Hanford Site workers have begun the push to empty most of the remaining waste in the final tank at the C Tank Farm, 19 years after preparations for waste retrieval at the farm began. If work goes as planned, Tank C-105 could be emptied to regulatory standards by the end of the year. C-105 is the last of 16 tanks in the C Tank Farm to have waste retrieved, putting the farm on track to be the first of Hanford’s 12 single-shell tank farms to be eligible for closure.
Sludge removal from Tank C-105 transfer to a double-shell tank began in June 2014, using the Mobile Arm Retrieval System (MARS) vacuum. It’s the largest retrieval system used at Hanford’s waste storage tanks and required cutting a hole in the top of the tank for insertion. The robotic arm had been used previously with a sluicing system to empty another tank in C Farm, but a vacuum attachment was used at Tank C-105, because it might have leaked in the past. Officials wanted to limit the amount of liquid introduced into the tank during retrieval.
“This has been a very challenging one,” said Doug Greenwell, manager of single-shell tank retrieval for Department of Energy tank farm contractor Washington River Protection Solutions. In 2014 and 2015, the MARS vacuum sucked up 92,000 gallons of waste from the tank before “it broke of old age,” Greenwell said. The hose on the vacuum end gave out after operating for 16,000 hours.
The major components of the MARS system were removed from the tank and replaced with two enhanced-reach sluicing systems. The switch took nearly two years, and retrieval operations resumed on Aug. 11. About 13,500 gallons of waste has been extracted since then, leaving an estimated 20,000 gallons still in the tank. The goal is to collect all but about 2,700 gallons of waste from the tank, which would equal about an inch of waste if it were spread evenly over the bottom of the tank.
Washington River Protection Solutions has planned a nine-step process to complete retrieval, alternating soaks of either hot water or a caustic solution with the operation of sluicers to spray liquid on the waste to break it up and move it toward a pump within the tank. The two kinds of soaking liquids are intended to partially dissolve different types of aluminum in the tank and soften the waste.
Sluicing remains a concern because of the tank’s history. Contamination has been found in the soil at the tank, but historical records show it was overfilled and spilled waste in the 1950s. Whether some waste also leaked is not known. If the tank does spring a leak during waste retrieval, it likely will be toward the last steps at the bottom of the tank, according to Greenwell. Work would be stopped if a leak is suspected and a decision made whether to proceed, he said.
As the first tank farm planned to be emptied to regulatory standards, the C Tank Farm has been considered a demonstration project. Lessons learned will be applied to other single-shell tank farms. One of the key changes being made as Hanford workers prepare to empty the next tanks, those in the A and AX tank farms, is earlier installation of infrastructure. Infrastructure is being installed across those tank farms, rather than tank by tank, with the intent of making waste retrieval faster and more efficient after pumping starts.
Early Tri-Party Agreement deadlines had required all C Farm tanks to be emptied to regulatory standards by 2006. That proved unrealistic and deadlines have been extended, with the most recent deadline set by a federal judge after consent decree litigation. It requires the C Farm tanks to be emptied to regulatory standards by 2020.