High entropy alloys (HEAs) or complex concentrated alloys (CCAs) represent a new paradigm in structural alloy design. Molten salt corrosion behavior was studied for single-phase HEAs such as TaTiVWZr and HfTaTiVZr, and multi-phase HEAs such as AlCoCrFeNi2.1. De-alloying with porosity formation along the exposed surface and fluxing of unstable oxides were found to be primary corrosion mechanisms. Potentiodynamic polarization study was combined with systematic mass–loss study for TaTiVWZr, HfTaTiVZr, and AlCoCrFeNi2.1 as a function of temperature. Electrochemical impedance spectroscopy (EIS) was used for monitoring the corrosion of TaTiVWZr and HfTaTiVZr in molten fluoride salt at 650 oC. TaTiVWZr and AlCoCrFeNi2.1 showed low corrosion rate in the range of 5.5-7.5 mm/year and low mass-loss in the range of 35-40 mg/cm2 in molten chloride salt at 650 oC. Both TaTiVWZr and HfTaTiVZr showed similar mass loss in the range of 31-33 mg/cm2, which was slightly higher than IN 718 (~ 28 mg/cm2) in molten fluoride salt at 650 oC. Ta-W rich dendrite region in TaTiVWZr showed higher corrosion resistance against dissolution of alloying elements in the molten salt environment. AlCoCrFeNi2.1 showed higher resistance to galvanic corrosion compared to Duplex steel 2205 in molten chloride salt environment. These results suggest the potential use of HEAs/CCAs as structural materials in the molten salt environment for concentrating solar power and nuclear reactor systems.

In the near future, Gen II, III, and IV nuclear reactors will be in operation. UO2 is a common fuel for reactors in each of these generations and molten salts are used as coolant/fuel in Gen IV molten salt reactors. This thesis investigates potential ways to measure thermal conductivity for these materials: Raman thermometry for UO2 and a needle probe for molten salts. Four Raman thermometry techniques are investigated in this thesis: The Two Laser Raman (TLR), Time Differential Domain Raman (TDDR), Frequency Resolved Raman (FRR), and Frequency Domain Raman (FDR). The TLR is a steady state method used with a thin film. The TDDR and FRR are both time domain methods used with thin cantilever samples. The FDR is a frequency domain method used with a thermally thick sample. Monte Carlo like simulations are performed for each technique. In the simulations, the affect introduced uncertainty has on the measurement of thermal conductivity and thermal diffusivity is measured. From the results, it is recommended that the TLR should be used for measuring thermal conductivity and the FRR used for measuring thermal diffusivity. The TDDR and FDR were heavily affected by the uncertainty which resulted in inconsistent measured thermal properties. For measuring the thermal conductivity of molten salt, a needle probe was designed and manufactured to withstand the corrosive environment found in using molten salts. The probe uses modulated joule heating and measures the temperature rise in a thermocouple. The phase delay and temperature amplitude of the thermocouple are used in determining the thermal conductivity. A new thermal quadrupole based analytical solution, which takes into consideration convection and radiation, to the temperature rise of the probe is presented. The analytical solution is verified using a numerical solution found using COMSOL. Preliminary data was obtained with the probe in water.

京都大学 / 0048 / 新制・課程博士 / 博士(エネルギー科学) / 甲第21192号 / エネ博第366号 / 新制||エネ||72(附属図書館) / 京都大学大学院エネルギー科学研究科エネルギー基礎科学専攻 / (主査)教授 野平 俊之, 教授 萩原 理加, 教授 佐川 尚 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM

No description available.

