Materials and Chemistry for Molten Salt Systems: Electrochemistry
Sponsored by: TMS Structural Materials Division, TMS: Corrosion and Environmental Effects Committee
Program Organizers: Stephen Raiman, University Of Michigan; Jinsuo Zhang, Virginia Polytechnic Institute and State University; Michael Short, Massachusetts Institute of Technology; Kumar Sridharan, University of Wisconsin-Madison; Nathaniel Hoyt, Argonne National Laboratory
Thursday 2:00 PM
February 27, 2020
Room: Theater A-6
Location: San Diego Convention Ctr
Session Chair: Nathaniel Hoyt, Argonne National Laboratory
2:00 PM Invited
Application of Molten Salts in Nuclear Energy: Mark Williamson1; 1Argonne National Laboratory
Molten salts play a central role in advanced nuclear energy systems with applications in used nuclear fuel processing and Generation IV molten salt reactor systems. These systems benefit from molten salt properties such as high actinide and fission product solubility, a wide electrochemical potential window, excellent heat transfer characteristics, radiation stability, and criticality safety. Building from our legacy in pyrochemical technology, Argonne is developing and demonstrating next-generation used nuclear fuel processing technologies. Specifically, these technologies employ electrochemical processes to separate the actinides from the fission products in used nuclear fuel and yield a metallic actinide product that can be used in fueling fast spectrum Generation IV nuclear reactors. The presentation will provide an overview of a pyrochemical processing system based on molten salt electrochemical processes, advantages of the approach to fuel recycling, and status of technology development.
2:30 PM
X-ray Absorption Studies to Elucidate Structure and Speciation of Metals in Molten Salt Systems: Simerjeet Gill1; Jiahao Huang2; Julia Mausz3; Kotaro Sasaki1; Mehmet Topsakal1; Ruchi Gakhar4; William Phillips4; Lingfeng He4; Shannon Mahurin5; Phillips Halstenberg6; Lynne Ecker1; Anatoly Frenkel2; 1Brookhaven National Laboratory; 2Stony Brook University; 3University of Montpellier; 4Idaho National Laboratory; 5Oak Ridge National Laboratory; 6The University of Tennesse
To facilitate robust and economical design of Molten Salt Reactor systems, fundamental understanding of structure and speciation of salts and metals is necessary. We utilize X-ray absorption to investigate local coordination environment and bonding of metal species in molten salt systems. Using a custom-designed in situ cell, we study Ni and Co dopants in ZnCl2 and their properties using Ni, Co and Zn K-edge spectra. Differences in behavior of Ni and Co systems are noted, where local structure around Ni varies from phase-segregated NiCl2 regions to single phase, substitutional Ni in ZnCl2 as temperature is increased above 500ºC to 700 ºC, while Co remains in single phase at same temperature range. XAS studies are complemented by optical absorption spectroscopy and transmission electron microscopy studies. This work was supported as part of Molten Salts in Extreme Environments, Energy Frontier Research Center, funded by the U.S. Department of Energy Office of Science.
2:50 PM
Application of Voltammetry for Investigation of Materials Corrosion in LiF-NaF-KF (FLiNaK): William Doniger1; Cody Falconer1; Evan Buxton1; Mohamed Elbakshwan1; Adrien Couet1; Kumar Sridharan1; 1University of Wisconsin, Madison
Corrosion of structural materials in molten salts is an important challenge to the deployment of molten salt reactors (MSRs). In-situ sensors for salt chemistry and materials degradation are key technologies needed to mitigate corrosion over the life of a MSR. Electrochemical sensors for investigating molten salt corrosion will be discussed including voltammetry and potentiodynamic polarization. Cyclic voltammetry has been used to probe for dissolved metal fluoride impurities in molten fluoride salts pre and post corrosion to track the dissolution of individual alloying elements of structural materials. Potentiodynamic polarization is a rapid way to predict and compare materials’ tendencies to be corroded in molten salts. These electrochemistry techniques have been used in concert with Inductively coupled plasma-mass spectroscopy (ICP-MS) and materials characterization to understand corrosion of candidate structural alloys, such as 316 stainless steel, Ni- and Mo-based alloys, in molten LiF-NaF-KF (46.5-11.5-42 mol %) salt at 700˚C.
