A Department of Energy proposal to fill a Hanford Site radioactive waste storage tunnel with grout will not preclude future cleanup options for the tunnel, according to information presented at a public meeting Wednesday in Richland, Wash. The meeting covered DOE’s proposed changes to the Hanford Dangerous Waste Permit issued by the state of Washington, including updates for both of the Hanford PUREX Plant waste storage tunnels.
On May 9, 2017, the older of the two tunnels was discovered partially collapsed. Hanford cleanup contractor CH2M Hill Plateau Remediation Co. and a subcontractor filled the 360-foot-long tunnel with concrete-like grout under a contingency plan. The risk of further collapse was considered great enough to fill the tunnel without taking time to update the state permit and hear public comment. However, the permit will be changed before the second tunnel is stabilized.
The Energy Department says also filling the 1,700-foot-long second tunnel with grout is the best option to stabilize it as an interim measure, based on input from an expert panel.
The proposed changes to the state permit allow for an eventual decision on permanent disposition of both tunnels and the highly radioactive waste they hold. The first tunnel, built from 1954-56, holds eight railcars loaded with failed equipment from the retired Plutonium Uranium Extraction Plant. The second tunnel, built in 1964, holds 28 railcars loaded with similar contaminated equipment and pushed into the tunnel as recently as 1996.
One future cleanup option would be to use water jets, wire saws, excavation equipment, or other technologies to cut up the grout and waste it encases for removal in pieces. The cutting could be guided by an excavation program with detailed plans for where specific cuts would be made. The PUREX Plant could be used for characterization and additional size reduction of the pieces of grout, according to the proposed permit changes.
A second option could be landfill closure of the tunnels, with the grouted waste left in place. A surface barrier could be constructed to prevent water from leaching into the tunnels and carrying contamination into the ground beneath.
The proposed permit changes also include information from CH2M on its investigation of the cause of the first tunnel’s partial collapse and why the second tunnel also is at risk.
The first tunnel was built of creosoted timbers with a flat roof, also consisting of timbers. Seventeen of the timbers fell into the tunnel, said Al Farabee, who represented DOE at the meeting. The 8 feet of soil that had topped the timbers fell into the hole on one of the railcars, almost sealing the collapsed area, he said. No airborne radiation was detected when the collapse was discovered.
A definitive reason for the collapse has not been found, according to DOE. But contributing factors likely included heavy rain on May 4 and 5 that would have added to the weight of the soil above the timbers. Exposure to high levels of radioactivity since the first railcar was pushed into the tunnel in 1960 might have caused the wood to deteriorate. Slight vibrations, whether from thunder or distant low-magnitude seismic activity, also could have contributed.
Engineers familiar with the tunnel have speculated that water that infiltrated and pooled along a concrete ledge support could have contributed to the collapse. CH2M concluded that was only a “moderately plausible” reason for the collapse because of the arid weather of Eastern Washington.
CH2M concluded the second tunnel also was at substantial risk of collapse and should be stabilized as soon as possible. Grouting could start this summer if funds are available and the permit changes are approved on schedule. A schedule included in the proposed changes said preparations for grouting, including construction of piping and ventilation systems, and the grouting itself could take about a year. The project is expected to be more complicated than grouting the first tunnel, which was completed in the fall, given the longer length that must be filled with flows of material in the second tunnel.
The CH2M report said the second tunnel was beyond the design life typical for projects with similar construction. It added that problems during construction of the tunnel might have made it less structurally sound than it should be.
The tunnel was built initially with steel ribs supporting corrugated steel plate roof panels in an arch shape. After two sections of the tunnel collapsed before construction was complete, the design was modified to add steel I-beams and reinforced, arched concrete girders over the top.
Comments on the proposed changes to the permit will be accepted until April 12 on the Washington state Ecology Department website.
