Development Of Molten Salt Reactor Research Articles

Corrosion Behavior of Binary Ni-Cr Alloys in Molten FLiNaK Salts at Different Electrochemical Potentials

The development of molten salt reactors (MSRs) is a collaborative effort among academia, government laboratories, and industry, with the aim of realizing the potential of clean nuclear energy. However, the corrosion of structural alloys presents a significant engineering challenge in this application. Addressing this issue requires investment not only in testing and characterizing candidate alloys, but also in developing a scientific framework to understand the fundamental mechanisms governing the corrosion of metallic materials in molten fluorides which can then guide alloy design [1].In this work, the corrosion behavior of binary Ni-Cr alloys (5-20 wt.% Cr) in molten LiF-NaF-KF (or FLiNaK) salts between 550oC and 750oC was interrogated using electrochemical methods coupled with thermo-kinetic analysis. Electrochemical techniques, including linear sweep voltammetry, electrochemical impedance spectroscopy (EIS) and chronoamperometry, were utilized to elucidate the rate-limiting kinetic factors and to identify the dependence of corrosion morphology on applied potential. Additionally, Ni-Cr alloys were also immersed 99.9% pure FLiNaK salts with a known oxidizer concentration (e.g., 0.1-5 wt.% EuF3) coupled with open-circuit potential measurement to provide a time-based description of morphological evolution in the case of a dynamically changing electrode potential. These results are corroborated by examination of the post-corrosion morphology using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD) techniques as well as salt composition analysis using inductively coupled plasma mass spectrometry (ICP-MS), X-ray diffraction (XRD) and in situ cyclic voltammetry.The systematic choice of potential revealed multiple potential regimes in which Ni-Cr alloys exhibit distinctly different corrosion morphologies. At intermediate potentials, a porous bicontinuous structure is formed by dealloying of the less noble element (Cr) and percolation into the Ni-rich alloy within grains and along grain boundaries. At other potentials, charge transfer control dissolution of both Cr and Ni leads to the formation of a crystallographic faceted structure. The morphological differences suggest the presence of multiple corrosion processes in Ni-Cr in FLiNaK controlled by either transport or interfacial reaction. These findings can be explained through electrochemical processes at the alloy-salt interface, as well as the roles of surface, grain boundary and bulk diffusion kinetics involving the alloying constituents. Furthermore, the effect of radiation damage on Ni20Cr alloys on these rate-limiting corrosion regimes will be investigated. Acknowledgment Research is primarily supported as part of the fundamental understanding of transport under reactor extremes (FUTURE), an Energy Frontier Research Center (EFRC) funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES).[1] Chan, H.L., et al. Npj Materials Degradation 6(1), 46 (2022).

Novel Calibration Approach for Monitoring Aerosol Hydrogen Isotopes Using Laser-Induced Breakdown Spectroscopy for Molten Salt Reactor Off-Gas Streams.

Online monitoring is a key challenge for the continued development of molten salt reactor (MSR) technology. Laser-induced breakdown spectroscopy (LIBS) has previously been demonstrated to be a viable tool for monitoring aerosolized species and noble gases in real time, but the ability to discern varying isotopes in these streams has not yet been investigated for MSR applications. Tritium will form in MSRs from ternary fission and from (n,α)-reactions occurring in lithium-containing salts. This study compares three spectrometers of varying resolutions and types for measuring hydrogen isotope shifts in LIBS spectra of wetted filter paper. For each spectrometer, multivariate models were built (i.e., principal component regression, partial least squares regression, and multivariate curve resolution) to quantify the isotope ratio. The top models were then modified and corrected to apply the models to aerosol samples with varying isotope ratios. This novel calibration strategy offers an 82% reduction in volume of the calibration samples needed and is a more viable pathway for calibrating deployable LIBS systems. Lastly, this calibration model was compared with an all-aerosol trained model for monitoring hydrogen isotopes during a real-time test where the protium/deuterium ratio, along with representative salt species (i.e., lithium, sodium, and potassium) were adjusted dynamically. Results of this test validated the predictive capabilities of the transferred model and highlighted the capabilities of LIBS for real-time monitoring of MSR effluent streams.

