An inspection of Hanford’s oldest double shell tank found not just one leak from the inner shell — it found seven.
Tank AY-102 had a slow leak of radioactive and hazardous chemical waste from its inner shell into the space between its inner and outer shells confirmed by the Department of Energy in 2012.
A settlement agreement reached by DOE with the Washington Department of Ecology required it to empty the tank of enough waste to determine the cause of the leak. No waste is believed to have breached the outer shell to contaminate the environment.
The tank, which had held 744,000 gallons of waste initially, had been emptied of all but 19,000 gallons in February, shortly before a March 4 deadline.
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The inspection was done with video cameras used to guide work to empty waste from the enclosed underground tank, plus new high-definition video cameras and improved lighting.
“We weren’t sure what we were going to find,” said Glyn Trenchard, DOE’s assistant manager of the Hanford tank farms. He spoke at a Richland meeting of the Hanford Advisory Board on Wednesday.
The tank was emptied enough that the welds in the plates at the bottom of the inner shell could be seen. In two places near where four plates met and were welded, the video showed bubbles, Trenchard said.
Hanford officials knew there were problems with the welds when the tank was built.
A review of construction records showed that 36 percent of the welds on the inner shell were initially rejected as not meeting quality standards. Weld maps showed welds being reworked as many as four times before passing radiography examination.
But before the tank was put into service in about 1971, all the the welds were repaired and passed inspection.
The high definition camera video showed more issues.
Corrosion appeared to have left pits not associated with the welds in the floor of the inner shell.
To determine if any of the pits went all the way through the shell, a small amount of liquid was pumped from the space between the shells into the inner shell.
The liquid drained through the pits in five places, bringing the number of leaks to at least seven, Trenchard said.
28 double shell tanks built
27 double shell tanks still in service
149 single shell tanks built
The tank, like all 177 of Hanford’s underground waste storage tanks, was constructed of carbon steel.
The chemistry of Tank AY-102 was not well controlled to protect the carbon steel during initial use, Trenchard said. Among chemistry that has to be controlled is the pH level, and the nitrates and nitrites in the waste.
The inspection data will inform discussions between DOE and Ecology on whether the tank can be repaired and returned to service, one of the initial goals of the inspection.
A return to service appears unlikely.
The inspection data also should give DOE useful information about its other tanks.
Hanford has 27 other double shell tanks that are being used to hold waste from double shell Tank AY-102 and the site’s 149 leak-prone single shell tanks as they are emptied.
The waste is left from the past production of plutonium at Hanford for the nation’s nuclear weapons program.