By SUSAN MONTOYA BRYANALBUQUERQUE, N.M. (AP) a¿¿ The U.S. Department of Energy and the operators of the nation's only underground nuclear waste dump said Monday they are making plans to allow specially trained workers to enter the site for the first time in weeks after more than a dozen employees were exposed to low levels of radiation during a mysterious leak. Officials acknowledge they are in uncharted territory in responding to something that has never happened since the Waste Isolation Pilot Plant opened in 1999. The site is important to the nation's efforts to clean up decades of Cold War-era waste, and administrators are eager to resume operations once they are convinced it's safe to do so. WIPP has been shuttered since early February. Shipments were halted after a truck hauling salt through the repository's tunnels caught fire, and nine days later the plant's alarms were triggered by the radiation release. The first major step in finding out what caused the radiation release happened over the weekend as crews a¿¿ covered from head to toe in special blue protective suits and booties a¿¿ slowly lowered a bundle of air and gas monitoring machines into the repository's air intake system and its salt shaft. Enclosed mostly in plastic and sealed with tape, the battery-powered monitors fed about an hour's worth of information about the air in the shafts to another machine that logged the data. The monitors detected no radioactive contamination in the shafts. "Once they've determined that space is clean and safe, they'll send people down and that will be kind of a base to operate from and they'll start moving forward, taking samples, until they get to the contaminated area," said Russell Hardy, director of the Carlsbad Environmental Monitoring and Research Center, which has been monitoring air quality above-ground at the location.