Computational Fluid Dynamics Analysis of the Molten Salt Tritium Transport Experiment Test Section

Several radioactive iodine isotopes are formed as fission products in the nuclear fuel, and retained within the fuel matrix by the fuel cladding as a containment. During reprocessing of the used fuels by the pyroprocessing route, a significant fraction of iodine was reported to be retained in the molten salt as iodide (I-). High concentration of iodine leads to significant deterioration of the Pt anode during electrolytic reduction of the oxide spent fuels. [[i]] The electrochemistry of iodine/iodide couple has been extensively studied in aqueous and non-aqueous conditions because of its application in dye-sensitized solar cells, and synthetic chemistry. The physical and electrical properties of the molten salt systems depend on the structure and interactions of the constituents. The structural arrangements that are present in molten salts can be viewed as intermediates between discrete chemical bonds and periodic crystalline lattices[[ii]]. The inter-atomic interactions determine the local ordering of the molten salt. The electrochemical properties are significantly influenced by the structural characteristics of the molten salt. The ion size and type in a molten salt system such as KX-LiX would affect the electrochemical stability. The ordering and structure of the melt could be described based on the entropy. The entropy (ΔS) of the electrolysis of melt can be determined using the relation: ΔS = nF(∂E/∂T)P (1) Where, n = number of electrons, (∂E/∂T)P = change in electrochemical window with temperature at constant pressure. Low entropy values indicate higher order and enhanced attractive interaction of the species in the molten salt. Generally, an increase in the entropy was observed with increase in the anion size of unary molten salts[[iii]]. The electrolysis potential turns out to be more anodic as the anion size increases due to weaker Coulombic interaction between the anion-cation pairs as well as increased repulsion between the larger anions[[iv]]. When two types of cations are present in molten salt with different sizes and different charge densities, an asymmetric polarization of anions is anticipated which may result in the electrostatic stability of the mixture. When two different types of anions such as chloride and iodide are present along with different cations, the asymmetric polarizations of the ions and their effect on the physical and electrochemical properties are not well documented. The Chemla effect[v] refers to a phenomenon where the internal mobility of a larger cation is higher than that of a smaller cation at high concentrations of the larger cation in a binary salt mixture such as LiCl-KCl that consists of large and small cations with a common anion. This effect could lead to compositional gradient of ions in an electrochemical cell where a local shift in the composition from liquidus range may cause solid precipitation of salt[[vi]]. Chemla effect has been reported for ternary cation systems such as (Na,K,Cs)Cl [[vii]] and also for anions in Li(Cl,NO3)[[viii]]. Presence of iodide at above a threshold concentration could result in anionic Chemla effect where the internal mobility of iodide could be higher than that of chloride. The other possible scenario could be at low concentration of chloride, the mobility of chloride could be larger than that of iodide, as observed in the Li(Cl,NO3). However, in normal pyroprocessing condition, the chloride concentration will not be lower than that of iodide. The redox reactions of iodide are given as: 2I- ↔ I2 + 2e- (2) 3I- ↔ I3 - + 2e- (3) With the chloride addition, the redox reactions are given as: I- + 2Cl- ↔ [ICl2]- + 2e- (4) I3 - + 6Cl- ↔ 3[ICl2]- + 4e- (5) This presentation will give an overview of the electrochemistry of iodide in the LiCl-KCl molten salt system that is relevant to the pyroprocessing of used nuclear fuels based on the existing database from the published literature, and new preliminary results obtained at the University of Idaho in collaboration with the Idaho National Laboratory. [i] S. M. Frank, P.K. Tripathy, S.D. Herrman, Global 2013, Salt Lake City, Sep.29 – Oct. 3, 2013 [ii] J.D. Martin, S.J. Goettler, N. Fosse, L. Iton, Nature, 419 (2002) 381 [iii] M. Chemla, I. Okada, Electrochimica Acta 35 (1990) 1761 [iv] I.K. Delimarskii, B.F. Markov, Electrochemistry of Fused Salts, The Sigma Press Publishing, Washington, DC, 1961 [v] J. Pdrie' and M. Chemla, C. R. Acad. Sci. 250, 3986 (1960). [vi] R. Takagi, H. Shimotake, K. J. Jensen, J. Electrochem. Soc., 131 (1984) 1280 [vii] M. Matsumiya, H. Matsuura, R.Takagi, Y. Okamoto, Journal of The Electrochemical Society, 147 (11) 4206-4211 (2000) [viii] A. Endoh, I.Okada, J. Electrochem. Soc., Vol. 137, No. 3, March 1990

