Department of Energy officials haven't updated an aging plan to fix leaks in tanks holding millions of gallons of highly radioactive waste they've known about for decades at a remote facility where many of America's nuclear bombs were once made.

DOE has suspended construction on a new waste treatment plant intended to dispose of the nuclear waste amid growing concerns over the safety of its present storage system, the Government Accountability Office said.

When DOE surveyed the tanks in Hanford, Wash., which hold 56 million gallons of nuclear waste, between 2012 and 2014, officials found them to be in worse condition than in 2011. That was the last year the agency was on schedule for emptying the tanks.

Hanford has been called the most contaminated site in the Western Hemisphere. Its tanks hold enough nuclear waste to cover an area the size of a football field with dangerous sludge 150 feet deep, GAO said.

DOE has two types of nuclear waste tanks — double-shell and single-shell — at the Hanford site, and water is seeping into both. The water causes problems by creating additional liquid waste in the tanks when it becomes contaminated.

Built between the 1940s and the 1960s, single-shell tanks have one concrete and steel wall and were only designed to last about 25 years.

“In the 1940s and 1950s, site contractors did not regard the tanks as a permanent solution to the waste produced at Hanford and viewed tank failures as inevitable,” according to GAO. Single-shell tanks still hold 29 million gallons of waste.

Double-shell tanks have two carbon-steel walls and were constructed between 1968 and 1986 after DOE discovered leaks in 61 single-shell tanks that released more than a million gallons of waste into the ground in the 1960s, GAO said. Each was designed to last somewhere between 20 and 50 years.

DOE began shifting its mixture of radioactive and hazardous waste from the single-shell tanks to the double-shells in the 1970s, a process that it has yet to finish. The department spends more than $1 billion each year on its Hanford nuclear waste program.

One single-shell tank that has been “actively leaking waste into the ground” has been the subject of concern for more than three decades. In 1979, DOE first worried about the tank leaking, although it did not confirm that assumption until 1994.

While the agency completed stabilization repairs on the tank in 1995, it has since discovered that a new leak, beginning sometime in 2010, has been spilling 640 gallons of nuclear waste into the ground every year, the report said.

DOE affirmed the soundness of its double-shell tanks and endorsed their fitness for continuing to store waste in a 2010 report.

But after discovering a leak in one of those tanks in 2012, the agency began an inspection of its tanks that revealed “corrosion” and “construction flaws” contributed to the leak, although DOE has yet to finish its investigation into whether those factors are present in other double-shell tanks.

“Given the current condition of the tanks, it is unclear how long they can safely store the waste,” GAO said.

After DOE announced in October 2012 that nuclear waste was beginning to leak from one of its double-shell tanks, Congress raised concerns that the other 27 tanks could have similar leaks.

For the double-shell tanks, the agency has stepped up its monitoring efforts, changed inspection procedures, hosted an “expert panel” on the leaks, and developed a plan to pump liquid out of the leaky tank, but has yet to take any action.

The expert panel concluded in May 2014 that due to clean-up delays and increasing concerns over the “integrity” of the tanks, “additional leaks cannot be ruled out,” GAO said.

DOE officials told the congressional watchdog that if another double-shell tank fails, they “may have nowhere to move the waste.”

The Washington State Department of Ecology tried to force DOE to begin pumping waste out of the double-shell tank earlier this year after the department said it wouldn't do so until 2016 even though its own guidelines suggest it do so “immediately."

The two sides reached an agreement in September 2014 that requires DOE to begin removing waste from the leaking double-shell tank by March 2015.

Energy Department officials are clinging to a 2011 plan for transferring waste to a treatment plant despite the recent evidence that conditions in the tanks are deteriorating. Construction of the plant, however, was put on hold “indefinitely” in March 2014.

"Technical uncertainties” make it impossible to predict when the plant will be completed despite a previous agreement to have the facility up and running by 2022, government officials claim.

Go here to read the full GAO report.