By Agence France-Presse
Employees at the sprawling Hanford Site plant, located about 170 miles (275 kilometers) southeast of Seattle, were sent an early morning alert by management telling them to take cover and 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 (six-meter) section of a tunnel containing rail cars filled with contaminated equipment had caved in.
A spokeswoman at the Department of Energy said nearly 5,000 employees were at the Hanford Site, which spreads across 586 square miles (1,518 square kilometers), when the emergency was declared and they were ordered to take cover.
She said fewer than a dozen employees were in close vicinity of the tunnel and were also ordered to take cover.
By early afternoon, non-essential employees were told to go home as crews prepared to fill the hole with clean soil, officials said.
"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, which is twice the size of Singapore, was used to produce plutonium for the bomb that brought an end to World War II.
Its last reactor closed down in 1987 and since then some 8,000 people at the facility have been working to clean up millions of gallons of leftover waste stored in aging tanks. The cleanup is expected to cost more than $100 billion dollars by the time it is completed in 2060.
Tom Carpenter, the executive director of the advocacy group Hanford Challenge, told AFP Tuesday's incident was a major "wake-up call."
"The takeaway message from this is: this is an old facility, it's not getting younger and ... this is a very dangerous place," he added.
Henderson said the alert was raised early Tuesday after employees at the Plutonium Uranium Extraction Facility (PUREX), a former chemical processing plant located at the center of the site, noticed during a routine inspection that soil had sunk over one of two tunnels in the area.
"At that point... an emergency was declared and about half a dozen employees at the facility were evacuated," he said.
He added that some 3,000 workers in the so-called "200 Area" near the wood and concrete tunnels were told to take cover after emergency crews discovered that the roof of the tunnel had collapsed. That order was later extended to the entire facility.
"After no contamination was detected, the shelter in place order was lifted, and employees were sent home from work early as a precaution," a statement by the Department of Energy said.
Henderson said emergency crews on the scene were trying to determine what caused the cave-in. Workers are also monitoring the air in the vicinity, using robots to detect any contamination.
A no-fly zone was meanwhile ordered over the site as a precaution.
There was speculation that the cave-in may have been caused by vibrations from nearby road works.
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.
"The approximately 360-foot-long tunnel where the partial collapse occurred contains eight rail cars loaded with contaminated equipment," the Energy department said. "That tunnel feeds into a longer tunnel that extends hundreds more feet and contains 28 rail cars loaded with contaminated equipment."
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.
The Hanford Site was part of the Manhattan Project, which led to the production of the first atomic bombs, including the ones dropped on Hiroshima and Nagasaki in World War II.