Hanford Site officials were hoping for little or no wind Friday to they can lay a plastic covering over the entire length of the PUREX plant underground waste tunnel where a breach was discovered on May 9.
Weather permitting, workers will stage a heavy piece of industrial plastic beside the soil berm that tops the 360-foot-long tunnel. The plastic measures 100 feet wide and about 400 feet long. “It is quite sizeable,” said Ty Blackford, president of CH2M Hill Plateau Remediation Co., one of the cleanup primes for the Department of Energy site in Washington state.
Workers have been practicing with smaller pieces of plastic elsewhere at Hanford this week, both to prepare to perform the work safely and to determine what wind speeds can be tolerated. “We don’t want it to sail and someone to get hurt,” Blackford said.
A lift will pull the plastic up and over the berm in one piece, if possible. The plastic will be held down on each side of the tunnel with about 150 concrete ecology blocks, each weighing roughly 3,800 pounds. The tunnel has several sampling ports with pipes sticking a few feet out of the ground that workers will have to maneuver the plastic around, Blackford said.
Once personnel being laying the plastic, work could be completed in eight to 10 hours. The plastic cover is intended to prevent airborne radioactive contamination if more of the tunnel collapses and also to prevent precipitation from soaking into the 8 feet of soil topping the tunnel, adding to its weight. The hole in the tunnel already has been filled with a mixture of sand and soil.
Hanford officials are investigating the cause of the collapse of the roof of the tunnel, which left a hole about 20 feet by 20 feet. The tunnel, constructed of creosoted timbers, was filled in the early 1960s with eight flatbed railcars loaded with large pieces of contaminated equipment and tanks from work at the Plutonium Uranium Extraction Plant.
The precise reason for the collapse might never be known, Blackford said. The timbers may have deteriorated over more than 50 years, losing strength due in part to exposure to gamma radiation. The latest winter was also unusually harsh, with about twice as much snow as usual, which could have contributed to the collapse.
“In all likelihood it was a combination of a lot of things,” said Doug Shoop, manager of DOE’s Richland Operations Office at Hanford. “I’m more interested in correcting conditions such that it is not possible to have a failure like this in the future.”
Representatives from DOE and CH2M plan to meet with officials from the Washington state Department of Ecology, the regulator for PUREX, as soon as Monday to discuss possible next steps to stabilize the tunnel, a schedule for the work, and a rough estimate of costs. DOE has declined to discuss what else might be done to stabilize the tunnel before it gets state agreement. However, state officials have said filling the tunnel with grout is among options discussed when Sen. Maria Cantwell (D-Wash.) was briefed and toured the area where the tunnel collapsed on May 13.
Officials have not determined whether a second, longer waste tunnel at PUREX also needs to be stabilized. It was built in 1964 of steel and concrete and holds 28 railcars with contaminated equipment.
No airborne radioactive particles have been detected since the collapse. Because DOE is not sure exactly when the roof collapsed, samples have been collected from nearby areas. No evidence of a spread of radioactive contamination has been found, Shoop said. The Department of Energy and the Washington state Department of Health plan to make monitoring data publicly available in the next few days.
Shoop credited Hanford radiological control technicians with discovering the breach after they questioned some unusual radiation readings on the morning of May 9. They were conducting safety monitoring to ensure the area was safe before planned maintenance work around the PUREX plant complex. When some readings were higher than expected, they began checking for the cause and noticed what appeared at first to be a depression in the berm above the older of the two PUREX waste tunnels.
Initial DOE statements during the chaotic hours of the emergency declaration that followed the discovery indicated there were no radiation readings above normal background levels. The radiation levels were not high enough to harm the technicians, Shoop said, correcting the initial report.
All Hanford employees had returned to work at the site as of Monday. However, waste tank farm workers based closest to the PUREX plant were assigned to alternate locations until the tunnel is further stabilized. Some roads in central Hanford remain closed.
“I’m thankful that no one has been hurt and (there was) no immediate harm to the environment,” Cantwell said. “But the incident is a harsh reminder that we need to stay on top of all our efforts at Hanford.”
