Pumping has stopped just shy of emptying the radioactive waste from the only Hanford tank where work was under way to retrieve waste.
Tank C-104, which has a capacity of 530,000 gallons, is considered empty to legal limits under a federal court consent decree when 360 cubic feet or less of waste remains spread across its bottom. But two technologies used to retrieve the waste have left about 650 cubic feet remaining.
Although the goal was not met, Washington River Protection Solutions was able to retrieve a significant amount of waste -- all but about 4,900 gallons of the 259,000 gallons of sludge and solid waste in the tank. Earlier pumping removed most of the liquid from the tank.
"Ecology is encouraged by the progress of retrieval from C-104," said Dieter Bohrmann, spokesman for the Washington State Department of Ecology, a Hanford regulator.
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The remaining waste, which resembles sand and fine gravel, will be sampled to determine the next step for the tank. In the meantime, work continues to ready additional tanks for retrieval.
Hanford is required to empty the waste from 16 single-shell tanks in the group called C Farm into sturdier double shell tanks by a legal deadline of 2014.
To date, six of those tanks have been emptied and this year work will be under way on nine more, including sampling, retrieval, engineering or buying of equipment, said Joanne Norton, Department of Energy project director for tank retrieval and closure.
Tank C-104 "was a real challenge in perseverance," said Kent Smith, deputy manager of tank retrieval and closure for Washington River Protection Solutions.
Workers had repeated equipment problems, the tank had a large volume of solid waste and the waste was high risk because of significant amounts of plutonium, cesium and strontium.
Work started to remove sludge from the tank in January 2010, using modified sluicing, a technology that relies on high-pressure nozzles lowered into the enclosed tank to spray liquid onto the sludge and wash it toward a pump lowered in the center of the tank.
But two problems occurred by spring. The pump in the double-shell tank receiving the waste began to have problems and needed to be replaced.
Then as pumping resumed in Tank C-104, workers hit an obstacle hidden in the 7-foot deep sludge that prevented them from lowering the pump farther as the level of waste dropped.
Workers solved that by inserting a small robotic arm left over from another project to push the obstacle out of the way. It turned out to be part of a pump used decades ago in the tank during a project to remove cesium and strontium from the tanks for storage elsewhere at Hanford.
Pumping of Tank C-104 restarted this year, but then the radioactive slurry in the tank bound up the pump, followed by a leak in the pump's piping.
Despite the series of problems, workers were able to retrieve all but about 6,500 gallons of the waste with modified sluicing.
Then they used a series of hot water flushes this spring to dissolve the remaining waste, removing all but about 4,900 gallons of it, before workers stopped seeing progress and halted pumping last week.
"We used as many retrieval methods as we have available to us," Norton said.
The remaining waste will be sampled to determine its chemical and radiological makeup. Then, DOE and the Department of Ecology will look at factors such as the risk it presents and the cost to retrieve more of it.
Washington River Protection Solutions will be moving ahead with work at Tank C-107 to install the Mobile Arm Retrieval System, a much larger and more robust robotic arm than has been used to date. In a new approach, the contractor cut into the top of the tank to insert a larger riser, so larger retrieval equipment could be inserted into the tank.
The new robotic arm is expected to be installed in the tank late this month and pumping could begin in August.
Also this summer, work is expected to resume on Tank C-108, where about 7,000 gallons of waste remain after most of the waste was removed in 2006. Caustic will be added to change aluminum salt into a form that will dissolve in water. That work could begin in early July.
-- Annette Cary: 582-1533; firstname.lastname@example.org; More Hanford news at hanfordnews.com.