SPOKANE, Wash. — Radiation warning alarms sounded Thursday at a former plutonium production plant in Washington state, prompting a take-cover order that sent about 350 workers seeking cover indoors during the demolition of a plant that for decades had helped make nuclear weapons.

The order was lifted less than four hours later after low levels of radiation were detected at the Hanford Nuclear Reservation, the site of a massive cleanup.

The Energy Department said no injuries were reported, and workers had applied an adhesive product to the contamination to prevent it from spreading from locations on sidewalks and near a vehicle access gate at the Plutonium Finishing Plant, which dates to the late 1940s.

“The contamination detected has been at very low levels, levels not harmful to human health,” CH2M Hill, the private contractor demolishing the plant, said in a statement.

Workers were continuing to monitor the air for any further contamination, the company said.

The Plutonium Finishing Plant is one of the most polluted facilities at the sprawling Hanford site, which is half the size of Rhode Island.

“Air monitoring alarms during demolition are not unexpected,” the Energy Department said in a news release, noting the alarms are used to ensure demolition of the plant proceeds safely.

The incident followed the collapse last month of a tunnel at a separate Hanford facility that contained nuclear waste, prompting another evacuation. The Energy Department has said no injuries or contamination was reported as a result of the collapse.

The alarm Thursday rang as crews were removing outdoor equipment at the highly contaminated 580-square-mile (1,502 square kilometer) Hanford site near Richland, Washington.

Hanford made about two-thirds of the plutonium for the nation’s nuclear arsenal, and now is engaged in cleaning up the huge volume of resulting radioactive waste at the site established by the Manhattan Project during World War II.

The tunnel collapse occurred on May 9 in a 360-foot long (110-meter) rail tunnel built in 1956 from timber, concrete and steel. Radioactive waste was stored inside, and the entrance was sealed in 1965.

The tunnel contains a mixture of radioactive and chemical waste and irradiated equipment, including eight contaminated rail cars.

The ground above the collapse has been entirely covered by a huge tarp and will be filled with a cement-like grout to prevent future problems.

The cleanup at Hanford is expected to last until 2060 and cost $100 billion more than the $19 billion already spent.

In his current spending plan, President Donald Trump has proposed cutting about $120 million from the cleanup budget.