Workers at the Department of Energy’s Hanford Site in Washington state on Monday completed filling with grout the PUREX Plant tunnel that was discovered partially collapsed on May 9.
The tunnel holds eight railcars loaded with highly radioactively contaminated equipment from the shuttered plutonium extraction plant. “Since a portion of the tunnel’s roof collapsed last spring, the tunnel has posed a threat of further collapse and the potential release of radiation into the environment,” said Alex Smith, Nuclear Waste Program manager for the state Department of Ecology.
Hanford cleanup contractor CH2M Hill Plateau Remediation Co. and subcontractor Intermech Inc., which has a location in nearby Richland, hauled about 521 truckloads of grout to the tunnel starting Oct. 3. Cameras in the tunnel were used to ensure that each layer of grout added flowed to the ends of the tunnel and surrounded the equipment and railcars.
The grout was poured in layers to help secure any lighter items that might float to the top of the concrete-like material. After the breach was discovered, a mix of sand and soil was used to fill the 20-by-20-foot opening in the top of the 360-foot-long tunnel, providing immediate stability and containment of radioactive contamination.
The grout was poured into the tunnel at the area where the fill was added, with the shorter south end of the tunnel requiring 12 layers of grout and the north end requiring 18 layers. Each layer was allowed to harden before another was added. As work started, fill material began to fall into the tunnel where a trench box had been placed for injection of the grout. Work stopped and the problem was resolved by adding more fill material. The project advanced safely and quickly after that, according to Hanford officials.
“Our contractors not only completed this work safely, but also ahead of the department’s projected completion timeframe of late December,” said Doug Shoop, manager of the DOE Richland Operations Office.
The grout protects the tunnel, constructed of timbers in 1956, from further collapse, Smith said, adding that it also protects people and the environment from the highly radioactive contents of the tunnel. A final remediation plan for the tunnel has not been developed, but the grout and equipment it surrounds could be cut out in pieces for permanent disposal.
“Although we recognize that grouting the tunnel may make eventual removal of the material in the tunnel more challenging, it was the best solution available to mitigate the short-term dangers posed by the aging timbers holding up Tunnel 1,” Smith said.
A second, longer tunnel at the PUREX plant was evaluated after the partial collapse of the first tunnel and determined also to be structurally unsound and at risk of collapse. No decision has been made on how to stabilize Tunnel 2, which was built in 1964 of concrete and steel. It is 1,700 feet long and holds 28 railcars loaded with highly radioactively contaminated equipment. The Department of Ecology ordered DOE to by Oct. 2 propose a plan for ensuring safe storage of the waste. However, it agreed to extend the deadline until Dec. 8 when DOE proposed convening an expert panel to evaluate options for stabilizing the tunnel.
