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At least six underground-storage tanks each holding tens of thousands of gallons of nuclear and chemical waste are leaking at the Hanford nuclear reservation, the nation’s most contaminated Cold War weapons-production facility.

And federal officials say more of the aging steel waste tanks also may be dribbling out nuclear material left over from a half-century of atomic bomb-building.

Gov. Jay Inslee made the announcement Friday after meeting in Washington, D.C., with outgoing Energy Secretary Steven Chu to discuss cleanup at Hanford.

Inslee told reporters that Energy officials recently figured out they had been inaccurately measuring the 56 million gallons of waste in Hanford’s tanks.

They were forced to conclude that a half-dozen of them had dropped in volume and were leaking anywhere from a few gallons to a few hundred gallons of radioactive material a year.

While the leaks pose no immediate health threat, “they are certainly levels that cause us concern and demand action,” Inslee said.

Energy Department officials would not answer questions Friday about how they planned to respond to the leaks, but Inslee said an investigation is under way.

A spokesman for Oregon Sen. Ron Wyden says the senator will ask the Government Accountability Office (GAO) to investigate a monitoring and maintenance program for underground-waste tanks at the nation’s most contaminated nuclear site.

Wyden is the new chairman of the Senate Energy and Natural Resources Committee. He has said he’ll seek a commitment during confirmation hearings for the next energy secretary that all radioactive waste at Hanford will be cleaned up.

Suzanne Dahl, who oversees Hanford’s tank-waste issues for the state, said it is too soon to say where, precisely, the radioactive material has gone, but that it would take many years for the waste to work its way to the groundwater that moves toward the Columbia River 5 miles away.

“Is it held up directly underneath the tank farms? Is it moving? It’s something we just don’t know at this point,” Dahl said. “We do know that there is 150 to 200 feet of dry soil between the tanks and the groundwater.”

Hanford’s single-shell and double-shell underground tanks, built between the 1940s and 1980s, hold such a complex and caustic brew of salts, gas, liquid and sludge that the precise mix of ingredients isn’t even known for all of them.

As a result, the tanks have long been the 586-square-mile site’s trickiest cleanup problem and are at the heart of the nation’s most
expensive construction project: a $13.4 billion treatment plant to stabilize that waste by turning it to glass.

Of the 177 tanks, 149 were built with a single-steel shell and are still in operation decades past their projected life span. Years ago, 67 were suspected to have leaked. At least 1 million to 1.5 million gallons of waste is known to have spilled onto the desert floor.

For nearly a decade, the Energy Department had claimed it had found all potential leaks and had been eliminating risk as workers drained liquid from the single-shell tanks into the newer and much safer double-shell tanks. A handful of the 149 are now nearly empty.

But last week, Energy officials acknowledged for the first time since 2005 that they’d found a new leaker.

The department admitted Friday it had found five more and that analysts hadn’t properly reviewed information about how tank volume had changed.

It appears “they were looking at the data over a more narrow time frame,” Dahl said.

Craig Welch: 206-464-2093 or or on Twitter: @craigawelch.

Material from Associated Press was used in this report.