The amount of radioactive waste held in Hanford's underground tanks has been reduced by 480,000 gallons through evaporation.
That creates more room in the nuclear reservation's 28 double-shell tanks to hold waste emptied from older leak-prone single-shell tanks, including Tank C-111. The double-shell tanks are near capacity.
Work began in mid-September to empty the nearly 57,000 gallons of solid waste in that tank after pumpable liquid waste was removed earlier. It's the only tank currently being emptied, although work is under way to prepare other tanks for waste retrieval soon.
The hard crust of waste on top of the sludge in Tank C-111 has proved difficult to break up using modified sluicing, said Steve Pfaff, Department of Energy project director for tank retrievals. The technology relies on high-pressure nozzles lowered into the enclosed tank to spray liquid onto the sludge and wash it toward a pump lowered into the center of the tank.
About 5,000 gallons of hot water were added to the tank at the beginning of the month to see if soaking the crust could soften it enough to allow the waste to be broken up through modified sluicing. Although liquid waste already retrieved from single-shell tanks is used when possible in the tanks, it is too saturated with salts to work in this case.
On Monday, the water is expected to be recirculated in the tank to stir it up as the soaking continues.
Each double-shell tank holds about 1.1 million gallons of waste, and the two campaigns just completed by Washington River Protection Solutions at the 242-A Evaporator created space equal to nearly half a tank.
"It's the evaporator's jobs to remove moisture from the waste, creating additional storage space in the double-shell tanks," Stacy Charboneau, DOE assistant manager for the tank farms, said in a statement. "Without the evaporator, we would have limited storage space and that impacts our ability to retrieve waste from the single-shell tanks."
In the evaporator, liquid tank waste is heated under vacuum so it will evaporate at a temperature of about 125 degrees Fahrenheit. Water vapor from the boiling waste is captured, condensed, filtered and sent to the nearby Liquid Effluent Retention Facility for further treatment and disposal. The concentrated waste is returned to the double-shell tanks.
The evaporator last ran in 2009, when it reduced waste in the double-shell tanks by about 940,000 gallons. About 53 million gallons of radioactive waste from the past production of plutonium are held in Hanford's underground tanks, but that varies as liquid waste is evaporated and water is used in some tanks to help empty them, as is being done in Tank C-111.
Since the evaporator began operating, it has reduced the volume of waste in the double-shell tanks by about 67 million gallons.
Reducing the volume of waste helps DOE avoid the cost of building new double-shell tanks to hold waste until it can be treated for permanent disposal at the vitrification plant, according to DOE. Building new tanks would cost hundreds of millions of dollars.