Thermodynamic measurements and assessments for LiCl-NaCl-KCl-UCl3 systems

Thermodynamic descriptions of higher-order molten salt systems are essential for the effective application of molten salt technologies. However, for pyroprocessing and molten salt reactor development, a complete description of the key Li-Na-K-U(III) chloride system has yet to be provided. To remedy this, we applied the CALculation of PHAse Diagrams (CALPHAD) approach to all available phase equilibria and related thermodynamic measurements that include new differential scanning calorimetry (DSC) observations for pseudo-binary and pseudo-ternary systems to obtain consistent and accurate sets of thermodynamic properties. An outcome of that effort was the apparent discovery of a previously unreported intermediate compound in the NaCl-UCl3 system. In this effort, we adopted a unique approach to the quantification of uncertainty in phase equilibria obtained from DSC measurements, which can provide distinct advantages in phase diagram analysis. The methodology indicates that reported values of uncertainty are often underestimated. The assessed systems of the current effort were incorporated in the publicly available Molten Salt Thermal Properties Database - Thermochemical.

Technical Note: Corrosion Behavior of 316 Stainless Steel With and Without an Ni Coating in Solid-State Fluoride Salt

With the rapid development of molten salt reactors, the problem of post-treatment of spent fuel salt discharged from the core becomes increasingly prominent. Like molten fluoride salts, solid fluoride salts are also highly corrosive, which poses a serious challenge to container materials. This paper describes the corrosion behavior of 316 stainless steel (316 SS) with and without Ni coating in low-temperature solid fluoride salt. Results show that 316 SS without Ni coating experiences pitting corrosion with the prolongation of corrosion time. 316 SS with Ni coating experiences intergranular corrosion. In addition, a loose hollow interlayer is formed between Ni coating and matrix. Corrosion of both samples increases with the extension of exposure time with solid fluoride salts.

Assessment of thermodynamic stability of sapphire in eutectic molten chloride environment

AbstractThe continued development of molten salt reactors and concentrated solar power plants requires highly efficient and stable instruments that can efficiently monitor the chemical conditions of the molten salt during long‐term operation in both the fuel and coolant/heat transfer fluid loops. Sapphire (Al2O3) fibers have shown tremendous potential due to inherent radiation resistance and a broader operational range of temperature. In this work, computational thermodynamic modeling (CALPHAD) using the ThermoCalc software in conjunction with the SGTE (Scientific Group Thermodata Europe) Molten Salts (SALT1) and Pure Substances (Pure5) databases is applied to understand the compatibility of Al2O3 fibers with NaCl‐MgCl2 eutectic molten salt in the temperature range of 1500–2500 K. The thermodynamic calculations show that sapphire fibers are not expected to be stable over the long‐term when exposed to molten chloride salts at these temperatures. To improve the stability of these diagnostic fibers in molten salt environments, various pure metallic elements were evaluated as potential cladding materials for Al2O3 fibers. Based on the thermodynamic analysis, molybdenum (Mo) and nickel (Ni) could be effective cladding materials to enhance the stability of Al2O3 in NaCl‐MgCl2 chloride salt molten bath in the desired temperature range. Thus, the presence of Mo and Ni cladding can provide a protective coating against the corrosive molten salts, thus improving the stability of Al2O3. Additionally, it is also shown that Al2O3 remains stable up to 2400 K in the presence of preexisting Al2MgO4 and Al2NiO4 in the eutectic molten chloride bath environment.

Reactor cell neutron dose for the molten salt breeder reactor conceptual design

The private sector’s recent interest in the active development of molten salt reactors has led to the need to develop and test advanced modeling and simulation tools to analyze various advanced reactor types under numerous conditions. This paper discusses the effort undertaken to model the Oak Ridge National Laboratory (ORNL) Molten Salt Breeder Reactor (MSBR) design using ORNL’s Shift Monte Carlo code. The MSBR model integrates a Monte Carlo N-Particle (MCNP) MSBR core model with an MCNP model that was generated from a CAD model of the external components and the reactor building, which was subsequently run in Shift. This paper focuses on development of the fully integrated model and its use in performing neutron transport calculations in the reactor cell area. This model is intended to aid in understanding radiological dose conditions during operation, as well as the iron dpa rates in the reactor vessel. The neutron biological dose rates and flux calculated in the reactor cell are much higher in the MSBR than in typical light-water reactors. The implications of these results and future work are also discussed in this paper.

