Temperature Effect of Cesium Exchange onto Crystalline Silicotitanate in AP-107 and AP-105 Hanford Tank Wastes and Two Simulants

Abstract

Washington River Protection Solutions, LLC (WRPS) is charged with the development of the Tank Side Cesium Removal (TSCR) system to process Hanford tank waste supernates in preparation for vitrification. In addition to a filtration step, TSCR will remove cesium (Cs) using ion exchange columns filled with crystalline silicotitanate (CST) ion exchange media. CST is produced by Honeywell UOP, LLC. The documented safety analysis (DSA) developed for the TSCR system limits a single column loading to 141,600 Ci 137Cs. Given a 137Cs isotopic mass fraction of 20% and the planned CST bed size of 596 L (157.5 gal) in a TSCR column, this equates to 0.10 mmole Cs per g CST (Cs distribution coefficient, Kd, 1400 mL/g). Factors that influence Cs uptake by CST include (but are not limited to) (1) CST production (lot-to-lot variations), (2) contact temperature, (3) contact duration, (4) competitors in the tank waste feed, (5) anionic composition of the tank waste feed, and (6) the 137Cs isotopic mass fraction (differs slightly among tank wastes and decreases with time). Recent testing using Hanford tank waste simulants has shown that bounding the worst-case matrix and test conditions to maximize Cs (137Cs) exchange onto CST will exceed the DSA limit.1 Therefore, actual tank waste testing was desirable to better determine if the DSA limit will be challenged under expected process conditions. Previous batch contact testing of AP-107 and AP-105 tank wastes with CST to develop Cs isotherms was conducted at ~30 °C (ambient hot cell temperature), which is higher than the expected 16 °C process temperature at TSCR. Therefore, a series of batch contact tests was conducted with these matrices to better assess Cs loading onto CST and to provide data to support modeling efforts at the expected process temperature. To reduce the radiation dose to personnel, AP-107 and AP-105 tank wastes were first processed through CST ion exchange beds to strip 137Cs (and Cs) allowing the matrices to be contact-handled during testing. Cs isotherms were then developed for each tank waste matrix at four different temperatures (12.7, 15.9, 21.0, and 34.5 °C). A Baseline simulant2 and Simple simulant (4.6 M NaNO3 and 1.0 M NaOH) were similarly processed at three different temperatures (12.7, 21.0, and 34.5 °C) to develop additional fundamental modeling data. From the isotherms and the equilibrium Cs concentrations, the Cs loading in terms of Q (mmoles Cs/g CST) and Kd (mL/g) were determined. Table S.1 provides a summary of the Q and Kd values for each matrix and temperature. All matrices resulted in a linear decrease of Cs loading with increasing temperature in the temperature range tested. Only AP-107 tank waste processing at 12.7 °C exceeded the DSA limit. The Baseline simulant processed at 12.7 °C reached the DSA limit.