Mark Ralston / AFP - Getty Images file Government officials said radioactive waste might be leaking into the soil from a nuclear site in Hanford, Washington state. Governor Jay Inslee said the situation should be treated with the "utmost seriousness."

An underground tank holding some of the worst radioactive waste at the nation's most contaminated nuclear site might be leaking into the soil.

The U.S. Energy Department said workers at Washington state's Hanford Nuclear Reservation detected higher radioactivity levels under tank AY-102 during a routine inspection Thursday.

Spokeswoman Lori Gamache said the department has notified Washington officials and is investigating the leak further. An engineering analysis team will conduct additional sampling and video inspection to determine the source of the contamination, she said.

State and federal officials have long said leaking tanks at Hanford do not pose an immediate threat to the environment or public health.

The largest waterway in the Pacific Northwest — the Columbia River — is still at least 5 miles away and the closest communities are several miles downstream.

However, if this dangerous waste escapes the tank into the soil, it raises concerns about it traveling to the groundwater and someday potentially reaching the river.

Washington Gov. Jay Inslee said in a statement that the situation "must be treated with the utmost seriousness."

Inslee said additional testing is expected to take several days.

"Our state experts confirm that there is no immediate public health threat. Given the relatively early detection of this potential leak, the river is not at immediate risk of contamination should it be determined that a leak has occurred outside the tank," he said.

Tom Carpenter, executive director of the Seattle-based advocacy group Hanford Challenge, said, "this is really, really bad. They are going to pollute the ground and the groundwater with some of the nastiest stuff, and they don't have a solution for it."

AY-102 is one of Hanford's 28 tanks with two walls, which were installed years ago when single-shell tanks began leaking. Some of the worst liquid in those tanks was pumped into the sturdier double-shell tanks.

The tanks are now beyond their intended life span. The Energy Department announced last year that AY-102 was leaking between its two walls, but it said then that no waste had escaped.

Two radionuclides comprise much of the radioactivity in Hanford's tanks: cesium-137 and strontium-90. Both take hundreds of years to decay, and exposure to either would increase a person's risk of developing cancer.

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, and it continued production through the Cold War.

Today, it is the nation's most contaminated nuclear site, with cleanup expected to last decades. The effort — with a price tag of about $2 billion annually — has cost taxpayers $40 billion to date and is estimated will cost $115 billion more.

The most challenging task so far has been the removal of highly radioactive waste from the 177 aging, underground tanks and construction of a plant to treat that waste.

The Energy Department recently notified Washington and Oregon that it may miss two upcoming deadlines to empty some tanks and to complete a key part of the plant to handle some of the worst waste.

Energy Secretary Ernest Moniz visited the site Wednesday for the first time since being confirmed by the Senate in May. He said he intends to have a new plan by the end of the summer for resolving the technical problems with the waste treatment plant.

Related:

Head of company overseeing leaking nuclear tanks at Hanford to step down

Six tanks now said to be leaking at contaminated Hanford nuclear site

Tank at Hanford nuclear site leaking radioactive liquids, Washington governor says

