LOS ALAMOS, New Mexico (Reuters) - A New Mexico wildfire raged largely unchecked for a fourth day near one of the nation's top nuclear arms production plants on Wednesday, but firefighters finally gained some ground in corralling the flames.

The so-called Las Conchas Fire has scorched at least 90,000 acres of pine-covered mountain slopes in the Santa Fe National Forest since erupting on Sunday and continued to lap at outskirts of the sprawling Los Alamos National Laboratory.

But newly reinforced ground crews managed by Wednesday to carve containment lines around 3 percent of the fire's perimeter on its eastern flank, marking the first headway they have made against a blaze, authorities said.

Los Alamos County Fire Chief Douglas Tucker said Wednesday afternoon that small spot blazes ignited by blowing embers were no longer encroaching on lab property, as they had earlier, but were still a factor in other areas of the fire zone.

Laboratory officials also sought to allay worries from some citizens and nuclear watchdog groups of the fire possibly unleashing residual ground contamination left from decades of experimental explosions or waste disposal in the area.

The first samples from special air monitoring conducted during the blaze revealed no signs of radiation or hazardous materials released into the environment, lab officials said.

"Those results show that what we see in this fire is the same as in any other fire in New Mexico," lab director Charles McMillan told reporters.

Under an agreement with New Mexico the laboratory is in the process of cleaning up some 800 remaining sites within the complex tainted by hazardous or radioactive substances, lab spokesman Heather Clark said.

She said the extent and nature of contamination varies widely from one site to another but includes areas fouled by trace amounts of plutonium, depleted uranium, industrial solvents and fuels.

Both the town of Los Alamos, home to about 10,000 residents, and the laboratory, with a work force of about 12,000, were evacuated on Monday. The lab is scheduled to be shut down at least through Thursday.


Nuclear watchdog groups have said they are concerned over the presence of 20,000 metal drums of plutonium-contaminated waste such as old clothing and equipment stored on a corner of the complex within about 3 miles of the edge of the fire.

But lab officials said the low-level radioactive waste site is clear of trees and other vegetation, and that fire-retardant foam could be quickly sprayed over the area in the unlikely event that flames reached the site.

Terry Wallace, director in charge of science and engineering at the lab, said on Wednesday the storage drums are certified as capable of withstanding heat "three times the temperature of a wildfire."

Situated on a hilltop 35 miles northwest of Santa Fe, the lab property covers 36 square miles and includes about 2,000 buildings, none of which have yet burned.

Established during World War Two as part of the top-secret Manhattan Project to build the first atomic bomb, the complex remains one of the leading nuclear weapons manufacturing facilities in the United States.

At a community meeting late on Tuesday, hundreds of evacuees gathered in a local gymnasium to hear updates and voice concern about the repercussions of the blaze.

Mai Ting, a doctor living in nearby Pojoaque, said she was frustrated by the lack of information on how to stay safe.

"I'm not a fear monger, but there's a reason this story is on the national and world news. It's because of the nuclear lab. I don't trust this fire," she said.

She added: "What do I tell my children and grandchildren? ... Well, they've left. I didn't want them around here."

The Los Alamos complex also contains 3 metric tons of highly radioactive weapons-grade plutonium, stored in concrete and steel vaults in the basement floor of a building near the center of the complex, with an air-containment system surrounding it. Lab officials say those storage structures are fire safe and pose no threat to public safety.

About 1,200 firefighters have been assigned to the blaze as of Wednesday evening, some from as far away as Wyoming, Texas, Colorado and Utah. The fire, believed to have started from a tree falling on a power line, grew by about 20,000 acres on Wednesday, fire information officer Jerome MacDonald said.

Tucker said fire crews on Wednesday had begun a controlled-burn operation along a 4-mile stretch of state Highway 501, creating a buffer zone to keep fire out of two thickly wooded canyons that lead straight into town.

(Writing and additional reporting by Steve Gorman, Editing by Greg McCune)

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