Materials and Chemistry for Molten Salt Systems: Poster Session
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
Monday 5:30 PM
February 24, 2020
Room: Sails Pavilion
Location: San Diego Convention Ctr
H-11 (Invited): The Chemistry of Graphite in FHRs and MSRs: Raluca Scarlat1; 1University of California, Berkeley
An overview of the chemical behavior of nuclear graphite upon exposure to molten fluoride salt is provided. Graphite fluorination, the effect of salt chemistry on surface tension and wetting, and the role of pore size distribution are presented. The role of chemical and physico-chemical interactions between graphite and salt on tritium uptake into graphite and tritium release from graphite are presented. A summary of differences in chemical behavior of nuclear graphite and graphite matrix is provided. Experimental methods for the study of salt-graphite and salt-graphite-tritium interactions are described and possible sources of error and variability in experimental data are discussed.
H-12: Ab-initio Thermodynamics of Molten Salts: Nicholas Winner1; Mark Asta1; 1University of California, Berkeley
Corrosion of structural metals by molten salts remains one of the principle challenges to the development of the Molten Salt Reactor (MSR). Studies of the processes by which corrosion occurs in molten salts is essential for the feasibility of these reactors; however, characterization of the corrosion process is hindered by the extreme environment, which makes in-situ monitoring of the exact mechanisms extremely difficult. Computational techniques are a promising pathway to help address these issues; however, higher length-scale models applicable to engineering systems must first be parameterized and validated by fundamental studies of the thermodynamics and kinetics of these systems. In this work, we present ab-initio thermodynamic calculations of model molten salts relevant to MSRs with applications including the study of phase stability at the metal-salt interface. Such atomistic studies are undertaken to further understanding of corrosion mechanisms, necessary for engineering corrosion resistant structural alloys.
H-13: Characterization of LiCl-KCl Eutectic and LiCl-KCl+H2O Mixtures Using Electrochemical Impedance Spectroscopy: Litun Swain1; Gurudas Pakhui1; Suddhasattwa Ghosh1; Bandi Prabhakara Reddy1; 1Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu
Molten salt electrorefining of alloy fuels, which is one of the batch operations in pyroprocessing, is carried out in LiCl-KCl eutectic melt, and its purity is of paramount importance as presence of any redox impurities would lead to deterioration of its long-term performance. Accidental ingress of moisture in eutectic mixtures is not ruled out in facilities where engineering scale demonstration of pyroprocessing technology is carried out requiring handling of large amounts of eutectic mixtures. Application of electrochemical techniques to investigate the purity of melts is an old subject, and several studies are reported in literature. In the present work, application of electrochemical impedance spectroscopy is explored to investigate redox mechanisms in binary eutectic melts with varying degrees of moisture content. Based on equivalent circuit fitting of complex impedance data, mechanism of reduction of moisture in eutectic melts is proposed, and formation of other secondary impurities is explained.
H-14: Effect of Mn and Zn Inhibitors on the Corrosion of Incoloy 800H in the MgCl2-KCl Molten Salt: Yuxiang Peng1; Ramana Reddy1; 1University of Alabama
The corrosion evaluations of Incoloy 800H (800H) alloys with Mn and Zn inhibitors separately were performed in MgCl2-KCl for 100 hours at 800oC under argon atmosphere. The corrosion rates were estimated based on the weight loss of the 800H samples, which were 0.003±0.038mg/cm^2/day with Mn and 0.152±0.022 mg/cm2/day with Zn. The negative value of the corrosion rate indicates the increase in weight of the 800H sample after corrosion. The increased weight was attributed to the deposition of inhibitor on the surface of 800H. Besides, the composition of deposition was investigated by using the scanning electron microscopy (SEM) equipped with energy dispersive spectrometer (EDS). In comparison, both inhibitors Mn and Zn can protect 800H alloy from further corrosion, especially the Mn.
H-15: Electrochemical Impedance Spectroscopic Study of Oxide Scales on 316L Stainless Steel in Molten FLiNaK Salt: Jie Qiu1; Peter Hosemann1; Digby Macdonald1; John Scully1; 1University of California, Berkeley
316L stainless steel (SS) were pre-oxidized in hot air and then immersed in molten FLiNaK salt at 700 oC for electrochemical experiments. The impedance of oxide films on 316L SS as a function of exposure time were investigated using electrochemical impedance spectroscopy (EIS). The results show that the resistance of the oxide film decreases with increasing the exposure time, due to the dissolution of the Fe and Cr elements into the molten FLiNaK salt. After experiments, the structures of the oxide film were characterized and compared. The defect density within the oxide film as a function of exposure time was estimated by interpretation of the EIS spectrums in the framework of the Point Defect Model (PDM), which has been successfully applied for extraction kinetic properties of oxide film. The dissolution mechanism of the oxide film on 316L SS in molten fluoride salt were calculated and discussed.
H-16: Hot Corrosion Behavior of Ni Based Inconel 939 Superalloy in Molten Salt: Ali Hemmasian1; Congyuan Zeng1; Shengmin Guo1; 1Louisiana State University
In this study, the hot corrosion performance of Inconel 939 superalloy was investigated in the mixture of Na2SO4−NaCl at 700 and 800 °C for up to 100h. Results indicated severe hot corrosion attack with increasing the temperature. A duplex microstructure corrosion layer was identified including Cr-depletion zone close to the inner layer. A plausible hot corrosion mechanism as well as the main corrosion products have been proposed based on the results of corrosion tests, XRD and microstructure analysis. Although the corrosion rate at 800 °C is higher than 700 °C, the corrosion products are almost the same at both studied temperatures.
