Waste and Solvent Composition Limits for Modular Caustic-Side Solvent Extraction Unit (MCU)
This study examined waste feed and solvent limits for the Modular Caustic-Side Solvent Extraction Unit (MCU) currently being designed and built at the Savannah River Site (SRS) to remove cesium from highly alkaline radioactive waste. The study involved proposing ranges for 12 waste feed components (i.e., Na{sup +}, K{sup +}, Cs{sup +}, OH{sup -}, NO{sub 3}{sup -}, NO{sub 2}{sup -}, Cl{sup -}, F{sup -}, SO{sub 4}{sup 2-}, PO{sub 4}{sup 3-}, and CO{sub 3}{sup 2-}, and AlO{sub 2}{sup -}) through a compilation of SRS waste data. Statistical design methods were used to generate numerous wastes with varying compositions from the proposed ranges. An Oak Ridge National Laboratory (ORNL) model called SXFIT was used to predict the cesium extraction distribution coefficients (D-values) between the organic (solvent) phase and the aqueous waste phase using the waste component concentrations as inputs. The D-values from the SXFIT model were used as input along with MCU base case process parameters to a SASSE (Spreadsheet Algorithm for Stagewise Solvent Extraction) model to calculate final cesium concentrations for the MCU. The SASSE model was developed at Argonne National Laboratory (ANL). The SXFIT D-value and the waste component concentration data were used to develop a handier alternative model (neural network model) to the SXFIT model that predicts D-values within 15% of the SXFIT D-values. Both the SXFIT and the neural network model revealed the following. The solvent extractant concentration ratios are approximately equal to the corresponding D-value ratios; a useful feature that could be used to predict extraction D-values when the extractant concentration in the solvent changes in the MCU operation. Also, potassium is the only waste component out of the 12 that shows a distinct relationship with the cesium extraction D-values; an indication of potassium's competition with cesium in the Caustic-Side Solvent Extraction (CSSX) process. A waste feed acceptance model suitable for assessing wastes within relatively wide ranges of D-values (0.6-40) and initial cesium-137 concentrations (0.2-12.8 Ci/gal) has been developed from the SASSE outputs. The waste feed acceptance model is an equation involving initial cesium-137 concentration and D-value that results in a final cesium-137 concentration of 0.1 Ci/gal, the target concentration for the MCU. For example, the waste feed acceptance model shows the minimum acceptable extraction D-value based on MCU base conditions is 5.73. The waste feed acceptance model is defined by a simple linear relationship for extraction D-values {ge} 7. This facilitates quicker calculations. For a given extraction D-value, final cesium-137 concentration (C{sub f}) and initial cesium-137 concentration (C{sub 0}) are linearly related; while for a given C{sub 0}, log (C{sub f}) and log (extraction D-value) are linear with a slope of -1.43. These two relationships allow one to quickly calculate C{sub f} at other MCU conditions without resorting to the SASSE model. The SASSE runs indicate that broad changes in the MCU process parameters for the extraction, scrub and strip stages (i.e., flow rate, temperature, fraction of interstage carryover, total liquid volume per contactor stage, and efficiency per contactor stage) will not result in C{sub f} exceeding target, at least for the MCU base conditions.
- Research Organization:
- Savannah River Site (SRS), Aiken, SC
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- DE-AC09-96SR18500
- OSTI ID:
- 881449
- Report Number(s):
- WSRC-TR-2005-00258; TRN: US0603145
- Country of Publication:
- United States
- Language:
- English
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