Experimental Verification of Caustic-side Solvent Extraction for Removal of Cesium from Tank Waste PDF Download

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Languages : en
Pages : 5

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The objectives of this report are: to demonstrate complete CSSX process flowsheet (proof of concept)--decontamination factor ≥ 40,000, and concentration factor ≈15; Scientific and technical issues evaluated--stage efficiency, temperature control, hydraulic performance, long time (multi-day) operation, short-term shutdown, effect of solids, and recovery from Cs moving through strip section.

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Languages : en
Pages : 22

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A caustic-side solvent extraction (CSSX) process to remove cesium from Savannah River Site (SRS) high-level waste was tested for 71 hours in a 33-stage minicontactor (2-cm centrifugal contactor). This multi-day demonstration used an average SRS simulant for the waste feed. The two key process goals were achieved: (1) the cesium was removed from the waste with decontamination factors greater than 40,000, and (2) the recovered cesium was concentrated by a factor of 15 in dilute nitric acid. These goals were maintained for 71 h as 1.4 L of solvent was recycled 42 times at 14 mL/min while processing 180 L of SRS simulant at 43 mL/min. The average decontamination factor was 159,000 for cesium and the average concentration factor was 14.9. The process had to be shut down twice for minor problems, which were fixed and testing resumed. The results confirmed that the CSSX process could be used to help decontaminate the millions of gallons of SRS waste now stored in underground tanks.

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Languages : en
Pages : 21

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A caustic-side solvent extraction (CSSX) process to remove cesium from Savannah River Site (SRS) high-level waste was tested in a minicontactor (2-cm centrifugal contactor). In the first phase of this effort, the minicontactor stage efficiency was improved from 60% to greater than 80% to meet the SRS process requirements using a 32-stage CSSX flowsheet. Then, the CSSX flowsheet was demonstrated in a 32-stage unit, first without solvent recycle, then with it. In both cases, the key process goals were achieved: (1) the cesium was removed from the waste with decontamination factors greater than 40,000 and (2) the recovered cesium was concentrated by a factor of 15 in dilute nitric acid. Oak Ridge National Laboratory (ORNL) analysis of the recycled solvent showed no evidence of impurity buildup.

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Languages : en
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Author: JR. J. F. Birdwell
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Languages : en
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This report presents the results of a conceptual design of a solvent extraction process for the selective removal of {sup 137}Cs from high-level radioactive waste currently stored in underground tanks at the U.S. Department of Energy's Savannah River Site (SRS). This study establishes the need for and feasibility of deploying a simplified version of the Caustic-Side Solvent Extraction (CSSX) process; cost/benefit ratios ranging from 33 to 55 strongly support the considered deployment. Based on projected compositions, 18 million gallons of dissolved salt cake waste has been identified as having {sup 137}Cs concentrations that are substantially lower than the worst-case design basis for the CSSX system that is to be deployed as part of the Salt Waste Processing Facility (SWPF) but that does not meet the waste acceptance criteria for immobilization as grout in the Saltstone Manufacturing and Disposal Facility at SRS. Absent deployment of an alternative cesium removal process, this material will require treatment in the SWPF CSSX system, even though the cesium decontamination factor required is far less than that provided by that system. A conceptual design of a CSSX processing system designed for rapid deployment and having reduced cesium decontamination factor capability has been performed. The proposed accelerated-deployment CSSX system (CSSX-A) has been designed to have a processing rate of 3 million gallons per year, assuming 90% availability. At a more conservative availability of 75% (reflecting the novelty of the process), the annual processing capacity is 2.5 million gallons. The primary component of the process is a 20-stage cascade of centrifugal solvent extraction contactors. The decontamination and concentration factors are 40 and 15, respectively. The solvent, scrub, strip, and wash solutions are to have the same compositions as those planned for the SWPF CSSX system. As in the SWPF CSSX system, the solvent and scrub flow rates are equal. The system is designed to facilitate remote operation and direct maintenance. Two general deployment concepts were considered: (1) deployment in an existing but unused SRS facility and (2) deployment in transportable containers. Deployment in three transportable containers was selected as the preferred option, based on concerns regarding facility availability (due to competition from other processing alternatives) and decontamination and renovation costs. A risk assessment identified environmental, safety, and health issues that exist. These concerns have been addressed in the conceptual design by inclusion of mitigating system features. Due to the highly developed state of CSSX technology, only a few technical issues remain unresolved; however, none of these issues have the potential to make the technology unviable. Recommended development tasks that need to be performed to address technical uncertainties are discussed in this report. Deployment of the proposed CSSX-A system provides significant qualitative and quantitative benefits. The qualitative benefits include (1) verification of full-scale contactor performance under CSSX conditions that will support SWPF CSSX design and deployment; (2) development of design, fabrication, and installation experience bases that will be at least partially applicable to the SWPF CSSX system; and (3) availability of the CSSX-A system as a means of providing contactor-based solvent extraction system operating experience to SWPF CSSX operating personnel. Estimates of fixed capital investment, development costs, and annual operating cost for SRS deployment of the CSSX-A system (in mid-2003 dollars) are $9,165,199, $2,734,801, and $2,108,820, respectively. When the economics of the CSSX-A system are compared with those of the baseline SWPF CSSX system, benefit-to-cost ratios ranging from 20 to 47 are obtained. The benefits in the cost/benefit comparison arise from expedited tank closure and reduced engineering, construction, and operating costs for the SWPF CSSX system. No significant impediments to deployment were determined in the reported analysis, and where technical uncertainties were identified, development tasks to mitigate them are indicated. It is recommended that deployment of the CSSX-A system be pursued in a timely manner in order to derive the greatest possible cost and accelerated treatment benefits.