Verification of multiphysics coupling techniques for modeling of molten salt reactors

Crucial to the development of molten salt reactor (MSR) designs is the application of multiphysics codes to model the tightly coupled neutronics and thermal-hydraulics behaviour of the liquid fuel. However, the verification and validation of such codes is not a trivial task, in particular for fast reactor designs, where no experimental data are available. In absence of experimental data, a benchmark was developed by LPSC/CNRS-Grenoble for multiphysics codes dedicated to MSR studies. In this study we present two independent multiphysics approaches and apply them to this benchmark. The first approach utilizes the Serpent2 multiphysics interface, allowing for high fidelity coupling of the finite volume computational fluid dynamics code OpenFOAM and Serpent2. In this approach, Serpent2 serves as the neutronics solver and is coupled to an OpenFOAM based thermal-hydraulics solver and supplemented by a delayed neutron precursors transport solver implemented in OpenFOAM. The main advantage of this coupling approach is that it allows for using a high accuracy Monte-Carlo approach to solve the neutron transport equations. The second approach is a novel approach that utilizes the SEALION framework. The SEALION code employs a specialized thermal hydraulics solver based on OpenFOAM, coupled with a custom-made modified point kinetics neutronics solver, that explicitly accounts for the altered neutron importance due to the transport of delayed neutron precursors. The main advantage of this approach is that it allows for a pre-determination of the temperature feedback effects using Monte-Carlo codes, such as Serpent2. Both approaches are verified against results from the benchmark and the overall agreement between the results demonstrates the validity of both approaches.

REACTOR PHYSICS EXPERIMENT IN A GRAPHITE-MODERATION SYSTEM FOR HTGR

The Japan Atomic Energy Agency (JAEA) started the Research and Development (R&D) to improve nuclear prediction techniques for High Temperature Gas-cooled Reactors (HTGRs). The objectives are to introduce a generalized bias factor method to avoid full mock-up experiment for the first commercial HTGR and to introduce reactor noise analysis to High Temperature Engineering Test Reactor (HTTR) experiment to observe sub-criticality. To achieve the objectives, the reactor core of graphite-moderation system named B7/4”G2/8”p8EUNU+3/8”p38EU(1) was newly composed in the B-rack of Kyoto University Critical Assembly (KUCA). The core is composed of the fuel assembly, driver fuel assembly, graphite reflector, and polyethylene reflector. The fuel assembly is composed of enriched uranium plate, natural uranium plate and graphite plates to realize the average fuel enrichment of HTTR and it’s spectrum. However, driver fuel assembly is necessary to achieve the criticality with the small-sized core. The core plays a role of the reference core of the bias factor method, and the reactor noise was measured to develop the noise analysis scheme. In this study, the overview of the criticality experiments is reported. The reactor configuration with graphite moderation system is rare case in the KUCA experiments, and this experiment is expected to contribute not only for an HTGR development but also for other types of a reactor in the graphite moderation system such as a molten salt reactor development.

Code development and analysis of heat pipe cooled passive residual heat removal system of Molten salt reactor

The high efficient and reliable Passive Residual Heat Removal System (PRHRS) is of great importance to the development of Molten Salt Reactor (MSR), whether the liquid fuel MSR or solid fuel MSR. In this research, based on the structure and operation performance of the new conceptual PRHRS, using high temperature heat pipe as heat transportation, a set of reasonable mathematical and physical models have been built, mainly including reactor core model, high temperature heat pipe model and PRHRS model. The new conceptual Passive Residual heat removal system Analysis Code (PRAC) has been developed using FORTRAN language. The verification and validation for PRAC is conducted by using existed Molten Salt Reactor Experiment (MSRE) data provided by Oak Ridge National Laboratory and the PRHRS experimental data in our previous work. The results show that the developed PRAC could predict the MSR transients reasonably and show high fidelity. The numerical results of reactor core neutronic and thermal–hydraulic model show a good agreement when compared with MSRE pump startup and shutdown transients. The numerical results by PRHRS model agree well with experimental data which the maximum error of the fluoride salt temperature and heat pipe wall temperature are within 1.2% and 2.8%. This research provides a valuable software basis for the design and analysis of PRHRS of MSR.

