U.S. Department Of Energy

The disposal facility for mixed and low-level radioactive waste at the Hanford Nuclear Reservation in Washington State is shown in an aerial image.

The head of the contracting company responsible for containment tanks found to be leaking radioactive liquid at the Hanford, Wash., Nuclear Reservation announced his retirement Wednesday.

The head of the contractor overseeing cleanup operations at the Savannah River Site in South Carolina will replace Mike Johnson, president and project manager of Washington River Protection Solutions, or WRPS, Johnson said in an email message to employees obtained by NBC station KING of Seattle.

Johnson gave no reason for his retirement, which he said would take effect at the end of June.

WRPS has been the subject of extensive federal scrutiny since it was learned in February that at least six of 177 underground tanks housing highly radioactive nuclear waste at the site were leaking. The tanks are believed to be losing about three gallons of waste liquid a day, the Tri-City Herald of Richland reported after Rep. Jaime Herrera Beutler, R-Wash., toured the site late last month.

The Herald also obtained Johnson's message Wednesday.

The Energy Department is building a $12.3 billion plant at Hanford to convert the dangerous waste to a stable form, but that project is years behind schedule.

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U.S. Department Of Energy

The disposal facility for mixed and low-level radioactive waste at the Hanford Nuclear Reservation in Washington state is shown in an aerial image.

OLYMPIA, Wash. -- A tank that holds radioactive liquids is leaking at the nation's most contaminated nuclear site, Wash. Gov. Jay Inslee said Friday, raising concerns about the integrity of other storage facilities at the Hanford Nuclear Reservation.

The U.S. Department of Energy said liquid levels are decreasing in one of 177 underground tanks at the nuclear reservation. Monitoring wells near the tank have not detected higher radiation levels, the agency said. Inslee said the leak could be in the range of 150 gallons to 300 gallons over the course of a year.

"I am alarmed about this on many levels," Inslee said at a Friday afternoon news conference. "This raises concerns, not only about the existing leak ... but also concerning the integrity of the other single shell tanks of this age."

The tanks hold millions of gallons of a highly radioactive stew left from decades of plutonium production for nuclear weapons.

Inslee said the state was told such problems had been dealt with years ago and were under control.

Energy Secretary Steven Chu said the federal government must not waiver in its commitment to clean up the highly contaminated site, Inslee told reporters.

The tank in question contains about 447,000 gallons of sludge, a mixture of solids and liquids with a mud-like consistency. The tank, built in the 1940s, is known to have leaked in the past, but was stabilized in 1995 when all liquids that could be pumped out of it were removed.

Inslee said the tank is the first to have been documented to be losing liquids since all Hanford tanks were stabilized in 2005.

At the height of World War II, the federal government created Hanford in the remote sagebrush of eastern Washington as part of a hush-hush project to build the atomic bomb. The site ultimately produced plutonium for the world's first atomic blast and for one of two atomic bombs dropped on Japan, effectively ending the war.

Plutonium production continued there through the Cold War, but today, Hanford is the nation's most contaminated nuclear site. Cleanup will cost billions of dollars and last decades.

Central to that cleanup is the removal of millions of gallons of a highly toxic, radioactive stew — enough to fill dozens of Olympic-size swimming pools — from 177 aging, underground tanks. Over time, many of those tanks have leaked, threatening the groundwater and the neighboring Columbia River, the largest waterway in the Pacific Northwest.

Construction of a $12.3 billion plant to convert the waste to a safe, stable form is years behind schedule and billions of dollars over budget. Technical problems have slowed the project, and several workers have raised lawsuits in recent months, claiming they were retaliated against for raising concerns about the plant's design and safety.