Hanford workers resumed pumping radioactive waste out of a leak-prone underground tank this week, after being plagued with technical problems in its waste retrieval program for almost a year.
"Every tank presents its own set of obstacles and challenges that we must be prepared to meet," said Scott Sax, Washington River Protection Solutions tank closure manager, in a statement.
It's working on the second single-shell tank the contractor has attempted to start emptying of sludge since taking over tank farm operations more than two years ago. Hanford has 149 single-shell tanks, seven of which already have had their radioactive waste emptied into newer and sturdier double-shell tanks.
Pumping resumed Tuesday night on Tank C-104, which has been a tough tank. Some work had been done toward retrieving the radioactive sludge from the 530,000-gallon-capacity tank as early as 2002, but efforts shifted to other tanks as difficulties were encountered.
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Not only did it have a lot of waste, but it also had one of the highest amounts of materials, such as plutonium, that emit alpha radiation in all the tank farms.
Washington River Protection Solutions tackled the 260,000 gallons of sludge in the tank in January 2010. But as progress was made and a pump was lowered further into the tank as the waste level dropped, the pump hit an obstacle hidden in 7-foot-deep sludge. About 75 percent of the waste already had been removed from the tank by then.
Workers tried to move the obstacle using a spray of liquid within the enclosed tank. But when that didn't work, a small robotic arm left over from another project was installed to push the obstacle out of the way in late 2010.
It had been modified with a head that included a pronged plate and high-pressure water nozzles to dilute the waste near the obstruction and drag it out of the way.
"The innovative modifications allowed us to use the arm in C-104 faster and at less cost than if we had tried to design a totally newsystem," Keith Carpenter, project manager for the C Tank Farm, said in a statement.
However, when workers tried to resume pumping, the pump would not turn on. A series of potential solutions failed, and a decision to replace the pump had been made when a final attempt to fix the problem was successful.
About 1,200 gallons of hot water were flushed through the pump, which freed waste that still plugged it, despite previous water flushes.
"We were all excited to get the pump running," Don Jones, a tank farm worker, said in a statement. "As operators, we would much rather be operating than doing anything else, and getting the pump going meant we would start moving waste again."
After several hours of pumping, workers could see what they believe is the obstacle that had stopped retrieval for about a year in its new location in the tank. The tank is being emptied with a technology called modified sluicing, which uses high pressure nozzles lowered into the enclosed tank to spray liquid onto the sludge and wash it to a central pump.
While waiting to resume work on Tank C-104 last year, tank farm workers also tried to retrieve waste from Tank C-111, which had about 57,000 gallons of waste at its bottom. However, work stopped after a few weeks when it became apparent that little progress was being made to break up or dissolve the hard crust of waste in the tank.
Work is being done on several other tanks to start retrieval of waste later this year or next year. That includes work to try a more robust waste retrieval system using a large robotic arm, the Mobile Arm Retrieval System.
-- Annette Cary: 582-1533; firstname.lastname@example.org