The Department of Energy should proceed cautiously if it decides to empty liquid radioactive waste from the Hanford double-shell tank that has an interior leak, according to a letter from the Defense Nuclear Facilities Safety Board.
It's not fully understood if removing some waste could increase corrosion within the tank, the board's staff said.
DOE's oldest of 28 double-shell tanks, Tank AY-102, has sprung a leak in its inner shell. The waste is believed to be contained by the outer shell, but it is thinner than the inner layer and the tank no longer complies with regulations.
The state of Washington, which regulates Hanford underground tanks holding 56 million gallons of radioactive waste, wants waste removed from the tank.
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DOE is prepared to begin pumping liquid waste from the tank, if warranted, said DOE spokeswoman Lori Gamache.
But as long as 151,000 gallons of radioactive sludge remain in that tank, some of the 680,000 gallons of liquid also will need to be left in the tank to help cool it. Another 22 months would be needed to prepare to also remove sludge from the tank.
The sludge generates heat as it radioactively decays, and heat can increase corrosion rates in the tank and contribute to generating potentially flammable hydrogen gas.
DOE has approved a report prepared by Washington River Protection Solutions that concluded that the liquid in the 75-foot-diameter tank could be lowered safely from 270 inches to 96 inches deep.
Liquid would continue to be lost to evaporation, but water could be added in the future to maintain the liquid level above 48 inches.
The defense board reviewed the report at DOE's request, but said there was not enough information to agree with the conclusion that removing the liquid would have little effect on tank waste temperatures or the estimated leaking rate.
The report does not address key technical uncertainties about the condition of the tank, which is more than 40 years old, the defense board said. That includes the specific cause of the leak and the leak rate.
The chemical and physical conditions at the leak site are uncertain and the impact from a change in temperature, pressure or chemistry on the rate of leaking cannot be predicted with certainty, according to the defense board.
If DOE decides to empty some liquid from the tank, it should watch closely for signs that the leaking is increasing or that the ventilation system that helps cool the waste is blocked.
The inner shell of the tank sits on a refractory, or heat-resistant material, on the bottom of the outer shell.
Solid deposits from a leak can plug the air distribution system along the tank bottom, limiting the movement of air to cool the tank bottom, the defense board staff report found.
The board's staff recommended close monitoring for changes in waste temperature at the tank bottom, which could indicate increased leaking or that channels for cooling air were being blocked by waste.
More analysis also could provide a better understanding of the significance of any changes that are observed, the staff said.
DOE is reviewing the defense board's recommendation and continues to monitor the interior leak, Gamache said. She described the leak as minimal.
w Annette Cary: 582-1533; firstname.lastname@example.org; Twitter: @HanfordNews