Hanford's new robotic arm resumed emptying radioactive waste from an underground tank this week after seven months of down time.
The success of the new technology, the Mobile Arm Retrieval System, or MARS, is critical to meet a court-enforced consent decree to empty the 16 tanks in the group called the C Tank Farm by fall 2014.
In more than eight years of work, six or possibly seven of the 16 tanks have had enough nonliquid waste removed to be classified as empty. Several more tanks have had some nonliquid waste removed.
MARS is the largest and most robust system yet tried to get millions of gallons of radioactive waste out of enclosed, underground tanks that date back to World War II, but so far it's only been used for a couple of weeks total.
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The waste is moved to newer and sturdier double-shell tanks for storage until it can be treated for disposal at the Hanford vitrification plant under construction.
MARS was operated by Department of Energy contractor Washington River Protection Solutions in Tank C-107 for a few weeks in October and then shut down because of issues not directly related to the new technology, including a broken pump in another tank.
MARS requires liquid to spray on waste to break it up and move it, but a pump that was providing liquid waste from a double-shell tank broke. Liquid waste rather than water is used when possible in emptying tanks to prevent the creation of more waste.
In a few weeks of operation in October, MARS retrieved 27 percent of the 253,000 gallons of waste in Tank C-107.
In its first six hours of operating as it resumed work this week, it removed 3,000 gallons of waste to bring the total volume of sludge removed from the tank to a little more than 70,000 gallons, said Dave Saueressig, C Farm retrieval manager for Washington River Protection Solutions.
The MARS is so large that workers had to dig down to the tank and then cut a hole in its top to replace a 12-inch-diameter riser with a 48-inch-diameter riser while protecting workers and the environment from the high-level radioactive waste in the tank.
MARS then could be lowered through the larger riser into the tank. Its robotic arm can be raised or lowered in the tank, rotated 360 degrees and unfolded and lengthened to reach 40 feet to the tank sides and bottom. The operating head is articulating, allowing it to reach around obstructions encountered in the tank.
It is equiped with a water cannon, high-pressure nozzles and fan nozzles to break up waste or sweep it toward a pump at the bottom of the MARS mast inside the tank.
The waste could have a hard layer hidden beneath the sludge, but the suite of sluicing tools MARS is equipped with and its ability to sluice so close to the waste, is expected to make it more effective than previous tools used to attack hard layers.
"We're pretty encouraged it can do some things we haven't been able to do in the past," Saueressig said.
In June, Washington River Protection Solutions also will be retrieving waste in a second tank, Tank C-104, at the same time. C-104 has a hard layer left at its bottom that will be attacked by soaking it with water and then a common industrial caustic. The caustic also will be used in Tank C-109.
"From here out we will operate two tanks at a time to meet the milestone" in 2014, Saueressig said.
Construction to prepare for retrieval also is being done on three more C Farm tanks. Old equipment is being removed from C-101 and C-102, and dirt is being removed from the top of C-105 to prepare to install a riser large enough to insert a MARS unit.
"The department is pleased with the progress that WRPS is making toward removing the remaining waste from the single-shell tanks in Hanford's C Farm," said Erik Olds, chief of staff for the DOE Hanford Office of River Protection.
Washington River Protection Solutions also tried an improvement of one of its most used technologies for retrieving tank waste, modified sluicing, this year with mixed success. Modified sluicing uses two high-pressure nozzles lowered into the top of a tank to spray liquid onto the waste and wash it to a central pump.
The enhanced-reach sluicing system replaced one of the nozzles with a new type that allows it to telescope down to get closer to the waste.
The system was used for the first time in Tank C-112, which had a hard crust on top of sludge. The system was able to break through the hard crust to retrieve some of the sludge beneath it.
However, it left large chunks of the hard pan that would not break up, and retrieval work stopped in the tank in April when no more progress was being made.
Before work stopped, about 70,000 gallons of waste had been removed but about 34,000 gallons remain. Samples of the remaining waste will be collected and studied before a decision is made on what to try next.