A partially collapsed Hanford waste storage tunnel will be filled with grout to further stabilize it, the Department of Energy said Wednesday.
“As we cannot quickly determine the exact cause of the partial collapse of the tunnel, the rest of the tunnel is still subject to collapse,” said Doug Shoop, manager of the DOE Hanford Richland Operations Office.
The state Department of Ecology, a Hanford regulator, agrees with the plan, said Alex Smith, the director of Ecology’s nuclear waste program.
On May 9, Hanford workers discovered a section of the tunnel roof about 20 feet long had collapsed, potentially exposing equipment with highly radioactive contamination stored in the tunnel.
An emergency was declared and initial steps taken to stabilize the 360-foot-long tunnel, which holds eight flatbed rail cars loaded with contaminated equipment from the nearby PUREX processing plant.
The breach was filled with a mixture of sand and soil and then a layer of heavy plastic was pulled over the length of the tunnel.
Both DOE and the state agree on the advantages of filling the tunnel with concrete-like grout.
It would help stabilize the tunnel, which is built mostly of creosoted timbers. It would encase the rail cars and the equipment they hold in place, and it would provide shielding from radiation.
The tunnel cannot be filled completely to the top with grout, but having it mostly full means that if another portion of the tunnel roof collapses, there would be far less risk, Smith said.
There was a concern after the partial tunnel collapse in May that some airborne radioactive contamination might be released, but none was detected by federal and state air monitoring.
Filling the tunnel with grout could be done reasonably quickly without hindering the eventual permanent cleanup of the tunnel and its contents, Shoop said.
“All options are still in play by using this methodology,” he said.
DOE and its contractor looked at many options, including permanently removing the waste from the tunnel in the near term to building a protective structure over the tunnel, Smith said.
Not only does grouting appear to be protective, but it allows more immediate protection, is less expensive and is more durable than some options considered, she said.
Grouting also is a radioactive waste stabilization method that Hanford has plenty of experience using.
The K East Basin, the area below the floor of the U Plant canyon and the hot cells at the Waste Encapsulation and Storage Facility all have been filled with grout.
In some cases when grout is used, the grout is then cut into pieces for removal and disposal.
No long-term decision on what to do with the waste at the tunnels has been made, but grouting it now would make it easier to remove eventually in blocks of grout, Smith said.
CH2M Hill Plateau Remediation Co. plans to hire a subcontractor with grouting experience to do the work at the tunnel, said Ty Blackford, CH2M president at Hanford.
His goal is to have the tunnel grouted by November, and DOE has told the state that the work will be completed by the end of the year.
“We’re trying to not subject the tunnel to another hard winter or any further stress more than we have to,” Blackford said. “We want to move out quickly, but move out smartly.”
Smith said ideally the state would like to have the tunnel grouted well before winter starts.
“We want to make sure they take the time to engineer the project safely and still do it as fast as they can get it done,” she said.
Most of the time between now and the completion of grouting will be used making preparations for the grouting.
The tunnel already has some sampling ports extending into the tunnel that may be used for adding grout down the length of the tunnel. CH2M is evaluating whether more openings may be needed.
The tunnel has an inactive ventilation system, but CH2M expects to add a simple, temporary system to control heat and humidity as the grout cures rather than get it up and running.
The grout will be laid in multiple layers the length of the tunnel.
Adding grout in layers, each possibly a few feet deep, will limit the pressure on the walls of the tunnel, Blackford said. It also will help keep any equipment with void spaces from floating to the top of the grout by cementing it in place at its base.
Experienced subcontractors can pump in grout that flows almost like water around the waste stored in the tunnel, Shoop said.
Each layer of grout will be allowed to cure for a few days and become part of the structure of the tunnel before the next layer is added, Blackford said.
Hanford officials continue to investigate the cause of the tunnel collapse, but have said that the exact reason may never be known.
The wooden tunnel has held waste for more than 50 years. Past reports have speculated about how quickly gamma radiation from the waste might weaken the wood.
The unusually cold and wet winter just past may have contributed to the tunnel collapse. The plastic placed over the tunnel is intended, in part, to keep precipitation from soaking into the eight feet of soil topping the tunnel and making it heavier.
Hanford officials continue to evaluate a second tunnel running parallel to the one that partially collapsed.
“There is nothing to indicate an immediate risk,” Blackford said.
The second tunnel was built in 1964 with more robust materials. It has internal steel I-beams attached to reinforced concrete arches and a steel liner.
It is much longer, at 1,700 feet, and has room for 40 rail cars, but was never filled to capacity. It was sealed up in the mid-1990s with 28 rail cars of contaminated equipment inside.
DOE and CH2M also continue to work to meet the requirements of a state legal order prompted by the tunnel breach.
Its first deadline is to submit a structural integrity evaluation for both PUREX tunnels by July 1. By Aug. 1, DOE must submit a a draft report on actions to ensure safe storage of waste in both tunnels. It has until Oct. 1 to submit a proposed plan for final cleanup of the tunnels.
Smith commended DOE for moving ahead of the August deadline to propose a plan for safe storage of waste in the tunnel most at risk.
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