No waste is believed to have leaked from the outer shell into the soil beneath the underground tank on the Washington reservation, the agency said.
The Tri-City Herald reported that earlier testing showed that some of the material seen between the two walls of the tank was radioactive waste. Now the agency has confirmed a leak. While the exact amount isn't known, the agency said perhaps a couple tablespoons of additional waste were seen between the two tank walls between Thursday and Sunday.
The tank is one of 28 double-shell containers used to hold waste from older, leak-prone single-shell tanks. They hold 56 million gallons of radioactive waste from the past production of weapons plutonium until it can be treated for disposal.
The tank is roughly 40 years old and contains about 850,000 gallons of sludge and liquid waste.
Plans call for continued twice-weekly visual inspections using cameras and regular checks of liquid levels inside the primary tank, said Tom Fletcher, assistant manager for the Tank Farms Project.
Fletcher added that his people are "working collaboratively" with the state Ecology Department to determine what to do next and to "ensure that we are effectively protecting the public and the environment."
Routine monitoring has not detected leaks in any of the other double-shell tanks, the Energy Department said. Expanded inspections are planned on six other tanks similar to the one that is leaking.
The federal government created Hanford in the 1940s as part of the top-secret Manhattan Project to build the atomic bomb. Today, it is the nation's most contaminated nuclear site, with cleanup expected to last decades.
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