The development, operation, and regulation of nuclear reactors that utilize molten salts as fuel or as heat transfer media require knowledge of the thermal properties of the salt systems and quantification of the corresponding uncertainties. Knowledge of molten salt properties is also necessary for applications in material synthesis, processing, separations, solar thermal power generation, and energy storage. A round robin was conducted with national laboratory and university participants from twenty-one laboratories in five countries to compare property measurements, to better understand uncertainties, and to identify possible best practices. Two salt mixtures, each from a common batch, were distributed to participants for evaluation: equimolar NaCl-KCl and 45.0LiF-13.7NaF-41.3KF mol % (FLiNaK). Measurements were performed to determine the major constituent composition, oxygen content, density, thermal expansivity, melting point, and thermal conductivity. Error analysis was performed on each measurement for uncertainty quantification for each type of property that was explored. The resulting discussion of the methodologies used in this work is meant to lay the groundwork for the development of standard methods and reference materials for future high-temperature property measurements on halide melts.

Solid-state NMR (SSNMR) spectroscopy is a powerful technique for studying actinide chemistry but has been significantly limited due to the complex paramagnetism, and radiological hazards presented by these materials. Lanthanide and actinide salts often feature magnetic ordering and can be paramagnetic, ferromagnetic, or antiferromagnetic depending on temperature and electronic structure. Paramagnetic interactions can manifest in SSNMR both as secular spectral shifts and/or couplings as well as contributions from non-secular relaxation. Both effects can be directly measured with NMR and used to extrapolate rich chemical information such as coordination environments, bonding characteristics, local molecular dynamics, and correlation times. Typically, these studies are carried out on high-γ and highly abundant NMR-active isotopes (e.g., 1H, 6/7Li, 19F, 23Na, etc.) or on enriched rare isotopes (e.g., 2H and 17O), which can be expensive. Herein, we present a facile methodology to measure the 35/37Cl electric-field gradient (EFG) and paramagnetic shift anisotropy (SA) tensor components using static wideline SSNMR measurements of LaCl3, NdCl3, UCl3, and UCl4. The static powder spectra were measured with both 35Cl and 37Cl SSNMR to increase the fidelity of the extracted tensor parameters. Variable temperature NMR of a select case confirms the Curie-Weiss paramagnetism. Relaxation measurements of both nuclei further corroborate observations owing to the paramagnetic relaxation enhancement and reveal simultaneous quadrupolar relaxation mechanisms. Density functional theory (DFT) calculations using Hubbard U corrections to the uranium valence orbitals show excellent agreement with experimental EFG tensor parameters and help describe the bonding characteristics in these lanthanide and actinide systems.

The extreme conditions of Molten Salt Reactors (MSRs) make even fundamental chemistry challenging for simple salts. However, an operating MSR comprises fission products, corrosion products, and actinides at various concentrations that have yet to be fully described in the literature. The primary objective of this work is to study complex molten salt systems while expanding the evidence for the impacts of solvent salt cation radii on coordination and oxidation state stabilization of species relevant to MSR fuel systems in a range of eutectic solvent salts with varying hardness. Cyclic voltammetry was performed in four solvent salt eutectics (LiCl-KCl, MgCl 2 -NaCl, CaCl 2 -MgCl 2 , and UCl 3 -NaCl) using lanthanide benchmarks as surrogates to inform future Pu experiments. An essential element in ensuring consistency and precision during such measurements is a novel, custom boron-doped diamond electrode for electrochemistry extreme environments. Diamond is known for low background capacitance, rapid electron transfer kinetics, wide potential windows, and resilience in chloride and fluoride molten salts. The present study used BDD to describe Eu 3+/2+ and Ce 4+/3+ via formal potentials, electron transfer kinetics, diffusion, and thermodynamics (ΔG o , ΔH o , and ΔS o ) in the various solvent systems. The success of these measurements informs fundamental chemistry by expanding our understanding of ion interactions in molten salts while characterizing systems applicable to an operating MSR. Additionally, it proves that BDD is an ideal electrochemical sensor for continued measurements in chloride and fluoride molten salts to support the safe, secure deployment of MSRs.

Despite more than 70 years of research on the thermodynamic properties of molten salts, there are still limited experimental data towards understanding the phase stability relations of molten mixtures containing PuCl3. While recent thermodynamic measurements of PuCl3-NaCl yielded some data, there have been significant differences in the reported values of heat capacity of PuCl3-NaCl in the liquid, or molten state. In this work, we conducted transpose temperature drop calorimetry of the PuCl3-NaCl eutectic using a commercial Tian-Calvet twin microcalorimeter and the Ni encapsulation technique developed by us previously. The transpose temperature drop enthalpy (ΔHttd) was measured to be 155.31 ± 8.03 kJ∙mol−1 at a temperature of 975.77 ± 0.08 K. To verify this value, a critical assessment of the literature was performed to determine that the enthalpy increment, ΔHT-298.15, of molten PuCl3-NaCl eutectic is 101.5 ± 8.1 kJ∙mol−1, which agrees well with the heat capacity (Cp) derived by Karlsson et al. [26] using differential scanning calorimetry. The original Cp equation of Karlsson et al. was then extended with confidence to 993 K (Cp = 101.9 ± 2.1 J∙mol−1∙K−1). In addition, the excess heat capacity (Cpex) of molten PuCl3-NaCl eutectic was determined to be 4.2 ± 2.1 J∙mol−1∙K−1, which was then used to determine the ΔHmix to be –5.3 kJ∙mol−1. These results provide the basis for modeling the thermodynamic stability of molten PuCl3-bearing chlorides for nuclear energy and other applications.

