Crews at the Hanford Site in Washington state by Thursday morning had injected 61 truckloads of grout into the PUREX Plant waste storage tunnel that was discovered on May 9 to have partially collapsed. Work was scheduled to continue Thursday night, when traffic would be light as cement trucks hauled the grout to the underground tunnel in central Hanford.
The grout is intended to help stabilize the tunnel and provide additional protection from the radiologically contaminated equipment inside. Injection of an estimated 650 truckloads of grout into the almost 360-foot-long tunnel began Tuesday night. But after 15 truckloads were added, workers noticed dirt starting to cave into the tunnel around a trench box. The trench box has lance-tipped piping reaching into the void areas of the tunnel for grout injection using a pumper truck, which causes vibrations, Hanford officials said.
A mix of sand and dirt was poured into the 20-by-20-foot hole at the top of the tunnel the day after the breach was discovered. Some of the fill material later was removed to fit the trench box into the collapsed area of the tunnel, and that sand and soil mixture was subsiding into the tunnel.
Work halted when the subsidence was discovered. No radiological readings above those anticipated were detected and none of the workers were at risk, according to the Department of Energy. Possible subsidence had been anticipated and fill mixture had been staged by the tunnel Tuesday night.
On Wednesday, crews added more fill material around the box, which appeared to stabilize it, allowing injections to resume, according to DOE. “The workers are highly skilled and prepared for situations like the subsidence encountered with injecting grout to stabilize the tunnel,” said Doug Shoop, manager of DOE’s Richland Operations Office at Hanford. “There is no question about the difficulty of the work, but we will work safely and methodically to fill up the tunnel.” The grouting is scheduled to be completed by the end of the year.
The grout is engineered to flow easily, allowing it to encapsulate the eight railcars in the tunnel, which hold equipment contaminated with radioactive waste from the former plutonium extraction plant. The grout is planned added in several layers, with each layer of the concrete-like substance allowed to harden before another is added.
Piping extending from the trench box is being used for ventilation and observation, in addition to grout injection. As the injected grout displaces air inside the tunnel, the air is filtered as it exits, as a precaution. To ensure worker safety, air monitoring stations have been set up around the tunnel. Workers are using video cameras to monitor the grouting and confirm the placement of the material.
“Our focus remains on safety,” said Ty Blackford, president of cleanup contractor CH2M Hill Hanford Plateau Remediation Co., which is in charge of the tunnel. In August the company awarded a $2.8 million subcontract to Intermech Inc. for the placement of the engineered grout.
The Energy Department and CH2M selected grouting for stabilization of the tunnel, in part, because Hanford officials believe it will not preclude future cleanup and closure work. DOE has said the grout and waste it surrounds could eventually be sawed up and removed in pieces from the tunnel, with the grout helping to shield contamination during cleanup. A second, longer waste tunnel at the PUREX Plant also is at risk of collapse, according to a DOE structural analysis, but a plan for its stabilization has yet to be selected.
