The Department of Energy is considering building an underground plant that would use the same technologies as the troubled Pretreatment Facility at Hanford's vitrification 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 plant's technical issues.
It outlined for discussion a possible phased startup of the $12.3 billion vitrification 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 Thursday 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.
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DOE has 56 million gallons of radioactive and hazardous chemical waste in a combination of liquid and solid forms held in underground tanks awaiting treatment for permanent disposal. The waste is left from the past production of plutonium for the nation's nuclear weapons program.
The vitrification plant is planned to separate waste at its Pretreatment Facility into low-activity radioactive waste and high-level radioactive waste for glassification at separate facilities.
To get glassification started sooner, DOE is looking at separating out some low-activity waste from the rest of the tank waste while it is still at the tank farms, where the 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. There it would be turned into glass logs within canisters that 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 of the liquids, similar to the two-stop process the vit plant's Pretreatment Facility is planned to use. The process would leave a largely decontaminated waste stream with some 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. That would 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.
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 to produce the low-activity waste stream 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 that look like solid metal, but would have tiny pores, just as in the ultrafiltration system that will be used at the vit plant's Pretreatment Facility, which covers a footprint larger than a football field.
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, which 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 as planned at 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 and also would provide more flexibility than in-tank treatment, he said.
DOE has studied in-tank pretreatment at its Savannah River nuclear site, 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 number of risers allowing access into underground tanks would limit how many systems could be fitted inside the tank.
DOE has instructed its 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. 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. DOE is emptying waste from 149 leak-prone single-shell tanks into 28 newer double-shell tanks. But those tanks are nearing capacity and the oldest of the double-shell tanks has a leak between its shells and may need to be emptied also.
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 offline for maintenance or other issues.
w Annette Cary: 582-1533; email@example.com; Twitter: @HanfordNews