HANFORD, Wash. -- Workers started repairingat the in Washington state on Wednesday.
The tunnel contains railcars filled with radioactive waste. The ground above them caved-in Tuesday. So far, there's no sign that radiation has leaked.
With a machine blowing mist to keep down dust, workers began filling the gaping 400-square-foot hole in the tunnel holding contaminated nuclear waste.
Mark Heeter is a spokesman for the Department of Energy at Hanford. We asked him if the plan was for crews to pour dirt back into the hole.
"They're refilling the hole that's exactly right," he said. "It's gotta be done carefully … that's part of the sophistication."
When the hole was discovered Tuesday, much of the vast Hanford site was put on lockdown.
Broadcastify provided the following audio from an emergency dispatch shortly after the incident: "Dirt collapsed into the tunnel. Nobody is involved, no people involved."
Welder Dennis Riste was working about a quarter mile away.
He said it was "very" unusual for the sirens go off and being told to take cover.
"I've never experienced it," Riste said, "and then when I see all the firetrucks and ambulances and everything over by PUREX (Plutonium Uranium Extraction Facility) ... that, that kinda sunk home like 'geez.'"
While the Department of Energy saysfrom the damaged tunnel, Hanford has a long history of problems.
Plutonium used to make nuclear weapons for 40 years left a contaminated mess of buildings and equipment. This includes 56 million gallons of radioactive waste stored in 177 underground tanks, some of which have leaked. Hanford is often described as the most polluted site in the country.
CBS News science contributor Michio Kaku weighed in on the incident.
"A major earthquake, a major fire at the site … with all these toxic -- much of them flammable liquids -- could cause a disaster beyond our imagination," he said.
The cleanup at Hanford started more than 25 years ago, but it's far from finished. The latest estimates show it's likely to take another 30 years before the work here is complete.