Hanford workers have cut a large hole in an underground tank holding radioactive waste for the second time since waste began to be added to the tanks in World War II.
The first cut was three years ago to install a larger pipe called a riser to insert a new robotic arm that's much bigger, tougher and more versatile than other technologies used to empty waste from enclosed underground tanks.
The second hole, which was cut early Wednesday, will allow another of the new robotic arms, the Mobile Arm Retrieval System, or MARS, to be installed in one of Hanford's 149 single-shell tanks.
This MARS is equipped with a waste vacuum system, rather than a sluicing system, to limit the liquid added to leak-prone tanks.
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The 17-inch thick concrete and rebar-reinforced dome of Tank C-105 plus the 6 feet of soil above it serve to protect workers from radiation from the 132,000 gallons of high-level radioactive waste in the tank.
But the hole was cut without spreading radioactive waste from the tank and with a negligible radiation dose to workers, according to Hanford officials.
It sets the stage to try the vacuum-equipped MARS, which shows promise to remove waste efficiently, particularly from tanks known to have leaked that could leak more if sluicing waste retrieval systems are used. In sluicing, liquid waste is sprayed to break up waste or move it toward a central pump.
The Department of Energy faces a September 2014 deadline under a court-enforced consent decree to have all 16 tanks in the group called C Tank Farm emptied to regulatory standards.
It has seven tanks to go, including Tank C-105, and it notified Washington state this week that it is one of two tanks that may not be emptied by the deadline. The MARS system is not expected to be ready to begin work to empty the tank until next year.
Work to empty tanks has been slow, in part, because waste retrieval systems inserted into many tanks have been unable to empty all the waste, requiring several technologies to be used to empty a single tank.
But in hundreds of hours of testing on a mock tank, the MARS equipped with a vacuum retrieval system has shown it can remove sludge, rocks and sand, and the hard-packed waste found at the bottom of some tanks, said Chris Burke, manager of the MARS program for contractor Washington River Protection Solutions.
The only drawback is that the more robust system will not fit down the 12-inch-diameter risers that provide the only access into the older, underground tanks.
To prepare to insert the second MARS into Tank C-105, workers had to dig up the dirt covering the top of the underground tank, and then cut a 55-inch-diameter circle to remove a portion of the tank dome.
Workers used remotely operated equipment to put distance between themselves and the radioactive waste in the tank. As the cut was made and a round plug of concrete lifted up, a shield plate was slipped over the hole to protect workers from radioactive "shine," said Tom Fletcher, DOE assistant manager of the tank farms.
Workers started the cut at 3:30 a.m. and were done in less than two hours, impressing Washington State Department of Ecology observers with their precision and attention to safety, said Jane Hedges, manager of Ecology's nuclear waste program.
The cut was made with a rotary cutting system, after concerns were raised that an abrasive spray with small particles of garnet used when the first tank was opened in 2010 would add garnet to the waste. The garnet might erode metal in the vitrification plant being built to treat the waste.
"This is the first time rotary core cutting technology has been used on this scale in Hanford's tank farms," Chris Kemp, DOE deputy federal project director for tank farm retrieval, said in a statement.
The commercially available technology, which was adapted for a nuclear environment, uses a laser-guided canister with teeth on the bottom to drill a circle into the concrete.
"There was substantial effort by our federal and contractor teams to plan, prepare and cut the dome on this tank," Kemp said. "The resulting cut was safely completed in one shift with no incidents."
Next a 47-inch diameter riser must be installed in the tank and then MARS will be inserted down the riser, with start-up planned in 2014.
It will use liquid already contaminated with radioactive waste in a closed loop to create a vacuum to suck up the waste. If it hits a hard layer of waste below the sludge in the tank, it will use a high-pressure water nozzle to break up the waste, with the water quickly vacuumed up.
Tank C-105, a 530,000-gallon capacity tank, has held waste since 1947, but is not on the list of 68 suspected leaking tanks. However, because of contaminated soil around the tank, Washington River Protection Solutions is taking a conservative approach and limiting the liquid used in waste retrieval, Burke said.
"Both the cutting technology and the MARS vacuum are potential technologies to use in the future," Burke said.