Toward a high-fidelity tritium transport modeling for retention and permeation experiments

Molten salt for advanced energy applications: A review

The goals of the Laser Inertial Fusion Fission Energy (LIFE) is to use fusion neutrons to fission materials with no enrichment and minimum processing and have greatly reduced wastes that are not of interest to making weapons. Fusion yields expected to be achieved in NIF a few times per day are called for with a high reliable shot rate of about 15 per second. We have found that the version of LIFE using TRISO fuel discussed in other volumes of this series can be modified by replacing the molten-flibe-cooled TRISO fuel zone with a molten salt in which the same actinides present in the TRISO particles are dissolved in the molten salt. Molten salts have the advantage that they are not subject to radiation damage, and hence overcome the radiation damage effects that may limit the lifetime of solid fuels such as TRISO-containing pebbles. This molten salt is pumped through the LIFE blanket, out to a heat exchanger and back into the blanket. To mitigate corrosion, steel structures in contact with the molten salt would be plated with tungsten or nickel. The salt will be processed during operation to remove certain fission products (volatile and noble and semi-noble fission products), impurities and corrosion products. In this way neutron absorbers (fission products) are removed and neutronics performance of the molten salt is somewhat better than that of the TRISO fuel case owing to the reduced parasitic absorption. In addition, the production of Pu and rare-earth elements (REE) causes these elements to build up in the salt, and leads to a requirement for a process to remove the REE during operation to insure that the solubility of a mixed (Pu,REE)F3 solid solution is not exceeded anywhere in the molten salt system. Removal of the REE will further enhance the neutronics performance. With molten salt fuels, the plant would need to be safeguarded because materials of interest for weapons are produced and could potentially be removed.

The gasification of coal with CO 2 in the molten salt (K 2 CO 3 /Na 2 CO 3 ) (CG-CO 2 /molten salt system) has been studied at 1123K. The gasification rate of the cola into CO was enhanced in the molten salt by 3.3 times. compared with that without the molten salt. The reason for the enhancement was discussed. Main factor would be the enhancement of the coal charring owing to the efficient heat flow from the molten salt tot the coal through the solid/liquid interface between the coal surface and the molten salt. Another reason would be the catalytic effect of the alkaline cations (K + , Na + ) on the Boudouard reaction (C + CO 2 = 2CO). This coal gasification process (CG-CO 2 /molten salt) is an endothermic process (coal charring and Boudouard reactions), and can be applied for the solar thermochemical process to convert solar energy into chemical energy and store it. Since the CG-CO 2 /molten salt system can be operated at 1123K. which is fairly lower than that for the conventional coal gasification with H 2 O. We have proposed an solar hybrid fuel production system, where the solar energy and the coal energy can be hybridized using the CG-CO 2 /molten salt system.

The aim of this research is to investigate mass and heat transfer of anodic aluminum oxide in packed-bed thermocline vessel with molten salt system (5% Lithium, 20% sodium, 50% potassium and 25% calcium). According to the computational fluid dynamic simulation, the ceramic ball that is packed inside the vessel does not have a significant impact on the mass transfer of anodic aluminum oxide in the molten salt system. Heat storage performance testing was conducted in a thermocline vessel (packed-bed zone 0.7 m in length and 0.3 m diameter) and molten salt flow rate between the ranges of 0.5–0.7 m3/h. Two different molten salt systems were studied including a normal molten salt system and a molten salt system with 0.5 wt% anodic aluminum oxide. An increase in molten salt flow rate have a positive impact on heat transfer inside the vessel due to the increase in turbulence of the flow. A decrease in charging time from 3.75 h to 3.5 h was observed for molten salt with 0.5 wt% anodic aluminum oxide. A seven-cycle charge/discharge test revealed that addition of 0.5 wt% anodic aluminum oxide resulted in a smaller reduction in heat transfer efficiency and actual energy storage. Heat storage decreased from 20.42 to 19.54 MJ corresponding heat transfer efficiency of only 87 to 85% for molten salt system consisting of anodic aluminum oxide.

