the contractor that runs the repository, Nuclear Waste Partnership LLC, have said there was no threat to the public or environment.
The source of the radiation leak has not been determined, but a DOE spokesman at the agency's Carlsbad office, Ben Williams, said one theory is that there might have been a structural collapse at one of the storage compartments, or panels.
Experts suspect the release was most likely to have originated in one compartment, Panel 7, where material had recently been added, Williams said.
The mishap came two weeks after a truck caught fire at the plant in an accident in which several workers suffered smoke inhalation. Plant officials have said the two incidents were not related.
Additional radiation sensors lowered into access shafts of the cavern since early March indicated no further radiation leaks in the surrounding air, paving the way for the re-entry by inspectors on Wednesday afternoon.
An eight-member inspection team consisting of radiological and mine-safety experts descended by elevator to the underground interior of the plant to establish a base of operations and measure radiation levels, DOE spokeswoman Carrie Meyer said.
There was no immediate word on what they found upon arrival. A second team was to follow later in the day. Inspectors were garbed in heavy protective gear and special lapel monitors to measure personal exposure.
The two groups were expected to spend a few hours underground before returning to the surface.
Officials say future teams would venture into the waste disposal area of the cavern once conditions at the bottom of the entry shafts are deemed safe enough. A plan will then be devised to pinpoint and remedy the source of the leak.
In the meantime, nuclear waste that had been due for shipment to the repository from the Los Alamos National Laboratory, about 300 miles across the state, is being shipped instead to a privately run facility in Texas for temporary storage there, Williams said. (Writing and additional reporting by Steve Gorman; Editing by Cynthia Johnston, Leslie Adler and Diane Craft)