PNNL

Abstract

The Tank Side Cesium Removal (TSCR) system, currently operational under Washington River Protection Solutions LLC (WRPS), sends initial low-activity Hanford waste tank supernate feed to the Hanford Waste Treatment and Immobilization Plant (WTP) Low-Activity Waste (LAW) Facility. In addition to entrained solids removal from the supernate, the primary goal of TSCR is to remove cesium-137 (137Cs) by ion exchange, allowing contact handling of the liquid effluent product at the WTP. Crystalline silicotitanate (CST) ion exchange media, manufactured by Honeywell UOP, LLC (product IONSIV™ R9140-B), was selected as the ion exchange media at TSCR. Laboratory-scale ion exchange processing using TSCR prototypic unit operations continues to contribute toward WRPS establishing accurate process flowsheets for the individual feed campaigns planned for TSCR. This report describes the small-scale ion exchange testing with 14.0 L of diluted and filtered supernate from tank 241-AP-101 (AP-101DF) at 16 °C (62 °F) to demonstrate processing at temperature conditions that are more prototypic of what the TSCR system could experience during colder seasons of the year. Since CST Cs capacity increases with decreasing contact temperature, testing at the lower operating temperature will help to predict the maximum 137Cs loading onto the CST in the TSCR system. One of the waste acceptance criteria (WAC) for the WTP Low-Activity Waste Facility is that the waste must contain less than 3.18×10-5 Ci 137Cs per mole of Na. For the AP-101DF tank waste to meet this criterion, only 0.144% of the influent 137Cs concentration may be delivered to the WTP; this requires a Cs decontamination factor of 694. Testing with AP-101DF matched TSCR prototypic operations where a lead-lag configuration was used until the lag column reached the WAC limit, then a polish column was brought online for continued processing in a lead-lag-polish column configuration. Feed was processed at 1.9 bed volumes (BVs) per hour; the flowrate, in terms of contact time with the CST bed, matched the expected flowrate at TSCR. The Cs-decontaminated product was retained for vitrification testing (to be reported separately). The lead column reached 62% Cs breakthrough after processing ~1400 BVs of feed; the 50% Cs breakthrough occurred at 1250 BVs. Testing compared to previous AP-107 testing at 16 °C showed ~80 BV increases in volume processed to reach the WAC limit for both lead and lag columns. A similar slope in breakthrough curves for both tests indicates similar kinetic behavior, with variations in feed matrices (Na and Cs concentrations) likely responsible for the deviations in reaching the WAC limit. The Cs effluent from the lag column reached the WAC limit after processing 875 BVs. Anticipating this breakthrough point, the polish column was preemptively installed at 770 BVs. Cs breakthrough from the lag column began at 300 BVs, reaching 5.32×100 µCi/mL, or 5.6 % Cs breakthrough, after processing all 1400 BVs of feed. Table S.1 and Figure S.1 summarize the observed column performance and relevant Cs loading characteristics.