The Hanford radioactive waste storage tunnel discovered partially collapsed in May has been filled with concrete-like grout to stabilize it.
Workers finished injecting layers of grout into the tunnel Monday, according to the Department of Energy.
“Our contractor not only completed this work safely, but also ahead of the department’s projected completion time frame of late December,” said Doug Shoop, manager of DOE’s Richland Operations Office at Hanford.
Stabilizing the tunnel meets one of the requirements of a May 10 order issued by the Washington State Department of Ecology, a Hanford nuclear reservation regulator.
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“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, manager of the Washington State Department of Ecology Nuclear Waste Program, a Hanford regulator.
The grout is intended to prevent further collapse of the tunnel until cleanup is planned and to provide protection from the radioactively contaminated pieces of equipment stored in the tunnel on eight rail cars.
No decision on eventual cleanup has been made, but grout was picked as an option because it has been used on other Hanford projects. The grout and waste it contained might eventually be sawed out in pieces for disposal, according to Hanford officials.
Smith said that grouting the tunnel will make eventual removal of the waste in the tunnel more challenging, but it was the best solution to short-term dangers posed by the aging timbers holding up the tunnel. The tunnel is built of creosoted timbers that likely have been weakened by radiation.
Work to fill the 360-foot-long tunnel began Oct. 3 with a type of engineered grout picked to flow easily through the void spaces in the tunnel and around the rail cars and the contaminated equipment they hold.
Grout was injected down the tunnel from each side of the collapsed area, which was filled with a mixture of sand and soil within a day of its discovery.
Each layer was allowed to harden before another layer was added to help keep any light materials from rising to the top of the grout. The breach was near one end of the tunnel, and that end had 12 layers of grout poured and the long north end of the tunnel had 18 layers.
Cameras in the tunnel were used to make sure that the grout flowed the length of the tunnel.
When work started to fill the tunnel, some of the sand and soil used to fill the tunnel collapsed into it around a trench box where grout was being injected.
Work stopped after the 15th truckload was injected, but the issue was resolved by adding more soil mix around the box.
About 521 truckloads of grout were added to the tunnel. Crews worked mostly at night as a precaution to keep the truck traffic off Hanford roads during the work day.
“The team did an amazing job,” said Ty Blackford, president of CH2M Hill Plateau Remediation Co., the DOE contractor in charge of the tunnel.
CH2M picked Intermech of Richland to inject the grout under a $2.8 million subcontract. Intermech built a mockup of the tunnel in south Richland to troubleshoot the work and allow workers to practice before Oct. 3.
Shoop credited the quick and safe grouting work on solid planning, extensive coordination and skilled workers.
The 20-by-20-foot hole in the top of the tunnel, which is covered with eight feet of soil, was discovered on May 9.
The tunnel was built in 1956 and holds eight rail cars loaded with contaminated equipment from the nearby PUREX plant. The plant chemically processed irradiated uranium fuel rods to remove plutonium for the nation’s nuclear weapons program during the Cold War.
The discovery of the breach resulted in a site-wide take cover issue at the Hanford nuclear reservation, with several thousand workers taking shelter indoors for several hours in buildings with ventilation shut off.
No spread of airborne radioactive contamination was detected and no worker was injured.
By the next night the hole in the tunnel had been filled with a mix of sand and dirt. In the following weeks a layer of thick plastic was draped over the soil berm that covers the tunnel to keep precipitation from adding to the weight of the soil.
A decision then was quickly made to fill the entire tunnel with grout to further stabilize it.
DOE also was required under the Department of Ecology to assess the structural integrity of a second, longer tunnel at PUREX. It holds 28 rail cars holding highly radioactive waste in its 1,700-foot length.
The assessment found that even though the second tunnel had sturdier steel and concrete construction, it also was at risk of collapse.
Initially, the state ordered DOE to propose a plan for ensuring the waste was safely stored by Oct. 2. It extended the deadline until Dec. 8 after DOE wanted to convene a panel of experts to analyze options to stabilize the second tunnel.
Grouting is among options being considered, along with covering the tunnel with a tent or building, performing a controlled collapse of the tunnel or retrieving the waste immediately.