Cutting a 55-in.-dia. opening into concrete is hardly rocket science.
But when it’s the concrete shell of a huge underground tank that’s been storing highly-radioactive plutonium waste for more than 60 years, the cutaway could be a doorway to the unknown.
At month’s end, however, a U.S. Dept. of Energy contractor’s crew at the Hanford nuclear waste cleanup site in Washington state will begin slicing into the C-107 tank. Then, the crew intends 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.
The cut will be the largest ever made into one of the site’s aging and leak-prone tanks, and 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 tanks at Hanford, and one of 67 that are known to have leaked waste contents.
Originally built to hold about 530,000 gallons of plutonium waste from former atomic bomb manufacture there, it 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.
At the bottoms of these tanks is what can only be described as a rock-like and hard-to-remove radioactive crust.
The new technology will dramatically decrease tank cleanup costs in the long run, site officials claim.
The $15-million estimate for the C-107 tank is slightly higher than the average tank cleanup cost, which is around $9 million, 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. But remediating the site’s first tank in 2004, the C-106, cost nearly $100 million.
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.”
Where Nozzles Can’t Reach
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 now have that technology with the Mobile Arm Retrieval System (MARS), being used at Hanford for the first time. But its installation on the C-107 tank was a tricky problem for workers.
Using the tank’s original 12-in-dia risers to reach the waste was like “building a ship inside a bottle,” Smith says.
That approach was impractical.
He says the new 55-in.-hole is large enough to accommodate the MARS system with its intricate hose configuration, but also small enough to maintain the tank’s structural integrity.
Nothing’s more important during the cutting than preventing the spread of contaminants.
Crews are hand-excavating 5 ft down 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 tank cutting tool, for speed and superior contamination control on the rebar-laced 15-in.-thick structure.
Saws Are No Help
He says traditional concrete saws would spread too many contaminated particles. Garnet particles are set 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 through remote video monitoring, while electronic sensors monitor 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.”
Three mock cuts have gone well, says Smith.
To make these test cuts, crews poured two simulated tank domes, practiced on one and then set up a full-blown mockup for a final testing. Smith expects the cut to be completed in 24 to 32 hours, with a new 52-in. steel riser put in place to immediately cap the hole.
By next March, crews will install piping, valves and ventilation needed for MARS installation. Operation of the robotic arm is set to begin in July and finish in September.
Blast-resistant Sludge
In the past, sluicing cannons with nozzles have had trouble clearing out the rock-hard bottom layer of sludge and sand-like waste in tanks because the cannons were still 17 ft away from the tank bottom, officials say. The telescoping arm of the MARS can rotate 360 degrees 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 flyaway sand. In effect, the high-pressure liquids “rake” the waste to the central pump. Officials estimate it can remove as much as 1,000 gallons of waste per hour.
DOE and its contractors are also developing a second MARS vacuum system technology for use in other tanks that are known leakers, to which more liquid cannot be added. The process is expected to be used first in Hanford’s C-105 tank at the end of 2013.