Kinetic study of the hydrogen charging reaction on the graphite e in aqueous solution and in room temperature ionic liquid (RTIL)

Electrochemical impedance spectroscopy and other electrochemical methods were used to study the kinetics of hydrogen entry into porous, isotropic graphite and a single crystal graphite surfaces upon cathodic hydrogen charging and to calculate the hydrogen adsorption efficiencies. This model was first applied to graphite in an aqueous, alkaline solution, then in a room-temperature ionic liquid (RTIL), and, ultimately will be applied to FliNaK molten salt systems in future work. The results demonstrate that our model accurately describes the entry of hydrogen into graphite in harsh environments, which are important for the development of molten salt reactors (MSRs).

Numerical study of tritium transport characteristics in Thorium Molten Salt Reactor with Solid Fuel (TMSR-SF)

Abstract Tritium control is one of the most key issues for the development of Molten Salt Reactor (MSR). The tritium transport characteristics in 2MWth Thorium Molten Salt Reactor-Solid Fuel (TMSR-SF) proposed by Shanghai INstitute of Applied Physics (SINAP) is studied in this paper. Three works are conducted: Firstly, an integrated numerical model is established, including tritium production, adsorption, diffusion, corrosion and permeation. Secondly, the Tritium trAnsPort chAracteristics analysiS code (TAPAS) for TMSR-SF is developed and benchmarked. The results prove the fidelity and accuracy of TAPAS. Finally, the distributions of the key variables including TF, T2 and Cr2+ in TMSR-SF system at both steady and transient state are obtained. In addition, some methods for tritium mitigation are evaluated by TAPAS. Numerical results show that tritium can be efficiently removed from the molten salt via graphite absorption. The amount of T2 at steady state is two orders of magnitude more than TF. Tritium permeation can be prevented by increasing 7Li enrichment and reducing redox potential. Tungsten or similar low-permeability material is a candidate for tritium barrier when covered on the surface of the heat exchanger wall. This work provides some valuable considerations for the tritium control in the MSR.

Electrochemical study of the corrosion of a Ni-based alloy GH3535 in molten (Li,Na,K)F at 700°C

The compatibility of structural materials with molten fluoride salts is an immediate challenge for the application and development of molten salt reactor (MSR) operating at high temperatures. In the present presentation, the corrosion behavior of a nickel-based alloy GH3535 has been examined in an eutectic (Li,Na,K)F melt at 700 degrees C under Ar and Ar-5%H2O, respectively, by some electrochemical techniques. Moreover, the effect of the additive CrF3 with concentrations of 0.09 and 0.18 M on the corrosion of the alloy in the melt has also been investigated in an attempt to understand the corrosion mechanism involving the multivalent transition metal Cr ions Cr2+/Cr3+. It is shown that the alloy is in active dissolution state at the corrosion potential and its corrosion occurs mainly through the preferential dissolution of Cr from the substrate, producing a Cr-depleted layer and some internal voids. The introduction of 5%H2O in Ar accelerates obviously the corrosion of the alloy, giving rise to the formation of a wider Cr-depleted layer where some Cr-rich internal oxides are formed over the internal voids. CrF3 itself acts as an oxidizer to accelerate the dissolution of Cr, with a more significant effectiveness observed for a higher CrF3 content. (C) 2015 Elsevier B.V. All rights reserved.

The effect of the microstructure on the corrosion behavior of N5 superalloy in a molten (Li,Na,K)F eutectic salt

The corrosion of structural materials in molten fluorides, influenced by the microstructure of the structural materials, is considered to be a great challenge for the development of molten salt reactors using fluorides as fuel carriers or coolants.

The Molten Salt Reactor Adventure

A personal history of the development of molten salt reactors in the United States is presented. The initial goal was an aircraft propulsion reactor, and a molten fluoride-fueled Aircraft Reactor E...