Caustic-Side Solvent Extraction: Prediction of Cesium Extraction for Actual Wastes and Actual Waste Simulants

Robustness of the CSSX Process to Feed Variation: Efficient Cesium Removal from the High Potassium Wastes at Hanford

“…Hence, it was a concern in the development of the Caustic Side Solvent Extraction (CSSX) process [1][2][3][4] for the removal of cesium from caustic high-level nuclear wastes that the process be forgiving of expected variations in the concentrations of waste components. [5,6] In this connection, it has been noted that the feed concentration of the chief competing cation, potassium, can have considerable impact when potassium begins to load the calixarenecrown extractant BOBCalixC6 in the currently formulated CSSX solvent, whereupon the cesium distribution ratio D Cs can decrease from the design values of 8-15 to values as low as 3. [7][8][9] This concern has raised questions regarding the applicability of CSSX for the treatment of Hanford tank wastes, known to possess potassium concentrations up to 1 M, [10] much higher than the average concentration of 0.015 M [11] for the Savannah River Site (SRS) waste application for which CSSX was designed.…”

“…Whereas the cesium/sodium separation factor (D Cs /D Na ) obtainable with calixarenecrown-6 compounds is excellent and can reach 5 orders of magnitude, [24][25][26] the selectivity for cesium over potassium is much lower (though still high), on the order of 100-200. [26] The wastes present at the SRS have a potassium concentration that usually does not exceed 0.030-0.040 M, though one waste type was identified as high as 0.067 M. [5,6] This is a high enough concentration to start showing significant loading effects leading to depressed values of D Cs , but the cesium distribution ratios remain adequate for the process. In the case of the Hanford wastes, the concentration of potassium can be as high as 1 M, while the concentration of cesium is submillimolar.…”

“…[7] A fairly sophisticated computer program, SXFIT, [35] had been successfully used to model the extraction of cesium from the very complicated mixtures present in the SRS tank wastes. [5,6,8] Given that the previous studies had indicated that just four species strongly impact the calculation of the cesium distribution ratios, we also tested the possibility that a simple analog equation might be developed for D Cs prediction. Assuming a target cesium decontamination factor (DF) of 5000 and a concentration factor (CF) in the range 5-15, we then use the experimental or SXFIT values of D Cs to calculate flowsheets specifying the minimum number of centrifugal-contactor stages and phase flow rates needed to meet the process targets.…”

“…[7] ; Savannah River compositions from ref. [5] . The ion concentrations listed here represent only a partial listing of the simulant constituents.…”

“…b Predicted from full SXFIT modeling; values reported previously in refs. [7] and [5] for Hanford and Savannah River compositions, respectively. c Calculated from the SXFIT model (this work).…”