Hanford workers have emptied waste to legal requirements from two more of the nuclear reservation's leak-prone underground tanks, including the first tank emptied with the Mobile Arm Retrieval, or MARS, system.
"MARS performed extremely well," said Tom Fletcher, the Department of Energy's assistant manager of the Hanford tank farms, where 56 million gallons of waste left from weapons plutonium production is stored.
They are the first two tanks declared emptied to regulatory requirements this calendar year and bring the total of tanks considered empty to 13 of 149 single-shell waste tanks.
However, DOE still looks to fall short of a court-enforced consent decree requirement to have all 16 tanks in the group called the C Tank Farm emptied of hazardous chemical and high level radioactive waste by the end of September. All but one of the emptied tanks are in the C Tank Farm, bringing the total emptied there to 12, with four more to go.
DOE earlier notified the state of Washington that it was at risk of not having all of the C Farm tanks emptied by the end of September, and negotiations are under way between DOE and the state on possible revisions to consent decree requirements on emptying tanks and finishing the Hanford vitrification plant to treat the waste for disposal.
Waste is removed from leak-prone single-shell tanks and stored in newer double-shell tanks until it can be treated for disposal.
Washington River Protection Solutions had proposed a larger and more robust system to empty waste from underground tanks when it won the tank farm contract and started work at Hanford almost six years ago.
In the past, systems to empty the tanks had to fit down 12-inch-diameter risers, or pipes that extend from the ground into the enclosed, underground tanks. But Washington River Protection Solutions used remotely operated equipment to cut a 55-inch hole into the top of Tank C-107 and then installed a 42-inch-diameter riser, showing that opening a high-level radioactive waste tank could be carefully done without harming workers or the environment.
The larger riser allowed a bigger and more powerful robotic arm equipped with multiple technologies to be inserted into the tank to remove waste.
The MARS robotic arm can be raised or lowered in the tank, rotated 360 degrees and unfolded and lengthened to reach 40 feet to the tank sides or bottom. The operating head, with multiple low- and high-pressure spray nozzles, is articulated, allowing it to reach around obstructions encountered in the tank.
The use of MARS was considered a demonstration project, and it began removing waste in fall 2011. Since then, its operation has been shut down multiple times, but mostly because of issues unrelated to MARS, including equipment problems at the double-shell tank used to receive waste from Tank C-107. Equipment can deteriorate quickly in the harsh environment of radioactive waste.
The first technology used by MARS in Tank C-107, which held 253,000 gallons of waste after pumpable liquids were removed, was a sluicing system. It sprayed liquid waste on the sludge in the tank to break it up and move it toward a pump for removal. It was able to remove about 90 percent of the waste in the tank.
Then MARS used high-pressure liquid to attack the hard waste beneath the sludge.
However, that stopped being effective with about 7 percent of the waste in the tank remaining.
Hard chunks of waste at the bottom of the tank were too large to be pumped out and a "bathtub ring" remained of hard, crusted waste on the tank's wall.
Hot water was used next to dissolve waste that had high levels of phosphates.
The goal was to have 2,700 gallons or less of waste remaining in the tank, which amounts to about one inch of waste if it were spread evenly across the bottom of the large tank. However, the consent decree also allows a tank to be considered emptied to regulatory standards if three technologies have been used to remove as much waste as possible.
In the case of Tank C-107, about 12,000 gallons of waste remains after three technologies were used.
"MARS has retrieved all the mobile components of the waste," Fletcher said. The remaining waste is too hard to break up or in chunks that cannot be pumped out.
Before the tank is closed in a future step of cleanup, the remaining waste will be analyzed to make sure that the closure is protective of human health and the environment.
The second tank now considered empty is Tank C-101, which has about 5,000 gallons left in it after two technologies were used to remove waste.
The tank had 77,500 gallons of solid waste when work to retrieve the waste started in December 2012. Hanford workers used a telescoping sluicing system and then a high-pressure spray as a second waste-retrieval technology.
DOE asked the state to approve the tank as meeting the consent-decree requirements for waste retrieval, believing it did not have a third technology that would substantially reduce the risk from the remaining waste in the tank. The state recently agreed, DOE said.
Waste now is being emptied from Tank C-105 using a MARS system equipped with a vacuum, rather than a sluicing system, because the tank might have leaked waste in the past.