Molten Salt Reactors (MSRs) are the leading candidates out of the six-generation IV advanced nuclear power reactor designs chosen for deployment for nuclear energy in US. MSR technology is particularly attractive due to better passive safety, operation at atmospheric pressure, high thermal efficiency, lower spent fuel per unit energy and increased solubility of fission products in molten salts.To facilitate robust and economical design of such systems, accurate knowledge and fundamental understanding of the structure and speciation of salts and metals in and near molten salt environments is necessary. Speciation of solutes is important because it determines how chemical behavior in solution and complex speciation from fission and corrosion products impact kinetic, thermodynamic, and transport properties of the molten salt systems. Investigating effects of radiation on metallic species and radiation-induced nucleation and growth of metallic nanoparticles under ionizing radiation is crucial for predicting the stability and reactivity of molten salts.Molten Salts in Extreme Environments (MSEE) EFRC aims to investigate speciation of metals and radiation-induced reactions in molten salt systems by utilizing synchrotron based XAS methods. In our work, Extended X-ray Absorption Fine Structure (EXAFS) and X-Ray Absorption Near Edge structure (XANES) are used to investigate local coordination environment and chemical structure of metal species such as Nickel in Zinc Chloride based molten salt systems. In addition, the effect of metal concentration and temperature on changes in local and chemical structure of metal is studied. XAS studies are complemented by optical ultra-violet spectroscopy studies of molten salts, enabling a direct correspondence between UV-Vis peak shape and coordination geometry and number, determined by EXAFS. Such knowledge of speciation of metals and radiation-induced nanoparticles in molten salt environments will provide critical understanding needed to predict and control the physical and chemical properties of molten salts and corrosion mechanisms in molten salt systems. 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.

The rise of molten salt use in green energy applications has led to increased efforts to model and understand the underlying thermochemical properties of active species within molten salts. Molten salts can become increasingly complex during the operational lifetime through corrosion or generation of new species, resulting in changes to the thermophysical and thermodynamic properties. For example, next-generation molten salt nuclear reactors start with a binary or ternary fuel salt, but generation of fission products during operation can significantly change the behavior of active species in the salt and interactions with container materials. While processes in established molten salt electrolytes such as LiCl-KCl eutectic can use well-characterized reference electrodes such as Ag/AgCl to quantify thermochemical properties1, emerging salt systems do not have well-established reference electrode systems. Using standard reference electrodes based upon anodic electrolyte decomposition (e.g., chlorine or fluorine gas-based electrodes) introduces additional engineering and safety challenges during construction and measurement of thermochemical properties. These experimental barriers have previously limited investigation of the thermodynamics of novel molten salt systems.Here, we use cathodic decomposition electrodes (CDEs) to measure the thermochemical properties of molten salt systems. The CDE defines 0 V as the cathodic decomposition of the pure solvent electrolyte at all temperatures. A CDE was used to make activity measurements in chloride and fluoride salts. Both steady-state emf measurements in concentration cells and transient measurements were made in an electrolyte containing active species such as GdCl3 or NiF2. The universal nature of a CDE reference can improve accuracy and repeatability of activity measurements in molten salt systems and fill the knowledge gaps that prevent prediction of thermochemical and thermophysical properties in molten salt systems. L. YANG and R. G. HUDSON, “Some Investigations of the Ag∕AgCl in LiCl-KCl Eutectic Reference Electrode,” J. Electrochem. Soc. 106 11, 986 (1959); https://doi.org/10.1149/1.2427195.

