Staff Reports
WC Monitor
1/8/2016
Another Hanford C Farm radioactive waste storage tank has been declared empty to regulatory standards, with work to continue on two additional C Farm tanks. The 2010 consent decree between Washington state and the Department of Energy required the last of the 16 Hanford C Farm tanks to be emptied 15 months ago. The most recent tank to be emptied, C-102, allowed the Department of Energy to count a tank emptied in 2015. It is the 15th of 149 single-shell tanks emptied at Hanford, all but one of them in the C Tank Farm. The target is to have no more than 360 cubic feet of waste remaining in a tank, or about 1 inch if it were spread evenly over the bottom of the 530,000-gallon C Farm container. However, the state can agree that a tank is emptied to regulatory standards if the limits of technology are met. In the case of Tank C-102, about 2,100 cubic feet, or 15,500 gallons, remain that is “all pretty hard,” said Jim Alzheimer, a tank engineer for the Washington State Department of Ecology.
Some chunks that look like concrete plus sandy-looking particles remain, as the state agreed enough work had been completed, he said. The bottom of the tank is fairly clear, but rock-hard waste remains mounded by the tank walls. Two technologies were used to empty the tank before the state agreed DOE had done sufficient work. Work began in April 2014 using an enhanced-reach sluicing system to remove nearly 300,000 gallons of waste from the tank. The system was lowered through a riser into the underground tank and then a nozzle at the end of an extendable boom sprayed liquid waste to break up hardened material and move it toward a pump. A high-pressure water system also was used. The waste was transferred to a double-shell container to prevent leaks that have plagued single-shell containers at Hanford.
Work is under way now to retrieve waste from Tank C-111, with about 29 percent of its waste removed. “Work is going fairly quickly,” Alzheimer said. The tank had about 35,000 gallons of waste when pumping was last done in 2010. Two months ago a caustic was added to the tank and left to soak and soften the waste, which proved to be effective in allowing pumping to proceed efficiently in December, Alzheimer said. Cold weather has interfered with sensor operation as condensation has collected in the ventilation system, but on days when retrieval is in full operation as much as 2 percent of the tank’s starting volume is retrieved.
Tank C-105 has been more difficult to empty and work is currently halted there. The Mobile Arm Retrieval System, or MARS, DOE’s largest and most robust tank retrieval technology, has been used in the tank. For the first time, it has been equipped with a vacuum system, rather than a sluicing system, to demonstrate MARS’ vacuum capabilities on Hanford tanks that are suspected of having leaked in the past. The vacuum system adds small volumes of liquid and quickly vacuums it up, rather than adding larger volumes of liquid as the sluicing system requires. However, after work with the MARS vacuum system started in June 2014, little progress was made until tank operations contractor Washington River Protection Solutions (WRPS) used a sluicing system to break through a hard crust of waste. Since then progress has been made with the MARS vacuum on the softer waste beneath the crust, but it has been “very, very slow,” Alzheimer said. Work halted when hoses for the vacuum system failed due to heavy use. DOE and WRPS considered either fixing the MARS vacuum system or removing the system and trying enhanced-reach sluicing. Because progress with the MARS vacuum system is so slow, DOE is expected to try sluicing systems, Alzheimer said. After work to remove waste from the final C Farm tanks is completed, retrieval will shift to the AX and then the A tank farms. The A Tank Farm has two ruptured tanks, Alzheimer said, and a vacuum retrieval or other low-liquid technology would be useful.
