Savannah River Site Tank Waste Research Articles

A newly proposed isotherm model to predict Cs exchange with crystalline silicotitanate in tank waste simulants

ABSTRACT The Zheng Anthony Miller (ZAM) computer model, a multicomponent ion exchange model used to predict the exchange of Group I metals onto crystalline silicotitanate (CST), has historically been used to predict Cs distribution coefficients from Hanford and Savannah River Site (SRS) tank waste simulants. Comparison of experimentally determined Cs distribution coefficients from tank waste simulants with ZAM isotherm model predictions indicate overprediction of Cs and K distribution coefficients for simple and complex simulants with the engineered form of CST. Additionally, recent changes in chemical composition/manufacturing of IONSIV TM R9140-B have resulted in increased Cs capacity from high-salt, highly alkaline solutions. This work served to assess different isotherm models and refine equilibrium parameters to develop a model that can be applied to Hanford and SRS tank waste Cs removal efforts. Toward this goal, the Campbell Westesen Peterson (CWP) model was developed. This model utilized the experimentally determined Cs capacity, and simplified ZAM equilibria expressions to include only the binary substitution of Cs+ or K+ on the Na+ sites. Equilibrium constants for these equations were refined using experimentally determined distribution coefficients. Overall, the CWP model significantly improved our ability to predict both Cs and K loading capacity from complex matrices.

Performance Testing of the Next-Generation CSSX Solvent with Actual SRS Tank Waste

Efforts are underway to qualify the Next-Generation Solvent for the Caustic Side Solvent Extraction (CSSX) process. Researchers at multiple national laboratories have been involved in this effort. As part of the effort to qualify the solvent extraction system at the Savannah River Site (SRS), Savannah River National Laboratory (SRNL) researchers performed a number of tests at various scales. A series of batch equilibrium, or Extraction-Scrub-Strip (ESS), tests were conducted first. These tests used ∼30 mL of Next-Generation Solvent and either actual SRS tank waste, or waste simulant solutions. The results from these cesium mass transfer tests were used to predict solvent behavior under a number of conditions. For larger-scale testing, twelve stages of 2-cm (diameter) centrifugal contactors were assembled. This rack of contactors is structurally similar to one tested in 2001 during the demonstration of the baseline CSSX process. No issues were encountered during assembly and mechanical testing. A non-radiological test was performed using 35 L of cesium-spiked caustic waste simulant followed by a test with 39 L of actual tank waste. Test results are discussed, particularly those related to the effectiveness of extraction.

Removal of Sludge Heels in Savannah River Site Waste Tanks with Oxalic Acid

The Savannah River Site (SRS) is preparing two tanks for closure. The first step in preparing the tank for closure is mechanical sludge removal. In mechanical sludge removal, a liquid such as inhibited water or salt solution is added to the tank, the liquid is mixed with the sludge to form a slurry, and the slurry is transported from the tank. Mechanical cleaning removes a large fraction of the sludge in the tank, but it leaves a sludge heel of several thousand gallons. SRS employs chemical cleaning to remove this sludge heel. In chemical cleaning, oxalic acid is added to the tank to dissolve the sludge, and the liquid, containing the dissolved sludge, is transported from the tank. The authors conducted demonstrations of the chemical cleaning process with simulated SRS waste and actual SRS waste to assess the effectiveness of oxalic acid in dissolving SRS sludge. Following these demonstrations, SRS conducted chemical cleaning in two waste tanks (referred to as Tank A and Tank B). During chemical cleaning, the authors analyzed samples to assess the effectiveness of the chemical cleaning in removing the sludge heel. The conclusions from this work follow. With the exception of iron, the dissolution of sludge components from Tank A agreed with results from the actual waste demonstration performed in 2007. The fraction of iron removed from Tank A by chemical cleaning was significantly less than the fraction removed in the SRNL demonstrations. The likely cause of this difference is the high pH following the first oxalic acid strike. The dissolution of sludge components from Tank B agreed with results from the actual waste demonstration performed in 2007. The fraction of plutonium removed from Tank B by chemical cleaning was slightly higher than the fraction removed in the SRNL demonstrations. Most of the sludge mass remaining in the tank is iron and nickel. The remaining sludge contains significant amounts of barium, chromium, and mercury. Most of the radioactivity remaining in the residual material is beta emitters and 90Sr. The chemical cleaning removed a large fraction of the uranium, aluminum, calcium, sodium, strontium, and cesium. The chemical cleaning was not effective at removing nickel, mercury, plutonium, americium, and curium.

