The tunnel was filled with more than 4,400 cubic yards of grout intended to prevent any additional collapse, the agency said.
The tunnel partially collapsed earlier this year, prompting a brief alarm at the site that required some 3,000 workers to shelter in place.
Since early October, crews worked mostly at night to inject the grout, which is intended to reduce the risk of further collapse and increase protection for workers, the public, and the environment from radiation releases, the agency said.
CH2M Hill Plateau Remediation Co., the DOE contractor in charge of the tunnel, began grouting on Oct. 3 and placed the last truckload on Saturday, without any injuries, the Energy Department said.
“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 the agency’s Richland Operations Office.
Hanford is a sprawling site located near Richland, in south-central Washington. The site was created during the Manhattan Project in World War II and made most of the plutonium in the nation’s nuclear stockpile, including the atomic bomb dropped on Nagasaki, Japan. Workers are now engaged in a massive cleanup of the wastes, which is expected to take decades and cost more than $2 billion a year.
About 520 truckloads of grout were placed in the tunnel, whose wastes included eight flatbed railroad cars. The tunnel, 360 feet long, was built in 1956 and contains wastes created during Cold War production of plutonium.
Cameras inside ensured the grout flowed the length of the tunnel and around the contaminated equipment, the agency said.
Attention now shifts to a second, much longer tunnel, also located next to the demolished Plutonium-Uranium Extraction Plant. That tunnel contains 28 rail cars filled with wastes and is 1,700 feet long.
An analysis completed earlier this year showed Tunnel 2, built in 1964, did not meet current codes for structural integrity, and may not be able to bear the weight of the soil above the tunnel.
Stabilizing the first tunnel meets one of the requirements of a May 10 order issued by the Washington state Department of Ecology, which helps regulate the site.
No decision on final cleanup of the wastes in the tunnels has been made.
Hanford officials said grouting was the best solution to short-term dangers posed by the aging timbers holding up the partially-collapsed tunnel. The tunnel is built of creosoted timbers that likely were weakened by radiation, the Energy Department has said.
A 20-by-20-foot hole in the top of the tunnel was discovered on May 9. The Energy Department said there was no airborne release of radiation and no workers were injured during the collapse.