Electrochemical synthesis of ammonia in molten salts

In the development of concentrated solar power systems (CSPs), the high-temperature corrosion of alloys in receivers and heat exchangers can affect the performance on CSPs and influence the apparent durability. [1-4] The extent to which high-temperature corrosion is reversible may improve the commercialization. To understand corrosion mechanisms and their impact on alloys, experimental coupon (1.2mm×12mm×30mm) studies were conducted to the 700-1000°C within halide and fluoride salts in a thermosiphon. The thermosiphon reactor was designed to allow exposure the coupons to the both isothermal and non-isothermal condition. That is, the thermal gradients between top and bottom of the reactor could induce natural convection of the salts. The corrosion experiments such as coupon test and post mortem analysis highlighted main corrosion mechanisms of Cr depletion and diffusion along the grain boundaries. [5-6] A multi-physics corrosion model that include the main corrosion mechanism of Cr oxidation along with diffusion-limited corrosion as the limiting step allow to predict corrosion rates (i.e., current density) and corrosion potentials. Also, a CFD model has shown that temperatures and fluid flows can be accurately predicted in molten salt heat transfer systems including coupons. The CFD predictions allow to characterize natural convection flows in the reactor by analyzing engineering parameters such as Rayleigh number, Nutssel number, and Grashof number, etc. As the temperature and mass transports are coupled in the natural convection system, the analysis of these dimensionless numbers helps to correlate different physics to understand the system better and to reduce complication to solve the problem. These dimensionless numbers around coupons incorporated into the corrosion model allow to predict mass transfer coefficients and boundary layer thickness that determine corrosion rates (i.e., current density). The model predictions and experiment results suggest that the use of thicker materials to improve the lifetime may not be a good strategy in molten salt systems due to the localized nature of the corrosion. Acknowledgements The authors gratefully acknowledge the support for this work by the DOE EERE SunShot Initiative under a subcontract from SRNL to the University of Alabama and to the University of South Carolina. R eferences [1] C. Forsberg, Progress in Nuclear Energy, 47, 32-43, 2005.[2] C. Forsberg, P.F. Peterson, and H.H. Zhao, J. Sol. Energy Eng. Trans.-ASME, 129, 141-146, 2007.[3] D. Ludwig, L. Olson, K. Sridahran, M. Anderson, and T. Allen, Corrosion Engineering Science and Technology, 46, 360-364, 2011.[4] P. Sabharwall, Matt Ebner, Manohar Sohal, Phil Sharpe, Mark Anderson, Kumar Sridharan, James Ambrosek, Luke Olson, Paul Brooks, Molten Salts for High Temperature Reactors: University of Wisconsin Molten Salt Corrosion and Flow Loop Experiments – Issues Identified and Path Forward, 2010. [5] B. L. Garcia-Diaz, L. Olson, M. Martinez-Rodriguez, R. Fuentens, J. Gray, “Electrochemical study of corrosion in high temperature molten salts,” paper #741, 2014 ECS and SMEQ Joint International Meeting, Cancun, Mexico, Oct. 08, 2014. [6] R. Fuentens, L. Olson, M. Martinez-Rodriguez, J. Gray, B. L. Garcia-Diaz, “Corrosion of high temperature materials in fluoride and chloride molten salts,” paper #743, 2014 ECS and SMEQ Joint International Meeting, Cancun, Mexico, Oct. 08, 2014.

