A rapidly growing blaze closed part of the Idaho National Laboratory, a sprawling complex for nuclear research in the southeastern Idaho desert, on Thursday.

The fire, which started Wednesday evening on the Idaho National Laboratory grounds, quickly swept across roughly 4,800 acres of sagebrush and grassland on the southeast side of the 890-square-mile nuclear research area. Its cause was not known, said John Epperson, a lab spokesman.

No facilities were in immediate danger, but 700 employees of the lab’s Materials and Fuels Complex were told to stay home Thursday.

The fire had burned within a mile of U.S. Highway 20, forcing the closure of part of the road for much of the night. By Thursday morning, the highway had reopened, Idaho State Police said, and the blaze was about 10 percent contained.

The Materials and Fuels Complex, about seven miles northeast of the edge of the fire, conducts research into nuclear reactor fuels, said Idaho National Laboratory spokesman Ethan Huffman.

Huffman said that, as of 5 p.m. on Thursday, the fire was roughly 50 percent contained.

He said the metal-roofed complex was surrounded by vast sand buffers and the wildfire posed no danger to it but operations were suspended. Other facilities at Idaho National Laboratory, which employs about 3,600 workers, remained open.

Mary Woollen, executive director of the Jackson-based Keep Yellowstone Nuclear Free, said the real threat to the public is that the fire might have burned over ground that has been used for tests in the past and is now contaminated by radionuclides, a harmful by-product of nuclear reactors.

“The re-suspension of those particles in the air is a short-term, immediate threat,” Woollen said.

The fire could also denude the ground of plants, increasing the speed that rainwater can carry those radionuclides into the aquifer, she said.

“The Department of Energy’s first line is always: ‘There is no threat to the public,’” Woollen said.

However, Huffman said, “The areas that the fire is burning in are not areas where INL research activities have ever taken place.”