Molten salt reactors (MSRs) using chloride-based salt-matrixes as coolants or fuels are a promising option for advanced nuclear reactors, but the extreme temperatures and corrosivity of molten salts pose a challenge for implementation. Molten MgCl2-NaCl-KCl is a viable candidate for MSRs that is considered in this work. Thermochemical properties are derived from electrochemical tests that aid in characterizing the properties of salts. To study these properties, some work has proposed using a three-electrode system with a reference electrode housed in a ceramic membrane. This research aims to develop a stable high-temperature reference electrode using a ceramic membrane that is then applied to develop an on-line monitoring system of Ni-Cr alloy corrosion in chloride salt. A mullite tube used as the membrane of a Ni(II)/Ni reference electrode in molten MgCl2-NaCl-KCl is studied. The performance of two different membrane thicknesses (1.325mm and 0.255mm) was studied in temperature ranges from 635oC to 835oC and data collected on the calculated formal potential of the Ni(II)/Ni system. Tests indicated that the results were stable and repeatable, and the formal potential for both systems differed from the previous experimental data by 0.12V at most, indicating that the system can be applied as an effective reference electrode. Using the reference electrode, on-line monitoring the corrosion of Ni-15wt.%Cr, Ni-20wt.%Cr, and Ni-30wt.%Cr was studied for 120 hours in MgCl2-NaCl-KCl. The on-line measurements showed the concentration changes of dissolved Cr and Ni by corrosion in the bulk molten salt. This work confirms that Ni(II)/Ni reference electrodes with a mullite tube membrane are stable and effective in molten chloride salt systems, particularly MgCl2-NaCl-KCl. The mullite membrane prepared by the manufacturer may be used directly for electrochemical applications without polishing, simplifying the reference electrode manufacturing process, and making it easier to replicate. The use of a Ni(II)/Ni reference electrode provides an avenue to study a different range of salt systems than previous reference electrodes allowed, particularly alloys in chloride salts at high temperatures. This work also confirms that the mullite tube may be used to perform on-line analysis of alloy corrosion in high temperature molten chloride salts. The study of Ni-Cr alloys in chloride salts better prepares the nuclear industry to select coolant salts and alloy containers with the best set of thermochemical and corrosion resistant characteristics for MSRs. / Master of Science / The United States receives approximately 18% of its energy from nuclear technology. Many of the reactors supplying this energy are at the end of their lifecycle and the decommissioning of some of these plants has already begun. In order to replace this older generation of nuclear reactors, a safer and cheaper option has been suggested: Molten Salt Reactors. Molten salt reactors (MSRs) using high temperature salts as a fuel or coolant are a promising option, but the extreme conditions of molten salts pose a challenge for construction and use of MSRs. Molten MgCl2-NaCl-KCl is a salt being considered for MSR application, and is considered in this work. Properties of the salts considered for MSRs are being studied diligently before implementation of these reactors. Electrochemical tests are used to study and monitor these properties. These electrochemical tests use a three-electrode system with a reference electrode housed in a membrane. In this work, a mullite tube is used as a ceramic membrane for a reference electrode in molten MgCl2-NaCl-KCl. The performance of two different membrane thicknesses (1.325mm and 0.255mm) was studied in temperature ranges from 635oC to 835oC. Results indicate that the system is an effective reference electrode. Using this innovative reference electrode, a method of monitoring on-line corrosion of Ni-15wt.%Cr, Ni-20wt.%Cr, and Ni-30wt.%Cr alloys was studied for 120-hour time periods during exposure to MgCl2-NaCl-KCl. This work confirms that reference electrodes with a mullite membrane may be used for electrochemical applications when studying molten chloride salts. The use of a Ni(II)/Ni reference electrode with a mullite membrane provides an avenue to study a different range of salt systems than previous reference electrodes and ceramics allowed, particularly chloride salts. Additionally, this mullite membrane Ni(II)/Ni reference electrode system may be used for monitoring on-line corrosion of Ni-Cr alloys in chloride salt systems.

No description available.

The present study attempts to find an alternate computational tool to model the complex physical interactions within the molten salt FLiNaK in a way that is both efficient and accurate. Additionally, this study seeks to describe the effects of several different types of impurities on the FLiNaK salt system. This study selects two different polarizable force fields, the AMOEBA polarizable approach and the polarizable ion model, to determine the density and the structure of the impure FLiNaK salt mixtures at typical operating temperatures in molten salt reactors (between 500-900 °C). This study conducts ab initio molecular dynamics (AIMD) simulations and classical molecular dynamics (CMD) for these salt mixtures to determine the correct parameter set for these two force fields. This study also uses an optimizer to minimize the difference between the forces calculated with AIMD and CMD simulation data. The AMOEBA polarizable approach is able to predict density for FLiNaK; however, it is unable to reliably predict other thermophysical properties due to the instability of its CMD simulations. The polarizable ion model is able to reliably determine density and salt structure for pure and impure FLiNaK mixtures. This model can be further used to determine other thermophysical properties. The polarizable ion model predicted densities for four impure salt mixtures: FLiNaK-MoF3, FLiNaK-UF3, FLiNaK-CsF, and FLiNaK-ZrF4. The predicted densities at 700 °C given in kg/m3 are 1929.94, 2454.15, 1650.67, and 1961.87, respectively with an error compared to the additive density model of -2.51%, -5.79%, -17.15%, and -1.67%, respectively. This study presents the radial distribution function and density correlation functions for each salt mixture. This study also presents a discussion of the shortcomings of the AMOEBA polarizable approach, as well as further work that may be done with these tools.