3:10 PM Student
Redox Potential Measurement of Ni2+/Ni in MgCl2-KCl-NaCl Molten Salt using Chronopotentiometry Method: Mingyang Zhang1; Jianbang Ge1; Jinsuo Zhang1; 1Virginia Polytechnic Institute and State University
Redox potential, E^0' vs. Cl2/Cl-, and activity coefficient, γ, of Ni2+/Ni are being investigated for the first time in MgCl2-KCl-NaCl (53.9-24.3-21.8 wt%) molten salt. In this study, an innovative application of in-situ electrochemical method, Chronopotentiometry (CP), is used to measure the redox potential for threeNiCl2 concentrations, 1 wt%, 3 wt% and 5 wt% at 873K - 1073K. In this temperature interval, the formal potentials of different NiCl2 concentrations appear to be the same, which is described as E^0'=-1.35591+0.00057*T, where T is the temperature in K. The activity coefficients of NiCl2ranges from 3.423 – 1.370 and are calculated using the supercooled NiCl2 Gibbs energy of formation. These results can be used to evaluate the corrosion behavior for Ni based alloy in this ternary molten salt.
3:30 PM
Multifunctional Voltammetry Sensors for Long-duration Process Monitoring and Control of Molten Salt Equipment: Nathaniel Hoyt1; Jicheng Guo1; Elizabeth Stricker1; Mark Williamson1; 1Argonne National Laboratory
Process monitoring of molten salts used in nuclear and solar energy applications requires accurate quantitative measurements of a variety of salt properties. Researchers at Argonne National Laboratory have developed a multifunctional sensor capable of simultaneous measurements of several key properties including salt species concentrations (including metal ion corrosion products and salt impurities), salt redox potential, and salt level. The sensor consists of an array of electrodes immersed in the salt, upon which electroanalytical measurements may be performed. These sensors have been used in flow loops and large-scale vessels for durations lasting beyond a year, providing crucial corrosion and chemical processing information.
3:50 PM Break
4:10 PM Invited
Determining Redox Potentials of Liquid Metal Electrodes for Recovery of Fission Products from Molten Salts: Hojong Kim1; 1Pennsylvania State University
In electrorefining process of used nuclear fuel in molten salts, fission products such as alkaline-earth and rare-earth elements accumulate in the process salt (e.g., LiCl-KCl) since they are electrochemically more active than uranium. In order to recover these fission products from the process salt, liquid metals (e.g., Bi, Sb, and Sn) were investigated as separating media through electrodeposition. The feasibility of this approach was rationalized by determining thermodynamic properties of binary alloys (e.g., Sr-Bi and Nd-Bi) through electromotive force (emf) measurements. While emf measurement is a straightforward practice by measuring the cell potential between two electrodes in equilibrium, the high reactivity of these electrodes resulted in technical challenges in stable emf measurements (e.g., early failure of electrochemical cells). In this presentation, several key aspects for successful emf measurements of reactive alloys are presented, including the selection of electrolyte system (chlorides vs. fluorides) and the use of less-reactive reference electrodes.
4:30 PM Student
High-throughput Electrochemical Methods Development to Accelerate Molten Salt Corrosion Resistant Alloy Design: Bonita Goh1; William Doniger1; Phalgun Nelaturu1; Michael Moorehead1; Dimitris Papailiopoulos1; Dan Thoma1; Kumar Sridharan1; Adrien Couet1; 1University of Wisconsin, Madison
High-entropy alloys (HEAs) are of interest in the development of next-generation molten salt-based energy systems for their corrosion resistance and thermal stability. The vast compositional and phase space of HEAs represent an immense opportunity to develop new alloys, but also a significant challenge as current high-temperature electrochemical methods are mostly low throughput. We recently developed a high-throughput method to fabricate HEA compositional arrays using in-situ alloying of elemental powders in a LENS system [Nelaturu et. al., TMS 2020]. In this work we demonstrate a mini-electrode assembly designed to perform high-throughput cyclic voltammetry on a molten salt drop on each point of the compositional array. We also present developments of a data structure with optimized efficiency for storing the large amounts data. This data structure will then be used as input for a feature recognition and categorization machine learning algorithm to automate the high-throughput electrochemical analysis of novel alloys.