Solubility of Uranium and Plutonium in Alkaline Savannah River Site High Level Waste Solutions

Five actual Savannah River Site tank waste samples and three chemically-modified samples were tested to determine solubility limits for uranium and plutonium over a one year time period. Observed final uranium concentrations ranged from 7 mg U/L to 4.5 g U/L. Final plutonium concentrations ranged from 4 µg Pu/L to 12 mg Pu/L. Actinide carbonate complexation is believed to result in the dramatic solubility increases observed for one sample over long time periods. Clarkeite, NaUO2(O)OH · H2O, was found to be the dominant uranium solid phase in equilibrium with the waste supernate in most cases.

Comparisons of Crystalline Silicotitanate and Resorcinol Formaldehyde Media for Cesium Removal by In-tank Column Processing

Chemical and thermal performance of crystalline silicotitanate (CST) and resorcinol-formaldehyde (RF) ion exchange columns are modeled for configurations that fit inside Savannah River Site waste tanks. Good cesium removal is predicted for both media. More waste can be processed per cycle with CST. Unlike RF, CST is not elutable and requires lead column replacement after each cycle. RF is subject to chemical and radiolytic damage reducing capacity each cycle. Fast cesium loading kinetics for RF allows minimization of exposure. Modeling shows that the baseline cooling system provides adequate cooling to maintain columns within process limits and indicates that removal of the outer cooling jackets may be possible.

Analysis of Turbulent Mixing Jets in a Large Scale Tank

Flow evolution models were developed to evaluate the performance of the new advanced design mixer pump for sludge mixing and removal operations with high-velocity liquid jets in one of the large-scale Savannah River Site waste tanks, Tank 18. This paper describes the computational model, the flow measurements used to provide validation data in the region far from the jet nozzle, and the extension of the computational results to real tank conditions through the use of existing sludge suspension data. A computational fluid dynamics approach was used to simulate the sludge removal operations. The models employed a three-dimensional representation of the tank with a two-equation turbulence model. Both the computational approach and the models were validated with onsite test data reported here and literature data. The model was then extended to actual conditions in Tank 18 through a velocity criterion to predict the ability of the new pump design to suspend settled sludge. A qualitative comparison with sludge removal operations in Tank 18 showed a reasonably good comparison with final results subject to significant uncertainties in actual sludge properties.

Recent studies of uranium and plutonium chemistry in alkaline radioactive waste solutions

Solubility studies of uranium and plutonium in a caustic, radioactive Savannah River Site tank waste solution revealed the existence of uranium supersaturation in the as-received sample. Comparison of the results to predictions generated from previously published models for solubility in these waste types revealed that the U model poorly predicts solubility while Pu model predictions are quite consistent with experimental observations. Separate studies using simulated Savannah River Site evaporator feed solution revealed that the known formation of sodium aluminosilicate solids in waste evaporators can promote rapid precipitation of uranium from supersaturated solutions.

Copper‐Catalyzed Peroxide Oxidation Testing for Tetraphenylborate Decomposition

A new processing option, copper‐catalyzed hydrogen peroxide oxidation of tetraphenylborate under alkaline conditions, was demonstrated in laboratory testing. Laboratory‐scale tests were conducted to evaluate the use of copper‐catalyzed hydrogen peroxide oxidation to treat simulants of the Savannah River Site tank waste. The oxidation process involves the reaction of hydrogen peroxide with a copper catalyst to form hydroxyl free radicals. With an oxidation potential of 2.8 volts, the hydroxyl free radical is a very powerful oxidant, second only to fluorine, and will react with a wide range of organic molecules. The goal is to oxidize the tetraphenylborate completely to carbon dioxide, with minimal benzene generation. Testing was completed in a lab‐scale demonstration apparatus at the Savannah River National Laboratory. Greater than 99.8% tetraphenylborate destruction was achieved in less than three weeks. Offgas benzene analysis by a gas chromatograph demonstrated low benzene generation. Analysis of the resulting slurry demonstrated >82.3% organic carbon destruction. The only carbon compounds detected were formate, oxalate, benzene (vapor), carbonate, p‐terphenyl, quaterphenyl, phenol, and phenol 3‐dimethylamino.

