By Annette Cary / Tri-City Herald
Work to fill a Hanford nuclear waste tunnel that partially collapsed started, and then stopped, overnight Tuesday after some of the dirt used to initially stabilize the tunnel began to cave into it.
The minor cave-in early Wednesday morning was around a trench box where workers were injecting grout, a concrete-like material, into the tunnel.
No radiological readings above those anticipated were detected, and none of the workers were at risk, according to the Department of Energy.
A PUREX plant tunnel holding eight rail cars loaded with highly radioactively contaminated equipment was discovered partially collapsed on May 9. A sand and soil mixture was poured into the 20-by-20-foot hole the next day.
Plans were made to next fill the tunnel with a concrete-like mixture to further stabilize it. That required some of the fill placed in the hole to be removed, creating an area to insert a trench box equipped with pipes for injecting the grout.
Dirt began to fall into the tunnel around the edges of the trench box after 15 truckloads of grout were injected into the tunnel, as the work causing vibrations.
Hanford officials knew some additional dirt fall was possible as grouting began, and had a soil and sand mixture and equipment staged to be used if needed.
More fill mixture was added around the trench box Wednesday morning.
Hanford leaders continue to discuss how to address the issue, but DOE said in a statement that it expects grouting to resume soon. An estimated 650 truckloads are expected to be required to fill the nearly 360-foot-long tunnel.
DOE still expects to have the tunnel grouted on schedule by the end of the year.
“The workers are highly skilled and prepared for situations like the subsidence encountered,” said Doug Shoop, manager of the DOE Hanford Richland Operations Office. “There is no question about the difficulty of the work, but we will work safely and methodically to fill up the tunnel.”
Additional fill material and equipment had been staged at the project Tuesday night in case it was needed
Sen. Maria Cantwell, D-Wash., said she had been informed of the issue.
“I have been in touch with labor leaders, and in contact with the Department of Energy and contractors at the site,” she said. “Worker safety must be our number one priority, and I will continue to monitor this situation to ensure worker and environmental safety in the area.”
Grouting was planned by DOE and its contractor, CH2M Hill Plateau Remediation Co., both to stabilize the tunnel and protect against radiation.
Hanford leaders said the grout should not preclude future cleanup of the tunnel and its contents. In other projects using grout, Hanford contractors have sawed grout and the contaminated equipment or material it contained into pieces and lifted it out in chunks for disposal.
CH2M awarded a $2.8 million subcontract to Intermech Inc. of Richland in August for placement of the grout.
The grouting was scheduled during the night to help the trucks supplying the grout avoid daytime traffic and ensure a steady supply of grout. The grout is transferred from the supply trucks to a pumper truck stationed near the tunnel and then the grout is inject at the trench box.
Lances were used to direct the piping through the fill added to the breached area and into void areas of the tunnel.
The grout is engineered to flow easily, allowing it to encapsulate the rail cars and waste inside the tunnel. It is planned to be placed in layers, with each layer hardening before another layer is added on top of it.
Workers will use video cameras to monitor the grouting to confirm the tunnel is filled.
As grout displaces air in the tunnel, the air will be filtered as it leaves the tunnel, as a precaution. Air monitoring stations have been set up around the tunnel to help protect workers.
“Our focus remains on safety,” said Ty Blackford, CH2M chief executive at Hanford.
A second, longer waste storage tunnel at PUREX also is at risk of collapse, according to a DOE structural analysis. A method for stabilizing the second tunnel has yet to be determined.
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