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Staff Accomplishments

Solidifying spent nuclear fuel sludge to help protect Columbia River

December 2004
Scientists at Pacific Northwest National Laboratory have successfully designed and tested a method to solidify the 6.3 cubic meters of the radioactive sludge in the North Loadout Pit of the K-East Basin, a leak-prone basin formerly used to store spent nuclear fuel rods, near the Columbia River. After PNNL's method to solidify the sludge is applied, the radioactive waste can be shipped to the Waste Isolation Pilot Plant, or WIPP, the world's first underground repository licensed to permanently dispose of transuranic waste from nuclear weapons production. Fluor Hanford, the contractor responsible for the K Basins, selected PNNL to characterize and design the treatment system "because we have made really strong technical contributions to sludge processing over the last several decades," said PNNL's Environmental Management Market Sector Manager Terry Walton. Characterizing the North Loadout Pit Sludge Starting in December 2003 in the Radiochemical Processing Laboratory, PNNL's scientists analyzed samples of the sludge provided by Fluor Hanford. The sludge consists of sand back-washed from filters in the basin's water treatment system along with uranium "rust" from the nuclear fuel rods, which were spent in powering N Reactor. Windblown sand, rust from structures in the basin, and ion exchange resin also added to the sludge layer. The radiochemical analyses revealed the concentration of the 17 isotopes in the sludge, including plutonium-241, strontium-90, and cesium-137. In addition, PNNL's scientists determined the sludge's density, water content, and other physical properties, including the amount and type of gases the material generates. These parameters were key to selecting a treatment option to meet regulators' requirements. Solidifying the Sludge Having characterized the sludge, the PNNL team evaluated several options for converting the sludge into a solid, eliminating drainable liquids. The WIPP does not accept waste with drainable liquids. Treatment options considered included
  • Granular, clay-based inorganic sorbent
  • Polymers
  • Portland cement and bentonite clay to form grout.
With PNNL's technical recommendations, Fluor Hanford factored in cost effectiveness, safety factors, and the waste acceptance criteria for the WIPP. Fluor Hanford decided to grout the waste in 55-gallon drums. The drums would then be accepted as contact-handled transuranic waste at the WIPP. With this decision, PNNL designed and tested the grouting system. The process, which will be conducted by Fluor Hanford, begins when the sludge is trucked from the K East Basin to T Plant, a large building with heavily shielded cells in Hanford's 200 West Area. The sludge is then transferred into a buffer tank in three to five 500-liter batches. Once in the buffer tank, 500 liters of water is added, and the mixture is stirred. Then, 10 to 15 liters of the mixture is pumped into a 55-gallon drum. Exact amounts of Portland cement and bentonite clay are stirred into the drum. The cement hardens, the drums are sealed, and final inspections and paperwork completed. The drums are then shipped to the WIPP, near Carlsbad, New Mexico. Innovative Solutions to Grout System Design The biggest challenge from a technical standpoint was to come up with a system that maximized the amount of sludge solidified in each drum while producing drums that meet the requirements for disposal at WIPP. "If you put too much sludge in a drum, it will exceed the WIPP limits for contact-handled waste and would have to be packaged and transported by much more expensive methods. On the other hand, if you put too little sludge in a drum, it becomes low-level waste and can't be sent to WIPP," said Project Manager George Mellinger. To determine, the correct amount of sludge to solidify in each drum the PNNL team added a gamma monitor to the buffer tank's recirculation loop. By adding this monitor, the team found a simple and elegant way to determine the amount of sludge for each barrel. During startup of the grout system, they will work with Fluor Hanford to determine the correlation between the monitor's readings and the surface dose rate of barrels of grouted sludge. This correlation will then be used to determine the amount of sludge to be added to each barrel. The Future Fluor Hanford plans to install the grout system in T Plant. PNNL staff will be part of the review team for the final installation. The Laboratory will also help with startup of the grout system, including development of the correlations needed for the gamma monitor. As a result of the Fluor Hanford/PNNL collaboration, Fluor Hanford will immobilize the 6.3 cubic meters of North Loadout Pit sludge some 30 years earlier than originally scheduled. This immobilization effort could be an important demonstration of the treatment for other sludges. In addition, it will help protect the Columbia River by removing waste from these leak-prone basins that sit about a quarter of a mile from the river. Contact: Wally Weimer, Process and Measurement Technology Product Line Manager, walter.weimer@pnl.gov

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