For a deuterium-tritium (D-T) fusion facility like Commonwealth Fusion Systems’ ARC or Xcimer Energy’s HYLIFE-III to operate at scale, considering that the current global tritium inventory is estimated at less than 27 kilograms1 compared to estimated start-up requirements alone (~10kg)1, it is clear that the viability of D-T fusion depends on the development of technologies that close the tritium fuel cycle. For instance, assuming cycle energies for the HYLIFE-III system, one day’s worth of 3 GJ fusion events at a 0.75Hz repetition rate2 would burn approximately 343.5 grams of tritium before including additional requirements to cover operational inefficiencies, radioactive decay (t1/2 = 12.32 years), or build a reserve inventory. Therefore, upcoming fusion facilities are planned to include "tritium breeding blankets", which generate the isotope through neutron capture reactions on lithium-6.The molten salt 2LiF-BeF2 (or FLiBe) is an attractive candidate for use as a liquid breeder. It combines a favorable neutron economy from the inclusion of beryllium as a neutron multiplier with compatibility as a heat transfer fluid to transport fusion energy to power generation at a relatively low corrosivity. However, a major research gap between today and the eventual scale-up of nuclear fusion facilities is the wide spread in the reported solubilities and diffusivities of tritium atoms (ions or otherwise) in FLiBe3, hindering the development of predictive engineering models for tritium retention, release, and accountancy.To address this, we will examine the behavior of hydrogen species in molten halides to build a mechanistic understanding of how a selected behavior depends on an equilibrium for or perturbation of a specific salt chemistry. We will review the body of literature examining hydrogen species (H-, Ho, and H+ of any isotope) behavior in the molten halides to assess how the cationic and anionic environments about hydrogen affect the solubility mechanisms and transport properties. Finally, we will extend this understanding to analyze the possible bond characteristics in Tritium-FLiBe systems incorporating the most recent electrochemical measurements of hydrogen speciation in FLiBe collected within the SALT group.We also reflect on strategies for communicating these complex chemical concepts to experts in other fields besides molten salts and ionic liquids. For example, words like species, behavior, or salt chemistry communicate a group of concepts to a reader which we cannot assume has the context to convert this terminology to actionable understandings to be implemented within system design of molten salt-facing technology. The behavior of hydrogen in molten FLiBe is not just a chemical curiosity; it directly impacts tritium safety, accountability, and supply. For these difficult to study systems (be it from the HF hazards, high temperatures, or strict beryllium safety protocols), it is important we make these chemical concepts accessible to the broader nuclear engineering community and extract as much value as possible from every measurement.To bridge the research and communication gaps, we propose a method to identify the excess chemical potential wells likely relevant to tritium-FLiBe chemistry like charge-dipole bonding between HF(d) and F-, BeF4 2-, or larger anions formed from tetrahedral crosslinking4 or covalent bonding character between elemental hydrogen and electropositive metals present in the melt. Using this method, we will define specific activity coefficients which relate input parameters (the exact LiF to BeF2 ratio, redox control additives, and tritium production rates) to the observed activities of T2 and TF in the melt from hypothetical tritium extraction rates for permeator or sparger systems. By demonstrating how the currently unpredictable data from tritium-FLiBe systems emerges from the chemistry at play in molten salts, we will build a case for molten salt chemical analysis as a core capability for fusion facilities through our exploration of charge-dipole and covalent bonding in our data and the body of literature on hydrogen-containing halide melts.(1) Abdou, M.; Riva, M.; Ying, A.; Day, C.; Loarte, A.; Baylor, L. R.; Humrickhouse, P.; Fuerst, T. F.; Cho, S. Physics and Technology Considerations for the Deuterium–Tritium Fuel Cycle and Conditions for Tritium Fuel Self Sufficiency. Nucl. Fusion 2020, 61 (1), 013001. https://doi.org/10.1088/1741-4326/abbf35.(2) Ogando, F.; Tobin, M. T.; Meier, W. R.; Farga-Niñoles, G.; Marian, J.; Reyes, S.; Sanz, J.; Galloway, C. Preliminary Nuclear Analysis of HYLIFE-III: A Thick-Liquid-Wall Chamber for Inertial Fusion Energy. Fusion Eng. Des. 2024, 202, 114333. https://doi.org/10.1016/j.fusengdes.2024.114333.(3) Humrickhouse, P. W.; Fuerst, T. F. Tritium Transport Phenomena in Molten-Salt Reactors; INL/EXT-20-59927-Rev000; Idaho National Lab. (INL), Idaho Falls, ID (United States), 2020. https://www.osti.gov/biblio/1777267 (accessed 2025-02-05).(4) Baes, C. F. A Polymer Model for BeF2 and SiO2 Melts. J. Solid State Chem. 1970, 1 (2), 159–169. https://doi.org/10.1016/0022-4596(70)90008-3.

