Workers at a Washington state nuclear site where a tunnel filled with nuclear waste in railroad cars partially collapsed have safely sealed off a large sinkhole that emerged as a result of the collapse, U.S. Energy Secretary Rick Perry said Thursday.
Authorities also revealed that the 400-square foot (37-square meter) sinkhole they filled with soil could have been there since last weekend before it was discovered Tuesday. That's because the area around the Hanford Nuclear Reservation's waste-filled tunnels is not observed every day by workers who patrol the site's sprawling grounds.
Authorities have detected no signs that radiation emanated from the collapsed tunnel, and the hole was filled with 53 truckloads of dirt delivered by workers wearing protective gear, Perry said.
Tuesday's discovery of the sinkhole prompted the evacuation of some nearby Hanford workers and an order for thousands more to stay inside buildings for several hours at the 500-square-mile (1,300-square-kilometer) expanse in Washington state's remote interior. No one was injured.
The plugging of the sinkhole "was accomplished swiftly and safely to help prevent any further complications," Perry said in a statement. "Our next step is to identify and implement longer-term measures to further reduce risks."
Hanford, created during the Manhattan Project to build an atomic bomb during World War II, for decades made plutonium for nuclear weapons. Now it is engaged in cleaning up the radioactive waste.
The cause of the partial roof cave-in of the tunnel is under investigation, said Mark Heeter, a spokesman for the Energy Department at the site in southcentral Washington state.
"We're not sure how long that will take," he said.
Also under investigation is when the cave-in happened. There is a massive volume of nuclear waste stored at the Hanford site, about the size of the U.S. state of Rhode Island, and not all of the storage sites are inspected daily, Heeter said.
Authorities believe the cave-in could have happened as many as four days before the hole was found, and Heeter said they "don't know exactly when it occurred."
But the agency said there was no sign that radiation escaped from the hole.
The state of emergency declared at Hanford ended late Wednesday night and most of the site's 9,000 workers were told to report back to work Thursday.
The tunnel collapse reinforced longstanding criticism that toxic remnants at Hanford are being stored in haphazard and unsafe conditions, and time is running out to deal with the problem.
Washington state officials on Wednesday demanded that the federal Energy Department immediately assess the integrity of all the Hanford tunnels.
"The infrastructure built to temporarily store radioactive waste is now more than a half-century old," said Maia Bellon, director of the state Department of Ecology, which oversees and regulates the federal government's Hanford cleanup.
The 360-foot long (110-meter) rail tunnel that collapsed was built in 1956 from timber, concrete and steel and covered with 8 feet (2.4 meters) of dirt. Eight flatbed railroad cars loaded with radioactive material were parked there in 1965.
A much larger nearby tunnel built in 1964 has 28 railroad cars with radioactive waste.
The Energy Department was warned in a 2015 report it commissioned that both tunnels were vulnerable to a collapse from an earthquake or deterioration of tunnel building materials caused by intense radiation, the report said.
The nearby Yakama Nation said it has warned about the safety of the tunnels for several years.
"No preventative action was taken," the tribe said in a statement.
The tribe also said the tunnels should be cleaned of radioactive waste and radiation long before a deadline of 2042 set by a cleanup agreement between the federal and state governments.
The cleanup of Hanford's waste is expected to last until 2060 and cost an additional $100 billion over the $19 billion already spent.