The production of ceramics from uranium coordination compounds can be achieved through thermal processing if an excess amount of the desired atoms (i.e., C or N), or reactive gaseous products (e.g., methane or nitrogen oxide) is made available to the reactive uranium metal core via decomposition/fragmentation of the surrounding ligand groups. Here, computational thermodynamic approaches were utilized to identify the temperatures necessary to produce uranium metal from some starting compounds─UI4(TMEDA)2, UCl4(TMEDA)2, UCl3(pyridine)x, and UI3(pyridine)4. Experimentally, precursors were irradiated by a laser under various gaseous environments (argon, nitrogen, and methane) creating extreme reaction conditions (i.e., fast heating, high temperature profile >2000 °C, and rapid cooling). Despite the fast dynamics associated with laser irradiation, the central uranium atom reacted with the thermal decomposition products of the ligands yielding uranium ceramics. Residual gas analysis identified vaporized products from the laser irradiation, and the final ceramic products were characterized by powder X-ray diffraction. The composition of the uranium precursor as well as the gaseous environment had a direct impact on the production of the final phases.

Open circuit potential (OCP) measurements were made to determine uranium chloride (UClx) species activity in NaCl-MgCl2, which is a candidate salt for use in a molten salt reactor fuel. The operating temperature ranged from 500 to 650( degrees)C, and the measurement system used a U-Zr working electrode (WE) and Ag/AgCl reference electrode (RE). The activity was calculated to range from 2.52x10(-6) to 2.99x10(-8) (with uncertainties of +/- 1.0x10(-7) and +/- 1.1x10(-9), respectively), dependent upon the temperature and concentration of UCl3 (ranging from 0.25 to 11.55 wt.%). Fit of the OCP data to the Nernst equation resulted in values of 3.1 to 4.8 electrons transferred per U atom. This indicates the possibility for codeposition of Mg2+ and U3+ under certain conditions. Scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photo-electron spectroscopy analyses of cathodic deposits confirm the presence of metallic magnesium.

Renewed interest in advanced nuclear reactors, such as Molten Salt Reactors (MSRs), has spurred studies in actinide halide chemistry and property measurements. Several proposed research-scale and commercial MSR designs incorporate uranium trichloride (UCl3) fuel. There are relatively few preparations for isolated actinide halides reported in the literature, therefore novel methods for the isolation of pure material are desired. This communication describes the thermal elimination of pyridine (py) from the coordination complex UCl3py2 to yield gram-scale quantities of UCl3. The purity of the UCl3 product was determined through powder X-ray Diffractometry (pXRD) and Elemental Analysis (EA). This research article describes a facile synthesis of uranium trichloride (UCl3) from a pyridine precursor UCl3py2 (py=pyridine) improving accessibility to this compound. It is envisioned that this preparation will be adopted by many laboratories, enabling broader studies of its properties for molten salt technologies and other applications. image

Molten salt reactors (MSRs) are a promising alternative to conventional nuclear reactors as they may offer more efficient fuel utilization, lower waste generation, and improved safety. The state of knowledge of the properties of liquid salts is far from complete. In order to develop the MSR concept, it is essential to develop a fundamental understanding of the thermodynamic properties, including the heat capacities (Cp) and enthalpies of mixing (ΔHmix), of molten salts at MSR operating conditions. Historically, the Cp values of molten salts were determined by drop-calorimetry or differential scanning calorimetry, whereas their ΔHmix values were typically measured using specialized high temperature calorimeters. In this work, a new methodology for measuring both the Cp and the ΔHmix of molten chloride salts was developed. This novel method involves sealing a chloride salt sample in a nickel capsule and performing conventional transposed temperature drop calorimetry using a commercially available Setaram AlexSYS-800 Tian–Calvet twin microcalorimeter. This methodology may be applied to calorimetric measurements of more complex salt mixtures, especially mixtures containing actinides and fission products.

The electrochemical behavior of UCl3 was investigated in molten MgCl2-NaCl at 550°C. Using electromotive force measurements, we have measured the formal reduction potential (Eo’) at 550°C of U3+/U0 as +0.276 V vs. Mg/MgCl2 (-1.400 V vs. Ag/AgCl 5 mol%) on the molal scale. We also present the design and construction of a sealed Mg/MgCl2 reference electrode and have determined this system demonstrates ideal nonpolarizable electrode behavior, confirming it as an appropriate choice of reference electrode in molten chloride salts.

