The Department of Energy will start buying equipment and making other plans to remove radioactive sludge from a double-shell tank with a leak between its shells. But pumping of waste would start no sooner than March 2016, according to a new DOE plan.
DOE delivered the plan for emptying Tank AY-102 to the state of Washington near the close of business Friday. The state has not yet reviewed the plan to comment on it.
DOE's previous plan, as the state understood it, was to take no action to remove waste from the tank until conditions worsened, the state said in a January letter that demanded that DOE come up with a pumping plan that met state requirements.
State regulations require DOE to pump the tank either within 24 hours of the discovery that it was not sound or as soon as possible, according to the state.
DOE already has equipment in place to remove liquid waste from the tank. But no waste would be pumped until DOE is ready to proceed with liquid and sludge removal, according to the plan.
Preparations for retrieving waste would be done in two phases. The first phase would including engineering, buying equipment and installing equipment outside the tank. That would be completed in November 2015.
Then DOE would evaluate the situation, determining when waste retrieval equipment should be installed inside the tank and when waste removal should begin, according to the plan. Construction of the waste retrieval and transfer system would be completed no earlier than February 2016, and authorization would be given no sooner than the next month to begin pumping.
Pumping liquid out now is technically feasible, but has risks because the liquid helps cool the waste, the pumping plan said. 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.
Any action to remove liquid waste also might affect the rate at which the inner shell is leaking, according to the plan. Potential causes for the leak from the inner shell of the tank have been identified, but not confirmed.
DOE suspects that a combination of difficulties in constructing the tank in 1969 and the type of waste it holds may be responsible for the leak. The bottom layer of waste did not have chemicals added to inhibit corrosion and high heat waste has been added above that layer.
The plan also points out that the leak is believed to be contained between the two shells of the tank, rather than reaching the soil beneath the underground tank.
The best course of action appears to be not to pump out liquid until sludge also can be removed, unless conditions significantly worsen, the plan said.
The level of liquid waste has been allowed to drop some because of evaporation but DOE has determined that a minimum of 48 inches of liquid waste must be kept above the sludge in the 75-foot-diameter tank, according to the plan. The last tank waste summary report released put the waste level of liquid and sludge combined at 293 inches at the end of November.
DOE confirmed in October 2012 that Tank AY-102, the oldest of its 28 double-shell tanks, had leaked waste from its inner shell. The waste was known to be in two places in the space between the two shells.
Wednesday, it learned that waste had been found in a third place between the shells. The waste had not been seen there when video had been shot of the same spot in September 2012.
The waste is evidently leaking from one or more places in the bottom of the inner shell and then flowing through channels in the mostly ceramic refractory on which the inner shell sits. It ends up in different spots on the floor of the circular space between the two shells.
The state previously has told DOE that there are risks to waiting to empty the tank.
DOE does not know the location of the leak, the rate of leakage or conditions at the leak site within the tank, the state said. It also does not know when or how the leak might worsen, it said.
Continued leaking could clog ventilation channels, undermining the ability to moderate the heat in the tank and leading to greater corrosion of the tank bottom, according to the state.
DOE has 28 double-shell tanks that are being used to hold waste emptied from 149 older leak-prone single-shell tanks.
Space in the double-shell tanks is nearing capacity as work continues to build the vitrification plant to start treating the waste for disposal. One single-shell tank is known to be leaking waste into the ground and 67 are suspected of leaking waste in the past before much of the liquid waste they held was removed.
-- Annette Cary: 582-1533; email@example.com; Twitter: @HanfordNews