A leak at the Hanford nuclear site in Washington state has prompted warnings of "catastrophic" consequences, as workers attempt to clean up more than eight inches of toxic waste from one of 28 underground tanks holding radioactive materials leftover from plutonium production.

Alarms on the site began sounding on Sunday, leading workers to discover 8.4 inches of toxic waste in between the inner and outer walls of tank AY-102, which has been slowly leaking since 2011 but has never accumulated that amount of waste before.

A former tank farm worker told local media that despite statements from the U.S. Department of Energy (DOE) that the spill does not pose a threat to public health, it should be considered a major problem.

"This is catastrophic," the worker, Mike Geffre, who first discovered that the tank was failing in 2011, told King-TV on Monday. "This is probably the biggest event ever to happen in tank farm history. The double shell tanks were supposed to be the savior of all saviors [to hold waste safely from people and the environment]."

DOE said Monday said the rupture was an "anticipated" result of ongoing efforts to fully decommission the most contaminated nuclear site in the nation.

However, the new leak at the site poses several problems, King-TV reporter Susannah Frame writes:

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The outer shell of AY-102 [the tank] does not have the exhaust or filtration system needed to keep the dangerous gases created by the waste in check. Workers have been ordered to wear full respiratory safety gear in the area, but the risk remains. “The hazards to workers just went up by a factor of 10,” said Geffre. In addition, the breakdown calls into question the viability of three other double-shell tanks at Hanford that have the exact design of AY-102.

Environmental groups also issued words of caution. Columbia Riverkeepers, an Oregon-based advocacy organization, said in a statement that the leak is "another reminder of the cost of nuclear waste, and the unexpected outcomes of handling radioactive material."

The AY-102 tank "holds some of the most dangerous nuclear waste on Earth," said the group's executive director Brett VandenHeuvel. "These tanks were not designed to hold waste for decades. It's past time to get the waste out of the unsafe tanks."

In 2012 and 2013, leaks were reported at seven of Hanford's 177 tanks, 149 of which are single-shelled.