Uranium trichloride (UCl3) has received growing interest for its use in uranium-fueled molten salt reactors and in the pyrochemical processing of used fuel. In this paper, we report for the first time the experimentally determined Raman spectra of UCl3, at both ambient condition and in situ high temperatures up to 871 K. The frequencies of five of the Raman-active vibrational modes (vi) of UCl3 exhibit a negative temperature derivative ((∂νi/∂T)P) with increasing temperature. This red-shift behavior is likely due to the elongation of U-Cl bonds. The average isobaric mode Grüneisen parameter (γiP = 0.91 ± 0.02) of UCl3 was determined through use of the coefficient of thermal expansion published in Vogel et al. (2021) and the (∂νi/∂T)P values determined in this study. These results are in general agreement with those calculated here by density functional theory (DFT+U). Finally, a comparison of the ambient band positions of UCl3 to those of isostructural lanthanide (La-Eu) and actinide chlorides (Am-Cf) has been made.

electricity demand, lessen dependence on imported fuels, expand energy access, and improve stressed infrastructure for fuel supply and electricity transmission. Energy efficiency (EE) and renewable energy (RE) technical solutions described in this paper can bridge action across climate change mitigation and resilience through reducing GHG emissions and supporting electric power sector adaptation to increasing climate risk. Integrated planning approaches, also highlighted in this paper, play an integral role in bringing together mitigation and resilience action under broader frameworks. Through supporting EE and RE deployment and integrated planning approaches, unique to specific national and local circumstances, countries can design and implement policies, strategies, and sectoral plans that unite development priorities, climate change mitigation, and resilience.

Molten salt mixtures containing LiCl–KCl and NaCl–MgCl2 have been infused with UCl3 via reaction of U metal and FeCl2. The process starts with base salt (LiCl–KCl or NaCl–MgCl2) drying/purification using hydrochlorination via bubbling anhydrous HCl. An auto-titrator running in pH-stat mode was used to determine the point at which there is no net reaction with the salt. U metal is contained in a porous stainless steel basket as it is submerged in the molten salt. The byproduct Fe metal forms dendrites on the basket walls, allowing for simple separation from the molten salt. Based on analysis of salt samples using inductively coupled plasma mass spectroscopy, UCl3 yield of 90

Melt point, enthalpy of fusion, and volumetric expansion of single- and multi-component liquid chlorides {NaCl, KCl, LiCl, MgCl2, CaCl2, UCl3} were measured experimentally. These properties and materials are relevant in applications such as heat transfer, liquid nuclear fuel, and pyrochemical processing. A novel method for density measurement by neutron radiography was shown to produce high-quality data, consistent with reference literature where available, and allowed measurement of some materials for the first time. This method is especially useful in the characterization of sealed sample crucibles, given the ability of neutrons to penetrate the containment and surrounding furnace material. The results of this study are presented within the context of a comprehensive review of the available published data. The purpose of this review is to integrate measurements of the thermophysical properties of liquid chlorides into empirical descriptions of the relationships between composition, temperature, and thermophysical properties. A model for the prediction of the density of mixtures of liquid chlorides is proposed and demonstrated within a case study of the {NaCl + x mol% UCl3} system. A discussion of saturated vapor pressure as a function of temperature is presented as a supplemental interpretation of potential deviation from ideal mixture behavior, which remains an area active research. (C) 2021 The Authors. Published by Elsevier B.V.

This study is a short communication on progress made in the adaptation of conventional push-rod dilatometry for the measurement of volumetric expansion of liquid salts up to 1300 K. A new crucible design is offered as a practical solution for measurement of liquid salts by this method. Proof-of-concept measurements of the volumetric expansion of sodium chloride (NaCl) are compared to the available data in literature. Measurement of the coefficient of thermal expansion (CTE) of solid NaCl compared favorably to the available data in literature, confirming both magnitude and trend. The average CTE of NaCl (solid) was $$\overline{\alpha }_{{{\text{NaCl}}\;{\text{Solid}}}} = 5.63 \cdot 10^{ - 5} {\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 K}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{$K$}}$$ . The volumetric expansion of liquid NaCl was measured as $$\beta_{{{\text{NaCl}}\;{\text{Liquid}}}} \approx 3.96 \cdot 10^{ - 5} 1/K$$ and is essentially constant from the melt point to 1300 K. The volumetric expansion produced by this method is in reasonable agreement with the available published data and previous measurements made by the authors by neutron radiographic technique.

Uranium trichloride (UCl3) is actively researched to develop and improve applications ranging from molten salt reactors to actinide processing, including spent fuel reprocessing. Here, we report for the first time the crystal structure evolution between room temperature and melting point from in situ high-temperature neutron diffraction to quantify, for example, the thermal expansion of the hexagonal a and c lattice parameters. The results are compared with density functional theory calculations. The melting point of UCl3 is determined by differential scanning calorimetry to be 1108.2 ± 0.2 K.