The Department of Energy ended 2015 able to claim another one of Hanford’s leak-prone single shell tanks emptied of waste to regulatory standards.
That leaves two more tanks in the group called C Tank Farm yet to be emptied of their mixtures of radioactive and hazardous chemical waste left from the past production of plutonium.
The court-enforced consent decree required that all 16 C Farm tanks, which were built during World War II, be emptied 15 months ago. A federal judge is expected to set new deadlines.
The most recently emptied tank, the only one completed in 2015, is Tank C-102. Rapid progress is being made on Tank C-111. But Tank C-105 remains a problem.
Two C Farm tanks still hold waste 15 months after deadline to empty them
Work has stalled there to demonstrate a robotic vacuum system that can be used on tanks suspected of leaking in the past. Work with the system is not expected to resume.
“It’s going to be an important issue,” said Jim Alzheimer, a tank engineer with the state Department of Ecology, a regulator on the project. Hanford needs a way to empty tanks that are in danger of leaking.
The vacuum system adds small volumes of liquid and quickly vacuums it up. The more commonly used sluicers use a larger volume of liquids to break up waste and move it toward a central pump.
The tally of single-shell tanks emptied to regulatory standards now stands at 15 of 149. All but one are in the C Tank Farm.
The waste is transferred to newer double-shell tanks, where it will be stored until the vitrification plant is ready to start treating the waste for permanent disposal.
The most recent tank to be emptied, C-102, had nearly 300,000 gallons of waste removed. It amounts to 95 percent of waste retrieved, although the goal is to retrieve an average of 99 percent of the waste from all single-shell tanks.
About 15,500 gallons of waste remains in Tank C-102, or about 2,100 cubic feet. The target was to get down to 360 cubic feet of waste, or the equivalent of about 1 inch of waste if it were evenly spread over the bottom of the 530,000-gallon-capacity tank.
However, the state can agree to consider a tank empty to regulatory standards when the limits of technology are reached.
What remains in the enclosed, underground tank is rock hard, Alzheimer said. Some of it is sandy and some is in concrete-like chunks. The bottom of the tank is fairly clear, but there are mounds of waste on the side of the tank that have been difficult to retrieve.
Retrieval was done with an enhanced-reach sluicer starting in April 2014. The tool was lowered into the tank and then extended to get a nozzle to spray liquid as close as possible to the waste. The liquid was used to break up the waste and move it toward a pump for removal. A high-pressure water system also was used.
When possible, recycled liquid waste was used, rather than water, for the retrieval work to prevent the creation of more waste.
“There was substantial effort from the workers to plan, prepare and retrieve this radioactive waste,” said Chris Kemp, deputy DOE project director for the tank farms, in a statement.
Work is ongoing to empty Tank C-111, with about 29 percent of the waste retrieved.
“Work is going fairly quickly,” Alzheimer said. As much as 2 percent of the waste has been retrieved on some days, although cold weather slows progress because of condensation issues in the ventilation system that result in false positive sensor readings.
The tank had 35,000 gallons of waste when pumping was last done in November 2010.
Caustic was added to the tank in November to soften the waste, speeding up retrieval in December.
“It’s been a very effective way to get going,” Alzheimer said.
Retrieval of waste from Tank C-105, which began in summer 2014, has been problematic.
Waste has been emptied with a Mobile Arm Retrieval System, or MARS, the largest and most robust system used to retrieve waste in Hanford’s underground tanks.
For the first time, MARS has been used with vacuum attachments, because Tank C-105 may have leaked in the past.
Retrieval started with little progress. Then a sluicing system was used to break through a hard crust of waste, and work then resumed on the softer waste beneath it using the vacuum system again.
“They had been making progress, but it was very, very slow,” Alzheimer said.
With retrieval about 45 percent complete and 67,000 gallons of waste remaining, the hoses used on the vacuum system failed due to heavy use.
DOE and its contractor, Washington River Protection Solutions, considered whether to fix the MARS vacuum system or remove MARS and put in enhanced reach sluicing systems, Alzheimer said.
Because the pace of work with MARS has been so slow, DOE is expected to try sluicing systems, Alzheimer said. No schedule for the work has been announced.
As work is completed on the C Tank Farm, DOE will next go to the A and AX Tank Farm.
At least two single-shell tanks are at risk of leaking during waste retrieval and could benefit from a vacuum retrieval system. One of the tanks was ruptured after high heat waste was added and an explosion occurred, Alzheimer said.