H-17: LIBS Investigation of Molten Salt Corrosion: William Ponder1; Kristian Myhre2; Stephen Raiman2; 1University of Tennessee Department of Nuclear Engineering; 2Oak Ridge National Laboratory
MSRs offer many advantages over LWRs. Corrosion is currently a lifetime-limiting challenge for MSRs. Understanding the mechanisms and processes behind the corrosion of structural materials would help to enable better preforming materials. To better understand the behavior of chloride salt systems, model Ni-Cr alloys were exposed to high temperature molten salt in static capsule tests. The chosen salt was a eutectic KCl-MgCl2. Metal alloys were exposed at 700-800 C for up to 2500 hours. De-alloying of Cr was observed as predicted by the thermodynamics of the system. Laser-Induced Breakdown Spectroscopy (LIBS) was used to study exposed samples. LIBS examination showed that during salt exposure salt constituents were deposited on the sample. This work will discuss the results of LIBS on molten chloride salt exposed alloys and develop the utility of LIBS for use in characterizing salt systems. Additionally, the corrosion behavior of the Ni-Cr alloy systems will be presented.
H-18: Maximization of Reduction Cell Efficiency for Spent Uranium Oxide Fuel: Jarom Chamberlain1; Michael Simpson1; 1University of Utah
In support of closing the nuclear fuel cycle, spent uranium oxide-based fuel undergoes a pyroprocessing operation to produce metallic fast reactor fuel. In this process UO2 is reduced either metallothermically after Li metal is generated or via direct electrolytic reduction. We report successful control over the mechanism by which the reduction proceeds. The mechanisms are controlled by configuring the electric lead in the cathode basket to contact either the UO2 or molten salt. To maximize current efficiency in the reduction cell, the reaction of O2 with reduced metals (Li or U) should be minimized along with elimination of hydroxides in the salt. An anode shroud has been tested to prevent oxygen from reoxidizing the reduced uranium metal or lithium. Salt purification has been developed to remove hydroxides that reduce current efficiency. We present cell efficiency data based on UO2 reduction measurements made using both thermogravimetric analysis and LECO oxygen analysis.
H-19: Molten Salt Electrolysis of Alkaline-earth Elements Using Liquid Metal and Alloy Electrodes: Thomas Nigl1; Timothy Lichtenstein1; Yuran Kong1; Hojong Kim1; 1Pennsylvania State University
Electrochemical separation of Sr2+ and Ba2+ ions from molten salt electrolytes (LiCl-KCl-SrCl2 and LiCl-KCl-BaCl2) into pure liquid metal electrodes (Bi, Sb, Sn, Pb) at 500–650°C was achieved due to strong chemical interactions between alkaline-earth metals and liquid metals. The liquid metal electrodes were cathodically discharged up to 200 C g-1 at a constant current density of 50 mA cm-2 in eutectic LiCl-KCl with the addition of either 5 mol% SrCl2 or BaCl2. Complex electrode reactions during separation resulted in co-deposition of Sr, Ba, and Li. To reduce the quantity of Li and increase the quantity of Sr/Ba removed from the electrolyte, Bi-Sb alloys were investigated as liquid metal alloy electrodes and cathodically discharged up to 200 C g-1 at 500°C.
H-20: Multimodal Characterization of the Morphological and Chemical Evolution of Ni and Ni-20Cr Microwires in Purified Molten KCl:MgCl2: Arthur Ronne1; Yi Xie2; Phillip Halstenberg3; Mingyuan Ge4; Xianghui Xiao4; Yachun Wang2; Wah-Keat Lee4; Lingfeng He2; Shannon Mahurin3; Yu-Chen Karen Chen-Wiegart5; 1Department of Materials Science and Chemical Engineering, Stony Brook University; 2Advanced Characterization Department, Idaho National Laboratory; 3Chemical Sciences Division, Oak Ridge National Laboratory; 4National Synchrotron Light Source - II, Brookhaven National Laboratory; 5National Synchrotron Light Source - II, Brookhaven National Laboratory and Department of Materials Science and Chemical Engineering, Stony Brook University
Molten salt reactors aim to provide more efficient, safer energy with less radiological waste. However, corrosion is a key challenge and the precise governing mechanism and kinetics are not fully understood. This study analyzes the fundamental mechanisms of corrosion for Ni and Ni-Cr alloys in purified KCl:MgCl2, focusing on the metal-salt interface. The multimodal approach of utilizing both X-ray nano-tomography and scanning transmission electron microscopy with energy dispersive X-ray spectroscopy in a complimentary fashion allows three-dimensional morphology and chemical composition to be thoroughly interrogated. The corroded Ni microwire forms bamboo-like structures, likely from corrosion along interfaces and grain boundaries. The Ni-Cr microwires show accelerated corrosion analogous to a dealloying mechanism. These results help to further knowledge of molten salt and alloy interactions. This work was supported as part of the Molten Salts in Extreme Environments Energy Frontier Research Center, funded by the U.S. Department of Energy Office of Science.