A novel concept has been developed for the direct carbon fuel cell (DCFC) based on molten carbonate recirculating electrolyte. In the cathode, co-current flow of electrolyte with entrained gases carbon dioxide and oxygen is sent in the upward direction through a porous bed grid. In the anode, co-current flow of a slurry of electrolyte entrained with carbon particles is sent in the downward direction through a porous bed grid. The gases carbon dioxide and oxygen in the cathode react on the grid surface to form carbonate ions. The carbonate ions are then transported via conduction to the anode for reaction with carbon to produce carbon dioxide for temperatures under 750 deg C. A mathematical model based on this novel DCFC concept has been developed. The model includes governing equations that describe the transport and electrochemical processes taking place in both the anode and cathode and a methodology for solving these equations. Literature correlations from multi-phase packed-bed chemical reactors were used to estimate phase hold-up and mass transfer coefficients. CO production and axial diffusion were neglected. The results demonstrated that activation and ohmic polarization were important to the cell output. The impact of concentration polarization to the cell output was comparatively small. The bed depths realized were of the order of 10cm which is not large enough to accommodate the economies of scale for a large scale plant, however thousands of smaller cells (10 m^2 area) in series could be built to scale up to a 10 MW industrial plant. Limiting current densities of the order of 1000-1500 A/m^2 were achieved for various operating conditions. Maximum power densities of 200-350 W/m^2 with current densities of 500-750 A/m^2, and cell voltages of 0.4-0.5 V have been achieved at a temperature of 700 deg C. Over temperatures ranging from 700 to 800 deg C, results from the modeled cell are comparable with results seen in the literature for direct carbon fuel cells that are similar in design and construction. / Ph. D.

Molten salts are high-temperature heat transfer fluids intended for cooling and/or storage purposes in a variety of energy applications. The current work seeks to ultimately study the thermophysical properties of fluoride and chloride salts, which are commonly considered for use in advanced nuclear reactors. Thermophysical properties like thermal conductivity are fundamental to ensuring safe, efficient, and competitive designs for advanced commercial nuclear reactors. Measurement challenges with liquid salts such as electrical conduction, corrosion, convection, and thermal radiation have hindered traditional approaches in their attempts to accurately quantify these properties at high temperatures. Here, a needle probe is developed, which modifies principles from existing instrumental techniques in order to experimentally measure the thermal conductivity of molten salts with reduced error. An analytical heat transfer model is developed to characterize 1D radial heat flow in a multilayered cylindrical system. This includes a thin layer of salt located between the needle probe and a crucible to limit natural convection. After being validated with finite-element methods, the needle probe is used to measure the thermal conductivity of several reference liquids, whose thermophysical properties are well-established at low temperatures. These seven samples are water, sodium nitrate (molten salt), potassium nitrate (molten salt), toluene, ethanol, propylene glycol, and galinstan. The needle probe was able to accurately measure thermal conductivity between 0.40-0.66W/mK for these samples with 3.5-10% uncertainty. Three eutectic halide molten salts (presented by molar composition) were selected for high-temperature testing. These include the ternary fluorides LiF(46.5%)-NaF(11.5%)-KF(42%) and NaF(34.5%)-KF(59%)-MgF2(6.5%), as well as the binary chloride NaCl(58.2%)-KCl(41.8%). Because testing temperatures range between 500-750C, the governing model is adapted to account for radiative heat transfer through the salt sample in parallel with conductive heat transfer. Improvements to the experimental apparatus are also made. For all three salts, the needle probe accurately measured thermal conductivity between 0.490-0.849W/mK with total uncertainty generally being less than 20%. A linear fit to the data demonstrates a clear negative relationship between thermal conductivity and an increase in temperature, which agrees with theoretical and computational predictions. These results indicate that the needle probe successfully handles the assortment of measurement challenges associated with high-temperature molten salts and provides reliable data to create correlations for thermophysical property databases.

This work 1) draws insights on molten salt structure and properties directly from ab initio molecular dynamics (AIMD) simulations, 2) demonstrates the advantageous symbiosis of computational and experimental collaborations on molten salt research, and 3) simultaneously generates ab initio data sets for fitting an interatomic potential model for classical molecular dynamics (MD) simulations. This work discusses the motivations for AIMD simulations of molten salts, thermophysical properties and structural characteristics of interest, advanced methodologies for AIMD simulations, and several completed AIMD studies on molten salts. Of import are the methodological contributions of this work to AIMD simulations, primarily the radical increase in generalized gradient planewave energy cutoff used to more accurately model the electron distribution across a highly-polarizable molten salt. Cutoffs of up to 2500 Rydbergs are used in this work, but 2000 Rydbergs is found to be sufficient for most AIMD NpT modelling of molten fluorides. The equilibrium liquid density of eutectic FLiNaK as a function of temperature is found to agree with the experimental density reported by Chrenkova et al. to within 0.2%, and the equilibrium liquid density of eutectic FMgNaK is found to agree with experimental measurements reported herein to within 4%. Self-diffusion coefficients in FMgNaK are also considered, with applicability to other halide salts. Molybdenum, Cesium, iodide, nickel, hydrogen, oxide, and uranium complexation are examined. It is found that solvation strength can be qualitatively determine via AIMD simulations, and that poorly solvated solutes will minimize the surface area of interaction with the salt solution. Cesium in particular is shown to be volatile or retainable in FLiBe at 500, 650, and 800 ËšC based on complexation and validated experimentally. It is shown that the chemical potential of an anion varies between melts as influenced by the different cations present in each melt. Hence, attempts to use a common electrochemical reference reaction for different salt mixtures are at best an approximation.