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, but Inslee said the leak could be in the range of 150 gallons to 300 gallons over the course of a year and creates a long-term threat that could impact groundwater or rivers.
"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 assured years ago that such problems had been dealt with and he warned that spending cuts - particularly due to a budget fight in Congress - would create further risks at Hanford. Inslee said the cleanup must be a priority for the federal government.
"We are willing to exercise our rights using the legal system at the appropriate time. That should be clear," Inslee said.
Inslee said the state has a good partner in Energy Secretary Steven Chu but that he's concerned about whether Congress is committed to clean up the highly contaminated site.
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. His staff said the federal government is working to assess other tanks.
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. 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.