Employees at the sprawling Hanford Site plant, located about 275 kilometers southeast of Seattle, were sent an early morning alert by management telling them to "secure ventilation" and refrain from "eating or drinking."
Federal officials said there was no sign that any radioactive material had leaked after crews discovered that a 20-foot section of a 100-foot long tunnel -- containing rail cars filled with nuclear waste -- had caved in.
"All employees have been accounted for, there were no injuries and there is no indication of a spread of radiological contamination," said Destry Henderson, a spokesman for the Hanford Emergency Center.
The Hanford nuclear site was used to produce plutonium for the bomb that brought an end to World War II. Its last reactor closed down in 1987 but millions of gallons of leftover waste are contained in tanks at the site.
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Henderson said the alert was raised early today after employees at the Plutonium Uranium Extraction Facility (PUREX), a former chemical processing plant, noticed during a routine inspection that soil had sunk over one of two tunnels in the area.
He added that workers in other areas were told to take cover after emergency crews discovered that the roof of the tunnel had collapsed.
"There is no indication of a release of contamination at this point," a statement by the US Department of Energy said. "Responders are getting closer to the area where the soil has subsided for further visual inspection."
A photo posted on the Hanford Emergency Center's website shows a large hole in the roof of the tunnel.
"Crews are using hand surveying techniques in the outer areas around the PUREX facility," the energy department said.
"At and near the area of subsidence crews have deployed a TALON, which is a remote operated surveying device that is capable of radiological and industrial hygiene monitoring as well as capturing video footage."
The two tunnels were used at the beginning of the 1950s to store contaminated equipment and the cave-in apparently took place in an area where the two join together. Both tunnels are covered with approximately eight feet of soil.
Beyond Nuclear, a watchdog group, said the incident was proof that such nuclear dump sites remain a hazard.
"The unfolding crisis at Hanford ... (shows) that radioactive waste management is out of control," Kevin Kamps, a spokesman for the group said in a statement, citing similar incidents across the country in recent years.
The Hanford Site suffered a leak in a massive nuclear waste storage tank in 2016 that was described as "catastrophic" by a former employee.
The US Department of Energy downplayed the incident at the time saying the leak had been "anticipated" amid ongoing efforts to empty the tank.