Scores of firefighters are still in the desert west of Idaho Falls battling what looks to be the biggest wildfire in the history of Idaho National Laboratory and one of the biggest in the lower 48 states so far this year.

About 105 firefighters from the Bureau of Land Management, INL and numerous local fire departments are battling the Sheep Fire, which was started by lightning 5 to 10 miles north of the Materials and Fuels Complex on Monday evening and had spread to consume more than 110,000 acres, or about 172 square miles, as of midday Wednesday. As of Wednesday, there were 20 fire engines out battling the blaze, INL Fire Chief Eric Gosswiller said at a news conference, of which 15 were from other agencies. Gosswiller said he expected to send some of the mutual aid home Wednesday, leaving about 15 engines total on the fire.

“We’ll be slowly releasing them today,” he said.

The fire started on the eastern end of the site, closer to Idaho Falls, and was spread by winds pushing it south and southwest and additional lightning strikes Monday that started more fires.

The major priorities Tuesday, Gossweiler said, were containing the northern edge of the fire and also making sure the southern end of the fire didn’t jump over U.S. Highway 20. On Wednesday, he said, firefighters were focused on the western edge of the fire.

“We did a lot of backfilling in that area this morning,” he said.

Gosswiller said he expected the fire to be 50 to 60 percent contained by the end of the day Wednesday, “if we survive the wind shifts this afternoon.”

The Idaho Falls and Ammon fire departments provided two engines each to help fight the fire, Gosswiller said. The Ucon, Inkom, West Jefferson, Hamer and Teton County departments, and the Idaho Department of Lands, have also provided engines and crews.

“We’ve had a lot of help,” Gosswiller said.

Although most of the INL facilities in the desert were closed as a precaution to all but a small number of essential employees, no INL buildings have been damaged by the fire, and neither has any of the lab’s electric infrastructure. The only structural damage, Gosswiller said, has been to some commercial power poles and other structures owned by Rocky Mountain Power. Rocky Mountain Power has already started repairs on these lines.

Most employees stayed home Tuesday and Wednesday from the Naval Reactors Facility, the Idaho Nuclear Technology and Engineering Center, the Materials and Fuels Complex, the Central Facilities Area, the Advanced Test Reactor, and Integrated Waste Treatment Unit. And, the Experimental Breeder Reactor-I Museum was closed. It is expected these facilities will return to normal operations Thursday.

The Advanced Mixed Waste Treatment Project and the Radioactive Waste Management Complex, which are operated by the cleanup contractor Fluor Idaho, were closed Tuesday but returned to normal operations Wednesday, said Fluor Idaho President Fred Hughes. Hughes said several shipments of transuranic waste were sent from the AMWTP to the Waste Isolation Pilot Plant in Carlsbad, N.M. on Wednesday.

INL Chief Operating Officer Juan Alvarez said INL has been monitoring both the air quality in the area and testing for the presence of radiation, and has found no spikes in radiation and that the air quality is safe for most people to return to work.

“I am happy to say we haven’t had a release of radioactive material,” he said.

Alvarez said INL expects wildfires and plans for them.

“Defensive actions are being implemented to keep the fire under control and to keep it away from key facilities,” he said.

Alvarez thanked the U.S. Department of Energy and the state for their help, and the firefighters “who have gone above and beyond to protect our people and our facilities.”

Although there have been bigger fires in Alaska so far this year, the Sheep Fire appears to be the second-biggest wildfire in the lower 48 states so far this year. The only bigger one, the Woodbury Fire near Superior, Ariz., grew to almost 124,000 acres before it was contained, according to the national fire information system InciWeb.

This one fire has also already made it the biggest fire year on INL land since at least 1994, if not earlier. The last fire of comparable size was the Jefferson Fire in 2010, which also burned about 110,000 acres total, of which about 79,000 acres was on INL land, according to a 2010 INL report. All of the land the Sheep Fire has burned has been INL’s, Alvarez said.

Gosswiller, who was also with the INL fire department in 2010, said the Sheep Fire has been a more unpredictable fire to fight than the Jefferson Fire was. The Sheep Fire, he said, has been one of the most erratic he has fought in all his time as an INL firefighter.

“We had heavy wind shifts that pushed this fire in a lot of different directions,” he said.

Reporter Nathan Brown can be reached at 208-542-6757. Follow him on Twitter: @NateBrownNews.