(289c) Investigation into Na and Cs Activity Coefficients in High Salt Solutions to Support Cs Removal in Hanford Tank Waste

The treatment of Hanford tank waste is one of the most technically challenging environmental cleanup activities for the Department of Energy to date. To expedite the processing of liquid waste stored in the underground tanks in southeastern Washington State and remove the significant dose contributor, Cs-137, ion exchange with crystalline silicotitanate (CST) has been employed as part of the Tank Side Cesium Removal (TSCR) system. The model used to predict Cs exchange onto CST was developed using activity coefficients calculated from the Bromley equation.

A series of batch contact tests from variable Na were conducted to look at the impact of Na concentration on Cs distribution. Experimental distribution ratios were compared to the distribution ratios predicted using three different activity coefficient models 1) commercially available HSC software 2) the Bromley equation and 3) a simplified approach adapted from Marcos-Arroyo et.al. Ultimately the Bromley method underpredicted the effect of ionic strength on the Na activity coefficient, HSC overestimated the impact of ionic strength on the expected performance due to the γ_Cs+, but the simplified approach predicted the experimental K_ds quite well in a binary matrix. Expansion of this approach in complex matrices is necessary for application to Hanford tank waste.