There has been a renewed interest over the past decade in the use of molten salts as coolants and fuels in next-generation nuclear reactors that are used for electricity generation and to supply high-temperature, low-pressure heat. For solid-fuel salt-cooled reactors, molten salts are used as the coolant for solid-fueled reactors that operate at high temperatures (>500 °C) while for liquid salt fueled reactors, fissile, fertile, and fission products are dissolved in a homogeneous molten salt which serves as both the fuel matrix and the primary coolant. Two key challenges with a liquid salt fueled reactor is to manage the solubility and oxidation state of the materials dissolved in the molten salt and to manage the interfacial interactions of the molten salt with the reactor materials to limit corrosion. To understand how the structure and dynamics of molten salts impact their physical and chemical properties, such as viscosity, solubility, and thermal conductivity, as well as chemical reactivity, it is necessary to determine the structure and speciation of the molten salt at the atomic/molecular scale. The Molten Salts in Extreme Environments (MSEE) Energy Frontier Research Center is addressing these challenges through a coordinated experimental and theoretical effort to elucidate the atomic and molecular basis of molten salt behavior, including interactions with solutes and interfaces and under coupled extremes of temperature and radiation. The structure of bulk salt mixtures, such as MgCl2/KCl, and solute speciation in molten salts has been studied to better understand behavior in these complex environments using combined X-ray scattering and spectroscopy, neutron scattering, optical spectroscopy and computational modeling. The chemical and morphological evolution of metal/molten salt interfaces has been examined using X-ray tomography and electron microscopy to better understand corrosion processes in molten salt systems. This presentation will discuss the challenges in molten salt chemistry for nuclear energy applications and highlight recent results in understanding the structure, properties and reactivity of high temperature molten salts.

Practical information is provided on electrochemical measurements in molten salt systems. The emphasis is on chloride and fluoride systems, but the principles are applicable to any high-temperature molten salt or molten oxide electrolyte system. Considerations are given to topics such as the functionality of electrochemical measurement equipment, reference electrodes, materials selection and chemical compatibility, interpretation of electrochemical measurement signals, molten salt properties, and laboratory practices.

Next-generation solar power conversion systems in concentrating solar power (CSP) applications require high-temperature advanced fluids in the range of 600° to 900°C. Molten salts are good candidates for CSP applications, but they are generally very corrosive to common alloys used in vessels, heat exchangers, and piping at these elevated temperatures. The majority of the molten-salt corrosion evaluations for sulfates with chlorides and some vanadium compounds have been performed for waste incinerators, gas turbine engines, and electric power generation (steam-generating equipment) applications for different materials and molten-salt systems. The majority of the molten-salt corrosion kinetic models under isothermal and thermal cyclic conditions have been established using the weight-loss method and metallographic cross-section analyses. Electrochemical techniques for molten salts have not been employed for CSP applications in the past. Recently, these techniques have been used for a better understanding of the fundamentals behind the hot corrosion mechanisms for thin-film molten salts in gas turbine engines and electric power generation. The chemical (or electrochemical) reactions and transport modes are complex for hot corrosion in systems involving multi-component alloys and salts; but some insight can be gained through thermochemical models to identify major reactions. Electrochemical evaluations were performed on 310SS and In800H in the molten eutectic NaCl-LiCl at 650°C using an open current potential followed by a potentiodynamic polarization sweep. Corrosion rates were determined using Tafel slopes and the Faraday law. The corrosion current density and the corrosion potentials using Pt wire as the reference electrode are reported.

Cuprous oxide-loaded AlPO4-5 for highly efficient iodide ions adsorption in chloride molten salt