There is a high risk that a second tunnel filled with radioactive waste might collapse at the Hanford Nuclear Reservation in Washington state, the U.S. Department of Energy said Friday.
A tunnel partially collapsed on May 9, forcing some 3,000 workers to shelter in place for several hours.
There were no injuries or release of airborne radiation from that accident, the agency said.
The Energy Department said it had completed an evaluation of a second tunnel on the former nuclear weapons production site and determined there is a high potential for the 53-year-old structure to collapse. The agency has an Aug. 1 deadline to develop plans to prevent that.
Hanford is located in south-central Washington state and for decades made plutonium for nuclear weapons. The site is now engaged in cleaning up a massive inventory of nuclear waste.
In a report released Friday, the Energy Department said the two sealed tunnels "do not meet current structural codes and standards."
"The report finds that Tunnel 2 is identified as presenting a high potential for localized collapse," it said.
Tunnel 2 was built of metal and concrete in 1964. It is approximately 1,700 feet (510 meters) long and is covered with eight feet (2.5 meters) of soil to prevent radiation from escaping. Inside are 28 flat-bed rail cars containing nuclear waste, including giant storage vessels and other large equipment from plutonium production.
The tunnel was sealed in 1996 and has not been entered since, said Doug Shoop, Hanford manager for the Energy Department.
The ceiling of Tunnel 1, which was built in 1956 and is 360 feet (110 meters) long, partially collapsed in May. That tunnel contains eight rail cars carrying radioactive waste, and it was sealed in 1965.
As a result of the partial collapse of Tunnel 1, Hanford officials have started daily inspections of the two tunnels and installed remote-controlled cameras to monitor them, Shoop said.
They are considering using remote-controlled equipment to go inside the tunnels and assess their condition, Shoop said.
"These tunnels are not safe and have literally been out of sight and out of mind," said U.S. Sen. Ron Wyden, D-Ore., a frequent Hanford critic. "I'm calling for action now in order to protect Hanford workers, the public and the environment from this ongoing threat."
Shoop said the Energy Department plans to fill the partially collapsed Tunnel 1 with grout by the end of the year, to stabilize the structure until a permanent disposal plan is created.
The new report said the partial collapse of the roof might have been caused by heavy rainfall a few days before, or deterioration of timber supports because of age and exposure to radiation.
The Energy Department believes the risk to employees and the environment from a collapse of Tunnel 2 is minimal, because the eight feet of soil atop the tunnel would sink into it and prevent or minimize radiation from escaping, the agency said.
Radioactive and chemical waste in the two tunnels was created by the nearby Plutonium Uranium Extraction Plant.
Hanford was built by the Manhattan Project during World War II as the U.S. raced to beat Germany to create an atomic bomb. Hanford made the plutonium for the atomic bomb dropped on Nagasaki, Japan, at the end of the war and went on to make about 60 percent of the nation's plutonium during the Cold War.
The site now contains the nation's greatest volume of radioactive defense wastes. Cleanup of the site is expected to last until 2060 and cost an additional $100 billion.