Retrieval of radioactive waste resumed at Hanford on Thursday, as the new tank farm contractor began pumping radioactive waste from one of the nuclear reservation's oldest underground tanks.

The pumping puts work "back on the road to reducing the risk posed by the waste in these aging tanks," said Bill Johnson, president of Washington River Protection Solutions, or WRPS, in a message to employees.

Tank C-110, which has 126,000 gallons of sludge and other radioactive and chemical waste, could be emptied to regulatory standards by late spring.

That would make it the first tank in which retrieval has been completed since spring 2007, in part because a spill of waste that summer stopped work for almost a year. Pumping has been done for only a few weeks in the months since safety issues related to the spill were resolved.

WRPS took over work at the Hanford tank farms Oct. 1 under a new contract worth $7.1 billion. The tank farms have 149 leak-prone single-shell tanks that have been emptied of pumpable liquids and 28 newer double-shell tanks.

Since then, the new Department of Energy contractor has conducted a safety review to prepare to resume retrieval of single-shell Tank C-110 and resolved issues at the double-shell tank where the waste will be transferred and held until it can be treated for disposal.

WRPS is using a technology called modified sluicing to retrieve waste from the single-shell tank. A nozzle is used to spray liquid on the waste inside the enclosed tank to break it up and move it toward a pump for removal from the tank. The technology uses a lower pressure and liquid volume than an earlier sluicing method to help protect the integrity of the tanks.

Rather than using clean water for the sluicing, liquid waste is being used as a spray to reduce the amount of new waste produced in the retrieval operation.

Tank C-110, which dates to 1946, is large enough to hold 530,000 gallons of waste. But all pumpable liquid waste was removed from C-110 and all the other single-shell tanks in 2004.

In late September 2008, former tank farm contractor CH2M Hill Hanford Group began retrieving sludge and other solids from Tank C-110 in the final weeks of its contract. Workers reduced the waste in the truck from about 150,000 gallons to the current 129,000 gallons, according to WRPS.

Waste from Tank C-110 is being transferred about 900 feet in an above-ground, hose-in-hose line to be stored in double-shell Tank AN-106.

However, as WRPS took over the contract, Tank AN-106 was nearly filled, and the solids had become so deep in the bottom of the tank that the pump was having difficulty removing liquid from the tank to make more space.

The plan inherited by WRPS called for replacing the pump, with work to be completed in June 2009 to allow pumping to resume on Tank C-110.

WRPS shaved six months off that schedule by having a collar built that would raise the pump about two feet higher to allow liquids to be removed from AN-106 and transferred to another double-shell tank.

To protect workers installing the collar on the pump in a pit above the underground tank, a 2-inch-thick sheet of steel with a hole large enough to pull the pump motor through was placed over the pit. It served as shielding against radiation from the tank while the pump pit cover block was removed.

Tank C-110 is the 11th single-shell tank at Hanford on which retrieval has been started. Retrieval has been completed on seven tanks.

"Removing the waste from the single-shell tanks and upgrading the aging infrastructure in the tank farms is a top priority for the Department of Energy," Shirley Olinger, manager of the DOE Hanford Office of River Protection, said in a statement.

It's also a necessary step toward having waste ready to feed to Hanford's vitrification plant, which is planned to start treating it for disposal in 2019, she said.