Cesium Removal from Savannah River Site Radioactive Waste using the Caustic-Side Solvent Extraction (CSSX) Process

Abstract

Researchers at the Savannah River Technology Center (SRTC) successfully demonstrated the Caustic-Side Solvent Extraction (CSSX) process flow sheet using a 33-stage, 2-cm centrifugal contactor apparatus in two 24-hour tests using actual high level waste. The CSSX process for removal of cesium from alkaline solutions is the reference process for decontamination of high level waste (HLW) at the Savannah River Site (SRS). The solvent consists of a calix [4]arene-crown-6 extractant (BOBCalix), an alkylphenoxy alcohol modifier, and trioctylamine (TOA) dissolved in an inert hydrocarbon matrix (Isopar ® L). Previously, we demonstrated the solvent extraction process with actual SRS HLW supernatant solution using a non-optimized solvent formulation. Following that test, the solvent system was optimized to enhance extractant solubility in the diluent by increasing the modifier concentration. We now report results of two tests with the new and optimized solvent. The first test used a composite of supernatant solutions from two waste tanks and the second test used a solution derived from dissolved salt cake. Test results showed that the CSSX process using the optimized solvent reduces 137 Cs in HLW supernatant solutions to concentrations below the waste acceptance criterion (WAC) of 45 nCi/g for disposal as low-level waste (called "Saltstone"). Waste decontamination factors as high as three million were achieved during testing. Test durations exceeded 24 hours of uninterrupted operation and demonstrated hydraulic stability of the contactor array while operating with the optimized solvent. Carryover of organic solvent in aqueous streams (and aqueous in organic streams) was found to be less than 1%. The concentration factor (i.e., the ratio of the cesium concentration in the strip raffinate to the concentration in the waste) averaged approximately 13 during both tests, slightly below the process requirement of 15. Uncertainties in process flow rate measurement and control prevented the test from achieving the target of 15.

Caustic-Side Solvent-Extraction Modeling for Hanford Interim Pretreatment System

“…The calculations employed arbitrary, but reasonable, stage efficiencies of 80% to estimate the "actual" number of stages shown; these may be compared with the slightly greater "theoretical" stages assuming that stage efficiencies are 100% (i.e., running at equilibrium). Four scrub stages were arbitrarily assumed, though two are used at the SRS and presumed sufficient [8][9][10]. No wash stages were included, as they would not be expected to influence the results; however, two wash stages with 10 mM NaOH, as used at the SRS, are likely necessary for long-term process stability.…”

“…The process as currently practiced has been optimized [2][3][4][5][6] and demonstrated [7][8][9] for removal of cesium from salt waste at the Savannah River Site (SRS). To meet the needs of the SRS Salt Waste Processing Facility (SWPF) [10], the CSSX process has been designed and demonstrated to remove cesium with a decontamination factor (DF) in excess of 40,000, concentrating it by a factor (CF) of 15 in a stream of 1 mM HNO 3 suitable for vitrification.…”

“…Cesium distribution ratios (D Cs = [Cs] org /[Cs] aq ) for extraction of the specified feed compositions by the CSSX solvent were to be either calculated using an existing thermodynamic model, provided it was found to be suitably parameterized, or determined experimentally for selected feeds. Assuming a target cesium decontamination factor (DF) of 5000 and a concentration factor (CF) in the range [5][6][7][8][9][10][11][12][13][14][15], the values of D Cs corresponding to each feed are needed to calculate a flowsheet specifying the minimum number of centrifugal-contactor stages and phase flow rates needed to meet the process targets. Based on the physical properties of the phases, the contactor sizes that would furnish a throughput of 20.8 L/min.…”

Evaluation of deep cavity imidazolylcalix[n]arenes for selective extraction of silver

“…Recognition of metal ions of biological [1,2], medicinal [3,4], radioactive [5,6] and environmental significance [7] is an area of intense research activity [8,9]. Owing to the presence of distinct hydrophobic and hydrophilic regions and adjustable cavity dimensions in their molecular architecture, calixarenes have attracted considerable attention for the design of useful receptors [10][11][12][13][14][15][16][17][18].…”

SPEC Process III. Synthesis of a Macroporous Silica-Based Crown Ether-Impregnated Polymeric Composite Modified with 1-Octanol and its Adsorption Capacity for Sr(II) Ions and Some Typical Co-Existent Metal Ions

“…Since both uranium and plutonium can be separated through the use of a 30% tri-n-butyl phosphate/kerosene mixture, the HNO 3 concentration in the HLW produced is usually ca. 3.0 M. A number of liquid-liquid solvent-extraction partitioning techniques, such as transuranic extraction (TRUEX) (Schulz and Horwitz 1988), universal solvent extraction (UNEX) Romanovskiy et al 2001;Herbst et al 2002), caustic-side solvent extraction (CSSX) (Walker et al 2003), strontium extraction (SREX) (Horwitz et al 1991;Wood and Law 1997), caesium separation by crown-calix extraction (CCCEX) (Simon et al 2000(Simon et al , 2004 and fission product extraction (FPEX) (Riddle et al 2005;Mincher et al 2007), have been reported as providing an effective separation of the long-lived minor actinides, Sr(II) and Cs(I) by utilizing a variety of chelating extractants in a hydrocarbon diluent. Some macrocyclic crown-calix[4]arene ether-containing compounds such as calix [4]arene-bis(t-octylbenzo-crown-6) (BOBcalixC6) and 4,4′,(5′)-di(t-butylcyclohexano)-18-crown-6 (DtBuCH18C6) dissolved in ISOPAR ® L diluent exhibit excellent extraction capabilities and selectivities for Sr(II) (Law et al 1997).…”