Scaled Tank Side Cesium Removal Test System for Assessing Performance at High Solids Loading - 21054

The Direct Feed Low-Activity Waste (DFLAW) process requires pretreatment of tank waste supernatant prior to vitrification in the Low-Activity Waste (LAW) Facility at the Hanford Waste Treatment and Immobilization Plant (WTP). Initially, this function will be performed by the Tank Side Cesium Removal (TSCR) system, which is designed to conduct two separations in order to prepare LAW for transfer to the WTP LAW Facility for vitrification. The separation steps are dead-end filtration using a Mott Grade 5 sintered stainless steel filter and ion exchange to remove Cs-137 using crystalline silicotitanate (CST). Testing of TSCR separation technologies at multiple scales with actual waste and simulant has demonstrated that unit operation performance at the nominal solids loading is sufficient to achieve the target throughput [3.15 x 10^-4 m³ s^-1 (5 gpm)]. Due to the nature of the waste feed process to the TSCR system, the nominal solids content is anticipated to be low (approximately 200 ppm solids); however, the TSCR waste acceptance criteria permits waste containing up to 15,000 ppm solids (1.5 wt%). As reported by the manufacturer, the Mott filter has a 90% collection efficiency for 5 μm particles, and 99.9% efficiency for 13 μm particles. Particles that comprise a waste feed with a higher-than-nominal solids loading may contain a significant fraction that are less than 13 μm in size and could depth foul (thereby increasing pressure drop and backwash frequency) or pass through the filter. Particles that pass through the filter may foul the CST ion exchange beds, increasing pressure drop and potentially restricting access to Cs-137 exchange sites via physical blockage or flow channeling. To address the performance uncertainties at higher-than-nominal solids loading, a scaled TSCR system was designed and assembled at Pacific Northwest National Laboratory. The scaled system was devised to perform most TSCR operations prototypically and integrate filtration and ion exchange equipment to dynamically observe the effect of processing at solids loadings up to 1.5 wt%. The key elements of the system are a full-height CST column [2.34 m (92-in) bed, 0.004 m³ (∼1.1-gal) bed volume] and a pair of back-washable filters with approximately 0.05 m² (0.54 ft²) of surface area each. In parallel, a focused simulant development effort was undertaken to refine an existing formulation to accommodate higher solids loadings (up to 1.5 wt%). The basis used to scale the test system and define prototypic test operations is presented in this paper. (authors)