DOE CONSIDERING UNDERGROUND PRETREAT SYSTEM
WC Monitor
2/21/2014
The Department of Energy is considering building an underground interim pretreatment system that would use the same technologies as the planned Pretreatment Facility at Hanford’s Waste Treatment Plant, but on a smaller scale. The possible interim pretreatment system is one of the solutions discussed in what DOE called a “framework” document released in September after a year of study of the vitrification plant’s technical issues. It outlined for discussion a possible phased startup of the plant to allow some waste to be treated for disposal while technical issues still are being addressed. A presentation at a Hanford Advisory Board committee meeting late last week was the first time DOE has discussed in detail how the framework proposes to bypass the Pretreatment Facility, where construction has stopped while technical issues are resolved. However, it’s too early to know what an interim pretreatment system outside the plant might cost or how long it would take to build and begin operating, according to DOE.
The interim pretreatment facility would separate out some low-activity waste from the rest of the Hanford tank waste while it is still at the tank farms, where 56 million gallons of waste are stored. Then it would be sent directly to the vitrification plant’s Low Activity Waste Facility, which could be finished next year. The glass-filled canisters that would be produced would be buried at a Hanford landfill already built for that waste, the Integrated Disposal Facility. DOE has looked at early treatment of low activity waste since the early 2000s, said Steve Pfaff, DOE project director for the vitrification plant. But it didn’t make substantial progress on the idea until about 2008, when an expert panel recommended a two-step process to get a low-activity waste stream out of the tanks. The panel recommended using filtering to get the suspended solids out of the tank liquids and then an ion exchange process to get dissolved cesium out the liquids, similar to the two-step process the vit plant’s Pretreatment Facility is planned to use. The process would leave a largely decontaminated waste stream with hazardous chemicals and some remaining radioactivity, Pfaff said.
But in a change from previous proposals for bypassing the Pretreatment Facility, DOE now wants an interim pretreatment system that has enough capacity to feed two melters for waste and glass at the Low Activity Waste Facility. The melters could produce five canisters of glass a day that stand 7.5 feet tall and are 4.5 feet wide when the facility is running at 100 percent efficiency.
Advantages of Going Underground
Earlier studies had looked at either building a new facility at the tank farms or inserting equipment inside tanks to produce a low activity waste stream to treat. No decision has been made, but creating an underground interim pretreatment system would allow a large enough system to be built to feed the Low Activity Waste Facility as it runs at full capacity, Pfaff said. The interim plant would be mostly underground to provide shielding from radiation. It would include a system of pipes with tiny pores, just as in the ultrafiltration system that will be used at the vit plant’s Pretreatment Facility. At the interim system, tank liquid would pass around the outside of the pipes at a high flow rate, allowing liquid waste to filter into the pipe without pulling along much of the solids that contain much of the high-level radioactive waste. Then the filtered liquid would be sent to a system of ion exchange columns, also underground. They would use the same resin planned for the vit plant’s Pretreatment Facility to strip out dissolved cesium, which also is being treated as high-level radioactive waste. The resulting liquid would be very similar to the low-activity waste stream that would come out of the vit plant’s Pretreatment Facility, Pfaff said.
The interim facility likely would be fed waste from Tank AP-107, one of Hanford’s 28 double-shell tanks. The technology is well developed because it has been studied for the vitrification plant’s Pretreatment Facility and also would provide more flexibility than in-tank pretreatment, he said. DOE has studied in-tank pretreatment at Savannah River, and it would have the advantage of radiation shielding provided by the tank. The technology relies on inserting a column with 25 microfiltration disks on it. The column would spin and liquid would leak inside the disks and be pumped up the column. An ion exchange system also could be fitted inside a tank. But the amount of risers allowing access into underground tanks would limit how many systems could be fitted inside the tank, Pfaff said.
DOE has instructed Hanford tank farms contractor Washington River Protection Solutions to prepare a cost and technical proposal for initial design activities of an interim pretreatment system. DOE’s typical design process takes seven years, but DOE would do everything it could to advance a design and make glass as quickly as possible, Pfaff said.
Getting WTP Operable Top Priority
DOE already has warned the state of Washington that it is at risk of not meeting court-enforced deadlines to have the plant at full operation in 2022. An interim pretreatment facility will increase costs for treating Hanford tank waste, Pfaff said. But getting the vitrification plant into operation is a top priority and there also are other advantages, he said. It will allow DOE to train vit plant operators starting with the least radioactive waste and also allow a ramp up of the Analytical Laboratory and the support facilities for the plant, he said. Eventually the interim pretreatment system also could free up some double-shell tank space, allowing continued retrievals of waste from leak-prone single-shell tanks. An interim pretreatment facility, which could be designed to be used for 25 years, could continue to be used after the vit plant’s Pretreatment Facility begins operating during periods when it is off-line for maintenance or other issues, he said.
