(146c) Evaluation of High Level Waste Sludge Processing Behavior

The U.S. Department of Energy’s (DOE) Hanford Site has 177 underground storage tanks that contain a complex and diverse mix of chemical and radioactive wastes from past nuclear fuel reprocessing and waste-management operations. The strategy of the DOE Office of River Protection is to retrieve and treat this waste. Approximately 20 vol% of this waste is in the form of insoluble undissolved solids (UDS) or sludge, and this waste may require modification prior to its transfer for treatment at the Waste Treatment and Immobilization Plant (WTP). Required modification may be due to specific WTP acceptance criteria for waste feed delivery which describe the physical and chemical properties of the waste that must be met before the waste is transferred to the WTP. Modification may also occur from incremental and incidental waste blending during retrieval from the individual waste tanks. One challenging requirement relates to the UDS composition in a waste feed because the solid particles settle, and their concentration and relative proportion can change during batch transfers of the waste to WTP. A key uncertainty is the ability to mix and transfer wastes with large variations in UDS concentrations and resulting settling rates. To address this uncertainty, a number of bench-scale mixing and settling tests have been conducted to determine the process performance for variable physical and chemical properties of tank waste sludge and simulant samples. Settling velocity, chemical characterization, shear strength measurements, and just suspended impeller speed (N_js) were determined for tank waste simulants and actual waste sludges to evaluate the mobilization of non-cohesive particles at various laboratory scales. This presentation details a comparison of the size and density of sludge particulate between simulants and actual waste sludge to help understand the behavior of high-level waste solids settling and transport. Variations in sludge chemistry are used to determine the possibility of potential solids segregation and uneven particle distribution. Understanding the behavior of high level waste sludge transport can provide potential remedies for addressing problematic behavior and technical guidance during high-level waste flowsheet planning.