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Languages : en
Pages : 5

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This paper describes a demonstration of a solvent extraction process for removal of 137Cs from alkaline high level radioactive waste (HLW) solutions from the Savannah River Site (SRS) tank farm. The process employed a calix[4]arene-crown-6 extractant dissolved in an inert hydrocarbon matrix, along with an alkylphenoxy alcohol modifier, and an additional additive, trioctylamine, to improve stripping performance and mitigate the effects of organic anions present in the feed stream. Testing included washing and recycle of the solvent. Tests conducted with two non-radioactive simulants of SRS tank waste verified proper hydraulic operation prior to the radioactive experiments. An additional 12-hour demonstration used simulated SRS tank waste spiked with 137Cs to confirm proper hydraulic operation, decontamination and volume reduction factors following installation of the equipment in a shielded cell facility. Finally, a 48-hour demonstration was performed employing a composite waste from SRS tank farms. Approximately 106 liters of waste (1.5E+09 d/min/mL 137Cs activity) was processed through the test unit. Average 137Cs decontamination factors for the decontaminated waste streams in all tests exceeded the target value of 40,000. Concentration factors in the strip section agreed to within 5 percent of the target value of 15.

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Languages : en
Pages : 85

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An alkaline-side solvent extraction process for cesium removal from Savannah River Site (SRS) tank waste was evaluated experimentally using a laboratory-scale centrifugal contactor. Single-stage and multistage tests were conducted with this contactor to determine hydraulic performance, stage efficiency, and general operability of the process flowsheet. The results and conclusions of these tests are reported along with those from various supporting tests. Also discussed is the ability to scale-up from laboratory- to plant-scale operation when centrifugal contractors are used to carry out the solvent extraction process. While some problems were encountered, a promising solution for each problem has been identified. Overall, this alkaline-side cesium extraction process appears to be an excellent candidate for removing cesium from SRS tank waste.

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Languages : en
Pages : 5

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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. 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.

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Languages : en
Pages : 5

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A modified solvent has been developed at Oak Ridge National laboratory (ORNL) for a caustic-side solvent extraction (CSSX) process that removes cesium (Cs) from Savannah River Site (SRS) tank waste. The modified solvent was evaluated using the same CSSX flowsheet, SRS simulant, and 33-stage minicontactor (2-cm centrifugal contactor) that had been used to test the previous CSSX solvent. As with the previous solvent, the key process goals were achieved: (1) the Cs was removed from the waste with decontamination factors greater than 40,000 and (2) the recovered Cs was concentrated by a factor of 15 in dilute nitric acid. Thus, the modified CSSX solvent can be used in place of the previous solvent while maintaining satisfactory hydraulic performance and still achieving process requirements at the bench scale.

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The Office of Waste Processing, within the Office of Technology Innovation and Development, is funding the development of an enhanced solvent - also known as the next generation solvent (NGS) - for deployment at the Savannah River Site to remove cesium from High Level Waste. The technical effort is a collaborative effort between Oak Ridge National Laboratory (ORNL) and Savannah River National Laboratory (SRNL). As part of the program, the Savannah River National Laboratory (SRNL) has performed a number of Extraction-Scrub-Strip (ESS) tests. These batch contact tests serve as first indicators of the cesium mass transfer solvent performance with actual or simulated waste. The test detailed in this report used simulated Tank 49H material, with the addition of extra potassium. The potassium was added at 1677 mg/L, the maximum projected (i.e., a worst case feed scenario) value for the Salt Waste Processing Facility (SWPF). The results of the test gave favorable results given that the potassium concentration was elevated (1677 mg/L compared to the current 513 mg/L). The cesium distribution value, DCs, for extraction was 57.1. As a comparison, a typical D{sub Cs} in an ESS test, using the baseline solvent formulation and the typical waste feed, is ≈15. The Modular Caustic Side Solvent Extraction Unit (MCU) uses the Caustic-Side Solvent Extraction (CSSX) process to remove cesium (Cs) from alkaline waste. This process involves the use of an organic extractant, BoBCalixC6, in an organic matrix to selectively remove cesium from the caustic waste. The organic solvent mixture flows counter-current to the caustic aqueous waste stream within centrifugal contactors. After extracting the cesium, the loaded solvent is stripped of cesium by contact with dilute nitric acid and the cesium concentrate is transferred to the Defense Waste Processing Facility (DWPF), while the organic solvent is cleaned and recycled for further use. The Salt Waste Processing Facility (SWPF), under construction, will use the same process chemistry. The Office of Waste Processing (EM-31) expressed an interest in investigating the further optimization of the organic solvent by replacing the BoBCalixC6 extractant with a more efficient extractant. This replacement should yield dividends in improving cesium removal from the caustic waste stream, and in the rate at which the caustic waste can be processed. To that end, EM-31 provided funding for both the Savannah River National Laboratory (SRNL) and the Oak Ridge National Laboratory (ORNL). SRNL wrote a Task Technical Quality and Assurance Plan for this work. As part of the envisioned testing regime, it was decided to perform an ESS test using a simulated waste that simulated a typical envisioned SWPF feed, but with added potassium to make the waste more challenging. Potassium interferes in the cesium removal, and its concentration is limited in the feed to