Three more double-shell tanks at the Department of Energy’s Hanford Site contain waste with chemistry that puts them them at heightened risk of corrosion, according to a new report from tank farm contractor Washington River Protection Solutions.
The report evaluated the waste chemistry of Hanford’s oldest double-shell tank, AY-102, which developed a leak from its inner shell, and compared it to the waste chemistry in the 27 double-shell tanks at Hanford still in use. Tank AY-102 has been emptied and taken out of service.
The study, dated June 2018, found that one of Hanford’s double-shell tanks, AY-101, has held waste that puts it at greater risk of corrosion than the tank that leaked waste between its shells. Two other double-shell tanks, AZ-101 and AZ-102, also were ranked among those most at risk, according to the report. Similarities among the waste in the four tanks, including the tank that was emptied, included high heat generation, which accelerates corrosion.
The Department of Energy initially suspected that problems during construction of Tank AY-102 caused the leak from its inner shell. Construction records showed that 36 percent of the welds on the inner shell were reworked up to four times before the tank was put into service around 1971. But after as much waste as possible was emptied from the tank by 2017, pitting likely caused by corrosion was found on the floor of the inner shell, allowing waste to leak between its shells.
The Washington state Department of Ecology said the report’s findings indicate three more double-shell tanks have “very high risk factors for tank bottom corrosion.” However, the findings do not mean the three tanks are leaking or that they will, said Jeremy Johnson, deputy federal project director for the Hanford tank farms.
The department’s independent Tank Integrity Expert Panel, which met on June 27-28, also is keeping an eye on two other findings related to double-shell tank integrity. Ultrasonic testing of the AY-101 and AZ-102 tanks found a ring with spots of corrosion where condensate was added to the tank. The condensate liquid was collected in the ventilation systems of the AY and AZ Tank Farms and then returned to the tanks. The ring where corrosion was found in the two tanks was at the top of the liquid where it interacted with air. An inspection this year found degradation was increasing, according to DOE.
In addition, analyses of the outer shell have found thinning in spots on the outer-shell bottoms of nine of 11 tanks checked. The most severe was in Tank AP-102, where thinning was up to 70 percent. However, DOE does not know whether the thinning is historical or recent, nor has it confirmed the cause. Corrosion might have been caused by liquid collecting in places on the concrete foundation on which the underground tank sits.
The Energy Department is taking multiple steps to address corrosion issues, including revising its waste chemistry control. It plans to collect core samples from the waste at the bottom of the tanks that might have waste chemistry issues to learn more about the material. Newly developed robots could be deployed later this year into the ventilation channels beneath the bottom of the inner shell to collect more information.
The agency also plans to start trucking condensate to the Hanford Effluent Treatment Facility, rather than adding it to tanks. To protect the outer shell from moisture, DOE will consider using corrosion inhibitors and will evaluate possible repair techniques.
Roughly 56 million gallons of chemical and radioactive waste remains stored at Hanford, the byproduct of decades of plutonium production for the U.S. nuclear arsenal.
Hanford’s double-shell tanks hold waste emptied from 149 leak-prone single-shell tanks until the waste can be treated for disposal. The Hanford Waste Treatment Plant could start treating some low-activity radioactive tank waste as soon as 2021.
The state Department of Ecology pointed to the findings about double-shell tank corrosion as another reason to build more double-shell tanks. “We need to start doing something, the sooner the better,” said Steve Lowe, Ecology’s double-shell tank lead engineer. “Cleanup depends on it.”
The Energy Department says the first 10 years of operation of the Waste Treatment Plant to treat low-activity radioactive waste will free up to 12 million gallons of double-shell tank space. Each tank has a capacity of roughly 1 million gallons. But the Department of Ecology said in a statement that when DOE starts treating waste, the oldest double-shell tanks will already be 50 years old and some of the tanks will need to remain in service for another four decades.
Any double-shell tank failure effectively makes two tanks unavailable, since waste emptied from the failed vessel must be transferred to one that remains in service, Lowe said. One of the three tanks at increased risk of corrosion because of waste chemistry, Tank AZ-101, holds material expected to be treated as high-level radioactive waste no sooner than 2036, when the Waste Treatment Plant is in full operation.