Demonstration Of The Caustic-side Solvent Extraction Process For The Removal Of 137Cs From Savannah River Site High Level Waste#

This article 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 nonradioactive 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 L 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% of the target value of 15. #The submitted manuscript has been authored by a contractor of the U.S. Government under contract No. DE-AC09-96SR18500. Accordingly, the U.S. Government retains a non-exclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for U.S. Government purposes.

Experimental Verification of Caustic‐Side Solvent Extraction for Removal of Cesium from Tank Waste#

A caustic‐side solvent extraction (CSSX) process was developed to remove Cs from Savannah River Site (SRS) high‐level waste. The CSSX process was verified in a series of flowsheet tests at Argonne National Laboratory (ANL) in a minicontactor (2‐cm centrifugal contactor) using simulant. The CSSX solvent, which was developed at Oak Ridge National Laboratory (ORNL), consists of a calixarene‐crown ether as the extractant, an alkyl aryl polyether as the modifier, trioctylamine as the suppressant, and Isopar®L as the diluent. For Cs removal from the SRS tank waste, the key process goals are that: (1) Cs is removed from the waste with a decontamination factor greater than 40,000 and (2) the recovered Cs is concentrated by a factor of 15 in dilute nitric acid. In the flowsheet verification tests, the objectives were to: (1) prove that these process goals could be met; (2) demonstrate that they could be maintained over a period of several days as the CSSX solvent is recycled; and (3) verify that the process goals could still be met after the solvent composition was adjusted. The change in composition eliminated the possibility that the calixarene‐crown ether could precipitate from the solvent. The process goals were met for each of the verification tests. The results of these tests, which are summarized here, show that the CSSX process is a very effective way to remove Cs from caustic‐side waste. #The submitted manuscript has been created by the University of Chicago as Operator of Argonne National Laboratory (“Argonne”) under Contract No. W‐31‐109‐ENG‐38 with the U.S. Department of Energy. The U.S. Government retains for itself, and others acting on its behalf, a paid‐up, nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.

Proof-of-Concept Flowsheet Tests for Cesium Removal from Tank Waste by Caustic-Side Solvent Extraction

ABSTRACTA caustic-side solvent extraction (CSSX) process to remove cesium from Savannah River Site (SRS) high-level waste has been developed through a joint program with Oak Ridge National Laboratory (ORNL), the Savannah River Technical Center (SRTC), and Argonne National Laboratory (ANL). The CSSX solvent consists of four components: (1) an extractant, a calixarene crown, calix[4]arene-bis(tert-octylbenzo-crown-6) designated BOBCalixC6, (2) a modifier, an alkyl aryl polyether, 1-(2,2,3,3,-tetrafluoropropoxy)-3-(4-sec-butylphenoxy)-2-propanol, also called Cs-7SB, (3) a suppressant, an alkyl amine, trioctylamine (TOA), and (4) a diluent, Isopar®L. The solvent composition is 0.01 M BOBCalixC6, 0.50 M Cs-7SB, and 0.001 M TOA in Isopar®L. In this program we have developed and demonstrated a flowsheet that can be used to process SRS tank waste. To this end, a series of flowsheet tests were completed using simulated waste in a 2-cm centrifugal contactor at ANL. Three short-term (3-4 hours) tests were completed to demonstrate various aspects of the flowsheet. These tests were followed by a 71-h test where the solvent was recycled 42 times. In each case, we met or exceeded the key process goals: (1) cesium removal from the waste with a decontamination factor greater than 40,000, (2) concentration of cesium in the aqueous strip effluent by a factor of 15 using dilute nitric acid, and (3) stripping the solvent sufficiently to allow it to be recycled many times. The results from the 71-h test are discussed.

DEVELOPMENT OF A SOLVENT EXTRACTION PROCESS FOR CESIUM REMOVAL FROM SRS TANK WASTE

An alkaline-side solvent extraction process was developed for cesium removal from Savannah River Site (SRS) tank waste. The process was invented at Oak Ridge National Laboratory and developed and tested at Argonne National Laboratory using singlestage and multistage tests in a laboratory-scale centrifugal contactor. The dispersion number, hydraulic performance, stage efficiency, and general operability of the process flowsheet were determined. Based on these tests, further solvent development work was done. The final solvent formulation appears to be an excellent candidate for removing cesium from SRS tank waste.