Story highlights Site produced plutonium for atomic bombs and uranium metal fuel for nuclear reactors

Government has spent billions trying to clean up the contaminated site

(CNN) A 20-foot hole in the roof of a tunnel at Washington state's Hanford nuclear waste site will be filled with clean soil, according to the US Department of Energy.

Workers noticed that a section of the tunnel had caved in Tuesday morning. The tunnel -- which is made of wood and concrete and covered in 8 feet of soil -- was constructed during the Cold War to hold rail cars loaded with equipment that had been contaminated in the process of plutonium production. It has been sealed since the mid-1990s, according to the Department of Energy.

The Hanford facility's 3,000 workers were ordered to shelter in place at one point Tuesday, but the order was lifted for non-essential employees hours later. Non-essential workers who live north of the site's Wye Barricade entrance were asked to stay home Wednesday.

"This hasn't happened before," Department of Energy spokesman Mark Heeter told CNN. "There are various projects in this site and occasionally there is spread of contamination."

But never, he said, has there been a tunnel collapse.

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