Work is under way to empty three of Hanford's underground tanks of high-level radioactive waste, which is believed to be a first since solid waste retrieval began, according to Washington River Protection Solutions.
The Department of Energy contractor has an ambitious schedule to meet a court-enforced legal deadline to have all 16 underground tanks in the group called C Tank Farm emptied two years and three months from now.
So far, the Hanford contractor believes that seven may be emptied enough for the state to approve. But emptying those tanks and a tank in the S Tank Farm has been done at an average rate of about one every 18 months.
"Our mandate is to increase that pace," said Kent Smith, Washington River Protection Solutions project manager.
To meet the deadline, workers will need to complete retrieval on three more tanks this years, three in 2013 and finish three more in 2014.
"WRPS is simultaneously retrieving waste from three tanks and performing construction activities on two additional tanks," said Lori Gamache, DOE spokeswoman. "Activities in the tank farm will continue at this pace as we work toward retrieving waste from the remaining tanks in C Farm to meet the consent degree milestone by September 2014."
Earlier this year, Washington River Protection Solutions announced it was retrieving waste from two tanks simultaneously, the first time that had been done in more than a decade.
"It is a great thing that they are in three tanks," said Jeff Lyon, the Washington State Department of Ecology manager for tank waste storage.
Meeting the deadline still will be difficult because of the complexity of the task, he said. Of the seven C Farm Tanks DOE has asked the state to approve as emptied to regulatory limits, four were relatively small 55,000-gallon tanks. The remainder of the tanks in the farm have capacities of 530,000 gallons.
They must be emptied of all but about 2,700 gallons, or about an inch of waste if spread evenly across the bottom of a tank.
"It's a very difficult thing taking waste out of the tanks," Lyon said.
One of the tanks being emptied now, Tank C-107, is using the new Mobile Arm Retrieval System, or MARS, a robotic arm that's much larger, stronger and more sophisticated than previous retrieval systems inserted down risers into the enclosed, underground tanks.
MARS has operated for about two months total -- after a lengthy downtime because of equipment problems in support systems -- and has pulled about 65 percent of the 253,000 gallons of solid waste out of the tank, Smith said.
It's projected to finish retrieval of sludgelike waste by late July, getting down to a hard layer of waste suspected to be at the bottom of the tank.
The sludge is being removed with a sluicing system that sprays liquid on the waste at about 100 pounds per square inch, then the waste is removed with a pump. But the hard layer will be attacked with a system that sprays liquid at 10 times that pressure and, unlike some previous systems, the nozzle can be placed very close to the waste.
The system already has been able to move out of the way a piece of old equipment found hidden in the sludge. When a similar piece of equipment was found in another tank, workers required weeks to move it out of the way.
"We were able to move it in six hours rather than six weeks," Smith said. "MARS is equipped to deal with it."
The other two tanks in which work is under way each have a hard layer remaining that earlier robotic or sluicing systems were no match for after bulk retrieval was completed. In both Tank C-104 and also Tank C-109, chemical dissolution is being used.
A series of water soaks is done first to help remove water-soluble salts. Then concentrated sodium hydroxide, a common industrial caustic already used in newer tanks to preserve the correct pH level, is added to the tanks. It converts the aluminum gibbsite salts in the waste to sodium aluminate, which also can be removed with water.