In addition to the leaks, the board noted concerns about the potential for hydrogen gas buildup within a tank, in particular those with a double wall, which contain deadly waste that was previously pumped out of the leaking single-shell tanks.
"All the double-shell tanks contain waste that continuously generates some flammable gas," the board said. "This gas will eventually reach flammable conditions if adequate ventilation is not provided."
All of the tanks are actively ventilated, which means they have blowers and fans to prevent a buildup of hydrogen gas, and those systems are monitored to ensure they are operating as intended, Energy Department spokeswoman Carrie Meyer said.
For even greater safety, she said, the agency implemented an improved monitoring system in February.
"DOE is absolutely committed to ensuring the safety of Hanford's underground tanks," Meyer said.
The board also noted technical challenges with the waste treatment plant, which is being built to encase the waste in glasslike logs for long-term disposal. Those challenges must be resolved before parts of the plant can be completed, the board said.
The federal government spends about $2 billion annually on Hanford cleanup — roughly one-third of its entire budget for nuclear cleanup nationally. About $690 million of that goes toward design and construction of the plant. Design of the plant, last estimated at more than $12.3 billion, is 85 percent complete, while construction is more than 50 percent complete.
The problems identified by the board show that the plant schedule will be delayed further and the cost will keep rising, Wyden said, adding: "There is a real question as to whether the plant, as currently designed, will work at all."