Some of the 1,000 firefighters at the scene lit backfires Thursday to remove brush and other fuels as well as coat a canyon slope with a thick line of fire retardant foam.
"For Los Alamos, it's been a great day. Everything is holding," Los Alamos County Fire Chief Doug Tucker said Thursday evening. "I'm very confident, after tonight that once they get done with this, this burn out, that the lower end is safe."
Los Alamos Canyon runs past 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, the Department of Energy's National Nuclear Security Administration site manager for Los Alamos, which over sees the lab. "Most of the materials have been dug up."
Los Alamos Canyon also 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. Fire had crept to within a half mile of homes in town.
Tucker said the area in the canyon was burning had been previously been thinned, providing a safe area for firefighters to attack it.
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.
Conditions in the area are so dry that the fire 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.
Firefighters, confident they can keep both the lab and town safe from the fire, 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.
"This is a fire like we've never seen before," Santa Clara Pueblo Gov. Walter Dasheno told The Associated Press in a telephone interview.
He said his people are devastated by the news coming in from the front lines of the firefighting efforts - cultural sites destroyed, forest resources lost and plants and animals that the pueblo's 2,800 residents depend on gone.
"We cried when we saw Mother Nature doing what she was doing to our canyon area. We were helpless," he 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.
Lab officials were trying to determine the extent to which thousands of 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.
The delayed projects include experiments run on two supercomputers. The lab 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.
Bryan reported from Albuquerque; Science Writer Seth Borenstein in Washington contributed to this report.