The Department of Energy is considering its next steps to empty waste from an underground tank after its most robust retrieval technology failed to work as expected when equipped with a vacuum attachment.
Work has temporarily stopped to empty Tank C-105 after the Mobile Arm Retrieval System, or MARS, worked very slowly to remove approximately 1,500 gallons of the 132,000 gallons of radioactive waste in the tank.
Washington River Protection Solutions proposed MARS when it was awarded the contract in 2008 to manage Hanford’s tanks holding 56 million gallons of waste left from the past production of radioactive material.
DOE has spent $42 million on the technology, according to documents filed in a federal court lawsuit brought by the state of Washington against DOE over deadlines related to retrieving waste from tanks and treating it for disposal.
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The system did work largely successfully in a demonstration project on a different tank, emptying waste from Tank C-107 and transferring it to a newer, double-shell tank for storage until it can be treated.
It began removing the 253,000 gallons of waste that tank once held in fall 2011 and the state of Washington agreed in summer 2014 that even though 12,000 gallons of waste remained, legal requirements had been met. The work took far longer than expected, but problems were unrelated to MARS, including equipment problems in the double-shell tank used to receive waste from Tank C-107.
That tank was emptied using a sluicing system that had multiple low- and high-pressure nozzles to spray liquid on the waste to break it up and move it toward a pump for removal.
Plans called for next using the MARS equipped with a vacuum system on Tank C-105, which is one of the 25 single-shell tanks at Hanford considered most likely to have leaked previously. Additional tanks have lost waste through a spill or overfill.
Hundreds of hours of tests of MARS with the vacuum system were conducted using mock waste to show that it could remove sludge, rocks and sand before it was inserted in Tank C-105.
“Eight months ago we thought we had it in the bag,” Kevin Smith, manager of the DOE Hanford Office of River Protection, told the Hanford Advisory Board in November.
But DOE and its contractor did not count on how firm the waste in Tank C-105 would be.
“It is clogging the pumps. It is a different animal,” Smith said. DOE was unsure if the waste was so hard because it was dry at the top or because of the metals it contained.
The mock waste to test the MARS vacuum system was chosen after core samples of waste were drawn from Tank C-105. But the samples “did not exhibit the high shear strength and ‘stickiness’ that the actual Tank C-105 waste has exhibited,” DOE said in a court document.
Vacuum retrieval has been used successfully, but slowly, in the past for some small tanks, holding much smaller volumes of waste — about 2,000 gallons. But this was even slower, said Jim Alzheimer, a single-shell tank engineer for the Washington State Department of Ecology, the regulator on the project.
Sluicing systems can add enough liquid to tanks that inches can sit on top of the waste. But the MARS vacuum system sprays liquid on the waste and immediately sucks it back up so liquid that might leak from damaged tanks does not escape, Alzheimer said.
The MARS technology is bigger, tougher and more versatile than other technologies used to empty waste from the tanks, because earlier technologies were designed to fit down the 12-inch diameter risers, or vertical pipes, that provide the only access to the older, underground tanks.
Part of the $42 million spent on MARS has been to develop technology to dig down to the top of a buried tank, cut a 55-inch diameter circular hole in the dome and then install a riser large enough to lower the MARS system into the tank without exposing workers or spreading contamination from the high-level radioactive waste.
The MARS technology can be raised or lowered inside the tank, rotated 360 degrees and unfolded and lengthened to reach 40 feet to the tank sides or bottom.
DOE said in court documents filed last week that it is exploring the use of other technologies, such as sluicing, in Tank C-105.
A sluicing system attached to technology small enough to fit down a 12-inch riser was used to empty another Hanford tank, Tank C-101, that is suspected of having leaked or spilled in the past, Alzheimer said. No additional leaking was detected during waste retrieval.
It is possible that the state could agree to allow a sluicing system to be used in Tank C-105, particularly for waste not near the walls or bottom of the tank, if an improved leak detection system is installed.
DOE also is looking at some possible reasons that the MARS vacuum technology did not work in Tank C-105, Alzheimer said.
“They are going to need it on other tanks down the line,” he said. DOE has 149 single shell tanks, with 14 considered emptied to regulatory requirements.
One issue could be the distributor that releases the waste retrieved from Tank C-105 into the bottom of a double-shell tank for storage. Its holes may be plugged up, and plans are being made to remove the waste-contaminated equipment and replace it, Alzheimer said.
To avoid creating new waste, liquid waste rather than clean water was used in the MARS vacuum retrieval. Trying the system using water would allow the liquid to be sprayed at a much higher pressure, he said.
Another problem area could be the screen that the liquid is sucked through by the vacuum in Tank C-105, he said.
In the meantime, DOE is continuing to do some work to empty another tank, Tank C-102, as equipment and resources are available. That work, when it is conducted, is going well, Alzheimer said.
The only other tank in the C Tank Farm that still has waste above regulatory standards is Tank C-111, where work has been stopped . A sluicing system that would fit down the small riser was being used there.
The system was equipped with carbon-impregnated hydraulic hoses specifically designed for the hostile radiation environment in Tank C-111, according to court documents. But they failed in August before the system began to operate.
DOE plans to install new sluicers this year. “Deployment of this more-robust technology will be of assistance in future retrievals,” DOE said in court documents.
DOE declined a request for an interview this week.