Methods tested to speed waste withdrawal from tanks
Washington River Protection Solutions is considering cutting new holes in Hanford’s single-shell tanks — openings that could potentially reach up to six feet in diameter.
Currently, the holes in the tops of Hanford’s underground tanks are very narrow with extremely long pipes connecting the surface with the radioactive sludge and fluids in the tanks. Collapsible equipment has to be inserted through those skinny tubes to be unfolded by remote control inside the tank to install sensors and pumps in those interiors.
Consequently, working inside the tanks and pumping out the waste is a very slow process.
This concept of cutting bigger holes could enable bigger and more complex machinery to be inserted in the site’s underground radioactive waste tanks to speed up pumping the material from the single-shell tanks into newer and safer double-shell tanks.
WRPS believes it is likely Hanford that will adopt this approach. But a timetable for making a decision on the concept and a creating a budget estimate has not been set yet, said WRPS Chief Engineer Karthik Subramanian. It will take an undetermined number of years to nail down and implement this approach, he said.
“We’re in the infancy of doing this,” Subramanian said.
Hanford has cut bigger holes in tanks twice in the past — Tank C-107 in 2010 and Tank C-105 in 2013.
In 2017, WRPS asked the Pacific Northwest National Laboratory to study whether cutting wide holes in the tops of the tanks would increase the likelihood of the tanks collapsing from the resulting underground tension and compression on the concrete and stainless steel of the tank. The physics is similar to that of a dome on top of church.
WRPS’ potential holes are much bigger than any existing openings in the tanks, said Ken Johnson, the PNNL engineer in charge for the structural analysis team.
A typical Hanford tank can hold up to 1.2 million gallons. Most tanks are 75 feet in diameter, 75 feet in height, with 15-inch-thick stainless steel walls, a concrete top and usually are under 7.5 feet of soil. Toxic and radioactive fumes are inside the tanks.
So far, 17 of Hanford’s 149 single-shell tanks — all well beyond their design lives — have been emptied into 28 newer and safer double-shell tanks that are flirting with the ends of their design lives. Overall, the 177 tanks hold about 56 million gallons of waste.
Johnson said the study showed in 2018 that cutting a new access hole of up to six feet in a 39-foot-in-diameter circle in the center of the top of the tank would have no effect on the structural integrity of a tank. Now the ball is back in WRPS’s court. “This is just the first step to see if this is a concept to be considered,” Johnson said.
WRPS still is hunting for the right equipment that can be inserted through wider holes to speed up pumping, Subramanian said. Also, safe ways of cutting new holes in the tanks need to be studied since sparks in a potentially flammable atmosphere and escaping fumes have to be considered, he added.
“This is a long-term process,” Subramanian said.