Cutting a 55-in.-dia. hole into concrete is hardly rocket science in construction, but when the hole is being cut into the concrete shell of a huge underground tank that has been storing highly radioactive plutonium waste for more than 60 years, that action could be the equivalent of a space-shuttle trip into the unknown.
At month’s end, however, a U.S. Dept. of Energy contractor crew at the Hanford nuclear- waste cleanup site in Washington state will begin slicing into the C-107 tank to install a new robotic arm that will sweep or chisel out waste remnants that have been there since the structure was built before the Cold War. It will be the largest cut ever made into one of the site’s aging and leak-prone tanks as well as one of the most significant cleanup efforts to date, officials say.
“A cut of this size has never been done on a tank already in service at Hanford, so this is a significant piece of work for the tank farms,” says Erik Olds, a DOE spokesman at the site.
The C-107 tank is one of 149 single-shell steel storage containers at Hanford It is one of 149 thinner-shelled tanks on-site, and while not one known to have previously leaked contents, about 67 other site tanks have.
Originally built to hold about 530,000 gallons of plutonium waste from former atomic-bomb manufacture there, the tank still contains an estimated 247,000 gallons of high-level nuclear and chemical sludges and solids. DOE contractors have completed cleanout of only seven tanks, with six now in process.
The larger C-107 tank hole will allow insertion of the longer robotic arm inside the tank that DOE and contractors hope will now remove 99% of waste remnants, some of it almost rock-like in consistency.
The new technology will decrease tank cleanup costs dramatically, from nearly $100 million for one of the first structures remediated in 2004 to $15 million for the C-107, says Kent Smith, deputy tank retrieval and closure manager for Washington River Protection Solutions (WRPS), the Hanford tank cleanup contractor venture led by URS Corp., San Francisco. Two more tank cleanups planned over the next several years could cost $5 million each, he adds.
“The challenge we have is gaining access to the tanks,” says Smith. “The tanks were designed to put waste in without a lot of consideration to getting the waste out. We are very excited about this new technology. We think this is a game-changer.”
WRPS has spent two years seeking to expedite tank-waste cleanout under an existing site cleanup agreement with federal and state environmental regulators. The preferred method is “modified sluicing,” in which nozzles spray high-pressure liquid to dissolve waste before it is pumped out. The method removes up to 95% of materials, but other technologies were needed to get at the toughest-to-remove waste remnants, officials say.
DOE and its contractors will have that technology with the Mobile Arm Retrieval System (MARS), which is being used at Hanford for the first time. But its installation on the C-107 tank was tricky for workers.
Using the tank’s original 12-in.-dia risers was like “building a ship inside a bottle,” Smith says, an impractical design. He says the new 55-in. hole is large enough to accommodate the MARS system and its intricate hose conglomeration but small enough to maintain the tank’s structural integrity.
Preventing the spread of contamination is a challenge in the cutting process, says WRPS. Crews are manually excavating five ft below the surface and 30 ft across to the 37-ft-dia C-107 tank’s existing riser and concrete pad, which will be demolished.
Smith says WRPS and its concrete subcontractor, Murray, Utah-based A-Core Concrete Cutting, opted to use high-pressure abrasive garnet as the cutting tool for its speed and superior contamination control on the rebar-laced 15-in.-thick tank. He says traditional concrete saws would spread too many contaminated particles; in this case, radioactive garnet particles are intended to fall into the tank, which is under vacuum pressure.
The cutting unit, akin to a high-pressure nozzle on a circular track, is placed atop the tank, enclosed in a containment tent and operated remotely. Crews watch the cut via remote video monitors, while electronic sensors track radiation levels.
Matt Verdun, manager of A-Core’s Kennewick, Wash., office, says employing remote control is rare but necessary because of the environment. “The C-107 tank project presented unique challenges that are not encountered on a daily basis in our industry,” he says. “The radioactive environment, the structural sensitivity of the concrete tank itself and the method of core-drilling by way of remote-controlled equipment proved to be quite the test.”
Smith says previous tests, in the form of three mock cuts, have gone well. Crews poured two simulated tank domes, practiced on one and then set up a full-blown mockup for a final test. He expects the cut to be completed in 24 to 32 hours, with a new 52-inch steel riser put in place immediately to cap the hole.
By next March, crews will place piping, valves and ventilation needed for MARS installation. Operation of the robotic arm is set to begin in July and finish in September.
In the past, sluicing cannons have had trouble clearing out the rock-hard bottom layer of sludge in some tanks and sand-like waste in others because they still sit 17 ft away from the tank bottom, officials say. The telescoping arm of the MARS can rotate 360� and reach out 40 ft.
Elaborate hose management systems give crews the ability to reach the tank bottom and the hard material. A “backstop” catches the sand. In effect, the high-pressure liquids “rake” the waste to the central pump. Officials estimate it can remove up to 1,000 gallons of waste per hour.
DOE and its contractors are developing a second MARS vacuum system technology for use in other tanks that are known to leak, thus more liquid cannot be added to them. The process is expected to be used first in Hanford’s C-105 tank at the end of 2013.
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