4:50 PM
Understanding the Effects of Operating Conditions on UO2 Electroreduction: Krista Hawthorne1; Augustus Merwin1; James Willit1; Mark Williamson1; 1Argonne National Laboratory
Reduction of uranium dioxide to uranium metal is a crucial step in the reprocessing of used oxide nuclear fuels. Typically, this process is carried out electrochemically in a molten LiCl-Li2O (1 wt %) salt at 650 oC with UO2 powder as the cathode and oxygen evolution occurring at a platinum anode. When using platinum anodes, the anode potential must be tightly controlled to ensure that the platinum does not undergo electrochemical dissolution. Alternatively, using graphite anodes produces CO and CO2 during operation in addition to O2; however, the introduction of carbon to the system may lead to carbon contamination in the product. We will present the measured effects of the cell operating conditions on the extent of UO2 reduction and product quality and discuss the roles of the anode material and surface area, applied current, and reduction time on the percent of UO2 converted to U metal.
5:10 PM Student
Cathodic Behavior of Moisture in LiCl-KCl Eutectic Melt: Applications to Pyroprocessing: Litun Swain1; Gurudas Pakhui1; Suddhasattwa Ghosh1; Bandi Prabhakara Reddy1; 1Indira Gandhi Centre for Atomic Research, Kalpakkam
In pyroprocessing technology, purity of LiCl-KCl eutectic melt with respect to moisture has to be ensured to avoid side reactions, corrosion of electrodes and low charge efficiency of electrodeposition process. Redox behavior of moisture in LiCl-KCl eutectic melt was investigated, and a two-point criterion was proposed to qualitatively assess purity of melts, which is based on Li+|Li onset potential and cathodic current density. This was subsequently applied to LiCl-KCl eutectic mixtures containing initially 2-8 wt.% moisture. This work gains importance in studying the influence of long-term storage on redox behavior of melts. It was observed that presence of moisture simultaneously led to formation of HCl, and both underwent a one-step one-electron reduction at similar potentials. Subsequent formation of hydroxide and oxide ions further deteriorated purity of the melt. Feasibility of preparing high purity eutectic melts from eutectic mixtures containing significant amounts of moisture was also explored in this work.
5:30 PM
Electrochemical Corrosion of Zircaloy-2 in Molten LiCl: William Ebert1; Evan Wu1; Vineeth Kumar Gattu1; James Willit1; 1Argonne National Laboratory
Electrochemical tests were conducted to assess the corrosion behavior of Zircaloy-2 in several molten LiCl-xLi2O salt mixtures at about 650 °C. Small pieces of Zircaloy-2 cladding secured by Ni leads were used as working electrodes in a standard three-electrode cell with a Ni counter electrode and Ni|NiO reference electrode. The currents were measured and electrochemical impedance spectroscopy analyses performed during 1-h to 7-d potentiostatic holds at voltages between -1.6 and -0.6 V vs Ni|NiO to assess the kinetics of Zircaloy corrosion, degradation and formation of ZrO2 oxide layers, reduction of Li2O, and the possible formation of secondary phases such as Li2ZrO3 in salts with different Li2O contents. Some of the corroded Zircaloy-2 specimens are being examined by using a scanning electron microscope to characterize the corroded surfaces and corrosion layers. The electrochemical results and analyses of corrosion products will be presented and discussed in terms of corrosion processes and kinetics.