The oldest double-shell tank at Hanford will be permanently closed, the Department of Energy has told the Washington Department of Ecology.
After emptying as much radioactive waste as possible from Tank AY-102 and inspecting one area of the bottom of the inner shell, Hanford officials suspect that it has widespread damage.
DOE was required by Ecology, a regulator for Hanford’s tanks storing radioactive and chemical waste, to empty enough waste from the tank to determine the cause of the leak by spring 2017.
DOE confirmed in 2012 that waste from the inner shell of the tank was slowly leaking into the space between its inner and outer shells. No waste is known to have breached the outer shell to contaminate the soil beneath the tank.
One of the goals of the inspection was to decide whether the tank could be repaired and returned to service, a scenario that appeared unlikely.
But it would have been helpful.
Hanford has just 27 double-shell tanks, excluding Tank AY-102, to hold waste emptied from 149 leak-prone single shell tanks until the waste can be treated for disposal.
Plans call for glassifying much of the waste at the vitrification plant under construction at Hanford.
The waste is left from World War II and Cold War production of plutonium for the nation’s nuclear weapons program.
With limited remaining space in Hanford’s double-shell tanks, the state of Washington asked U.S. Judge Rosanna Malouf Peterson to require more double-shell tanks to be built at Hanford as part of new court-enforced deadlines for environmental cleanup at the nuclear reservation.
The judge declined to require more tanks in a 2016 ruling, but said that the state may repeat its request if DOE misses deadlines for emptying single-shell tanks because of a lack of space in double-shell tanks to hold the waste.
After removing most of the waste from Tank AY-102, DOE contractor Washington River Protection Solutions used new high-definition video cameras to take a close look at an area of the inner shell that was suspected of being the primary leak site.
Washington River Protection Solutions found not just one leak in the area, but seven.
Initially, workers sprayed residual waste from the area using water and saw air bubbling up in three places, a clear indication of leaks, said Doug Greenwell, the contractor manager of tank waste retrieval.
After the bottom of the tank dried out, workers pumped some more liquid into it and could see it swirling down in seven places, all leak sites. The floor of the tank appeared to be pitted.
DOE and its tank farm contractor have ween working with a panel of academics and industry experts on tank integrity.
Initially the Tank Integrity Expert Panel’s theory was that the leaks could be associated with the troubled construction history of the first of the Hanford double-shell tanks. Some of the leaking spots in the inner shell appeared to be at tank welds.
A review of construction records showed that 36 percent of the welds on the inner shell were initially rejected as not meeting quality standards. Some welds were reworked up to four times before the tank was put into service in about 1971.
But the primary cause of the leaks appears to be pitting of the bottom of the carbon steel tank, the expert panel said after reviewing inspection results and records for the tank’s initial use.
Contents of the tank were not managed in its early years to the standards now used, Greenwell said.
“The science of corrosion got better over the years,” he said.
A couple of feet of water was left in the bottom of the tank for months or possibly years before waste was added to it. Now engineers know that adding caustic to the water would have helped prevent corrosion of the carbon steel.
The initial waste added to the tank also was not chemically managed well to prevent corrosion.
Although only a small area of the inner shell was inspected, the panel said widespread corrosion on the bottom of the inner shell was likely.
DOE evaluated information from Washington River Protection Solutions, looking at factors such as the cost and time needed to repair the tank and other ways to manage tank waste, said Reggie Eakins, Jr., DOE technical program manager.
“We’ve determined the best decision is to close the tank,” he said.
Greenwell said waste management plans include continuing to evaporate as much liquid waste as possible to free up space in double shell tanks and starting to glassify some of the tank’s waste as soon as 2022.
The state Department of Ecology is looking forward to moving into the next phase of the closure process, said Alex Smith, Ecology’s Nuclear Waste Program manager.
“(It) will include setting measures in place to secure the AY-102 Tank system in a manner that prevents threats to human health and the environment, until the final closure of the AY Tank Farm can take place,” she said.
No plan has been made for how to close Hanford’s waste storage tanks after they are emptied to regulatory standards.
A likely scenario would be filling them with concrete-like grout rather than trying to dig up tanks, some of them massive enough to hold 1.1 million gallons of waste.
Smith said she hoped that the lessons learned by DOE as it emptied Tank AY-102 would help it deal with any future leaks more quickly.
When work started to empty it in March 2016, it held about 744,000 gallons of waste.
A wealth of information has been gained from emptying Tank AY-102 and inspecting the tank, according to Hanford officials.
It will help preserve DOE’s other tanks until waste can be treated for disposal, Greenwell said.