RICHLAND, Wash. - It’s nearly impossible to see the integrity of the metal plates that form the bottom of the double-shell waste tanks (DSTs) buried about 50 feet underground at the Hanford Site, but hearing them may help researchers understand how they fare.
It may now be possible to inspect the plates using a novel sensor technology in which sound waves detect small pits or defects. Such inspection hasn’t been possible before because about 90 percent of the tank bottom is blocked by insulating concrete.
EM Office of River Protection tank farms contractor Washington River Protection Solutions (WRPS) and researchers at DOE’s Pacific Northwest National Laboratory (PNNL) just finished testing the technology on a 12-by-20-foot mock tank bottom.
The tank bottom is made of steel plates welded together. The primary tank rests on an insulating concrete refractory pad within the secondary liner. The pad contains tiny air-cooling channels that provide limited access to the tank bottoms.
The sensor technology was developed by a Pennsylvania-based company, Guidedwave, which adapted it to fit inside the tiny slots.
“It’s one thing to have an examination sensor, it’s another to have one capable of scanning wide areas from a single location and then modifying it for deployment through small air slots the size of a 3-ounce paper cup between the primary and secondary tanks," said Kayte Denslow, PNNL project manager.
PNNL experts evaluated the technology, creating corrosion pitting, wall thinning, and weld seam openings or cracks to see if the sensors would detect them.
“The first adapted sensor performed very well through the air slots. They detected the same flaws that were detected last year with their original sensor, which had to be size-reduced to fit in the air channels. Based on the test outcomes, they know what design changes need to be incorporated next year to produce the final sensor that will go under the tanks," Denslow said. “It will be like having excellent peripheral X-ray vision across large areas of the tank bottom. The system was able to detect flaws that represent degradation types of concern for the tanks."
The sensor remains stationary under the tank, but there are 26 elements within it that go off at different times to activate sound waves. The output is almost like a sonar or radar image - bright colored blips indicate defects and their distance and direction from the sensor.
A team of Washington State University Tri-Cities engineering faculty members and students are testing a small crawler robot by Inuktun that will deploy the sensor through the tiny air slots and press it against the tank bottom.
“As the tanks go beyond their design lives, these are the kinds of novel yet scientifically supported solutions that we need to help determine the best course of action for these waste tanks," said Kayle Boomer with the WRPS Chief Technology Office. “Whether it’s prevention of degradation, management or potentially even repair, being able to better understand the actual physical condition of the tanks is invaluable to our mission."
The sensor could be deployed under a Hanford tank as early as 2019.
Source: U.S. Dept. of Energy, Office of Environmental Management
