Firefighters were confident Thursday they had stopped the advance of a wildfire that headed toward the Los Alamos nuclear lab and the nearby town that now sits empty for the second time in 11 years, even as they battled the blaze that crept into a canyon that descends into the town and parts of the lab.

Of 1,000 firefighters on the scene, 200 were battling the blaze in Los Alamos Canyon, which runs past the old Manhattan Project site in town and a 1940s era dump site where workers are near the end of a clean-up project of low-level radioactive waste. The World War II Manhattan Project developed the first atomic bomb, and workers from the era dumped hazardous and radioactive waste in trenches along six acres atop the mesa where the town sits.

"The threat is pretty limited," said Kevin Smith, site manager for Los Alamos for the Department of Energy's National Nuclear Security Administration, which over sees the lab. "Most of the materials have been dug up."

Los Alamos Canyon runs through town and a portion of the northern end of the lab, where a weapons research nuclear reactor was located until it was demolished in 2003. The fire burned upslope at least three miles from the sites and didn't pose an immediate threat.

Los Alamos County Fire Chief Doug Tucker said the area in the canyon was burning had been previously been thinned, providing a safe area for firefighters to attack it.

"Am I concerned? Yes. Do I feel threatened? No. But it's fire and it's dangerous," Tucker said.

In an evening briefing, Tucker said efforts that included burning out brush and other fuels and laying down a line of foam down a slope to keep the fire up the canyon appeared to be successful.

"I'll feel better about it in the morning," he said.

Tucker noted that conditions in the area are so dry that the fire, which had charred nearly 145 square miles, was burning downed trees that were scorched in the huge Cerro Grande fire in 2000. The fire also burned through moisture-rich aspen trees to push into the canyon.

Meanwhile, residents of Los Alamos, who fled the town earlier in the week under an evacuation order, wouldn't be allowed back home until Sunday at the earliest, Tucker said.

Despite the erratic nature of the blaze, fire officials remained confident that they could keep it from spreading onto the Los Alamos National Laboratory or into the town. They made progress on some fronts along its southern border Thursday even as the fire pushed northward toward land considered sacred by a Native American tribe.

"Today is a good day for parts of this fire. It's a bad day for other parts of this fire. Our hearts go out to the folks that are suffering the bad part," Tucker said.

The fire has chewed up tens of thousands of acres a day since it started Sunday, becoming among the largest forest fires in New Mexico history. Crews have contained only 3 percent of the fire. Fire officials believe the blaze will soon surpass the Dry Lakes fire, which burned more than 94,000 acres of the Gila National Forest in 2003.

Thunderstorms bringing erratic winds and some rain moved over the fire area Thursday afternoon, as crews braced for gusts of up to 40 mph that could spark spot fires ahead of the blaze.

Lab officials were trying to determine the extent to which experiments at the facility have been affected by a shutdown caused by the fast-moving fire. Lab Director Charles McMillan said teams will quickly figure out how things stand as soon as they're able to return.

Though the physical risk to the lab from the fire apparently had lessened Thursday, McMillan said "the laboratory is not just a bunch of buildings."

"It's not just a bunch of equipment. The laboratory is the people of the laboratory. That is the fundamental asset that this laboratory has and those people live all over northern New Mexico," he said.

The lab has been closed since Monday, when the city of Los Alamos and some of its surrounding areas — 12,000 people in all — were evacuated. There was no word on when it would reopen, but it was expected to remain idle at least through Friday.

Officials said the lab has some 10,000 experiments running at the same time that have been put on hold.

"We have a range of projects, some of them have shorter time deliverable, some of them are years to decades," said McMillan, who last month took over management of the lab that sits atop desert mesas.

The delayed projects include experiments run on two supercomputers, the Roadrunner and Cielo. The National Nuclear Security Administration's three national laboratories — Los Alamos, Sandia, and Lawrence Livermore — all share computing time on Cielo, which is among the world's fastest computers.

The lab also works on such topics as renewable energy and particle physics, solar flares, forensics on terrorist attacks, and studying the AIDS virus at the molecular level to help scientists develop strategies for developing vaccines.

Recent discoveries at the lab include a cheaper method of producing the element thorium, which is viewed as a potential sustainable energy source; so-called NanoBeacons that are silver atoms that glow different colors when they attach to certain acids and can help in diagnosing disease; and a special drilling fluid to help prevent massive oil spills, such as the one that happened last year in The Gulf.

Work under way at the lab now on hold because of the fire include studies on materials needed to extend the life of 1960s-era B61 nuclear bombs, and better understanding of how the ocean currents affect phytoplankton in the ocean that produce large amounts of oxygen.

On Monday, about an acre of lab property burned, raising concerns about possible contamination from material stored or buried on lab grounds. As a precaution, the government sent a plane equipped with radiation monitors over the lab. Samples analyzed so far from some of the lab's monitors show nothing abnormal in the smoke.

Lab authorities described the monitoring from the air as a precaution, and they, along with outside experts on nuclear engineering, expressed confidence that the blaze would not scatter radioactive material, as some in surrounding communities feared.

Anti-nuclear groups have sounded the alarm about thousands of 55-gallon drums containing low-grade nuclear waste — gloves, tools, even paper notes and other contaminated items — about two miles from the fire.

Lab officials said it was highly unlikely the blaze would reach the drums, and that the steel containers can in any case withstand flames and will be sprayed with fire-resistant foam if necessary.

Meanwhile, the economic impact of shutting down the town was already weighing on the minds of Los Alamos officials and business owners. Tucker said that unlike last time a fire forced the town's evacuation, none of the utilities have been shut off and no structures have been lost.

The state is allowing businesses to delay their sales tax payments and the local chamber of commerce and a local economic development group is offer businesses help making interest payments on loans.

"I don't see us in any significant danger because of the fire," said Kent Peg, owner of Los Alamos Fitness Center. "We have been in business nine years. We have a very loyal clientele. The town will rally."


Bryan reported from Albuquerque; Science Writer Seth Borenstein in Washington contributed to this report.