In this study, new insights into the solid state chemistry and melting behaviour of the Image 1001 system are presented, building on results in the simulant system Image 1002. Our studies have revealed the solubility of U in the high-temperature β[jls-end-space/]-phase of BaCl₂ (i.e Image 1003 ) and an intermediate compound, Ba₃U₂Cl₁₂, which has led us to revisit or present for the first time the phase diagrams of this system accordingly. Furthermore, we present revised thermodynamic models for the systems Image 1004, Image 1005, Image 1006 and Image 1007 (AE = Sr, Ba) based on existing literature data. With the constructed multi-component database Image 1008, the effect of fission products on the melting behaviour of molten chloride salts containing uranium and thorium is investigated through higher order phase equilibria calculations.

Four fluoride fuel salt samples (78LiF-22ThF₄) in graphite crucibles were irradiated in the HFR Petten for a duration of 508 Full Power Days under the name SALIENT-01 (SALt Irradiation ExperimeNT). Goal of the experiment was to gain experience with the design of liquid salt experiments and the handling of the salts before and after irradiation. Specific research goals for SALIENT-01 are (i) to confirm claims of good fission product retention in the salt, (ii) to obtain size distributions for noble metal particles using Transmission Electron Microscopy and (iii) to assess possible interactions between fuel salt and fine-grained nuclear graphite, as well as possible uptake of fission products by the graphite. Here the design and irradiation history of the experiment are discussed together with plans for post-irradiation examinations. Limitations in representativeness of this experiment and capsule irradiations in general are discussed as well as follow-up actions to improve the quality of future irradiations.

The authors regret to inform that the thermodynamic data [Formula presented] of the pure compound CsI(l) was not reported correctly in Table 3. The corresponding enthalpy of formation should read: [Formula presented]

The (Li,Be)F_x fluoride salt is an ionic liquid with complex non-ideal thermodynamic behaviour due to the formation of short-range order. In this work, we explore the relationship between local structure, thermo-physical and thermodynamic properties in this system using a multidisciplinary approach that couples molecular dynamics simulations using the Polarizable Ion Model (PIM) and thermodynamic modelling assessment using the CALPHAD method. The density, thermal expansion, viscosity, thermal conductivity, molar and mixing enthalpies and heat capacity of the (Li,Be)F_x melt are extracted from the polarizable ionic interaction potentials and investigated across a wide range of compositions and temperatures. The agreement with the available experimental data is generally very good. The local structure is also examined in detail, in particular the transition between a molecular liquid with Li⁺, BeF₄ ²⁻ and F⁻ predominant species at low BeF₂ content, and a polymeric liquid at high BeF₂ content, with the formation of polymers (Be₂F₇ ³⁻, Be₃F₁₀ ⁴⁻, Be₄F₁₃ ⁵⁻, etc.), and finally of a three-dimensional network of corner-sharing tetrahedrally coordinated Be²⁺ cations for pure BeF₂. Based on the available experimental information and the output of the MD simulations, we moreover develop for the first time a coupled structural-thermodynamic model for the LiF-BeF₂ system based on the quasi-chemical formalism in the quadruplet approximation, that provides a physical description of the melt and reproduces (in addition to the thermodynamic data) the chemical speciation of beryllium polymeric species predicted from the simulations.

NRG together with JRC Karlsruhe has set out to perform a series of molten fuel salt irradiations in the High Flux Reactor (HFR) Petten, to build up irradiation experience with molten salt samples, the handling of irradiated salt and the treatment of salt waste produced by these irradiations. As a first step, an irradiation of small 78LiF-22ThF₄ fuel samples in graphite crucibles is being conducted under the name SALIENT-01 (SALt Irradiation ExperimeNT). The specific goals for SALIENT-01 are (i) to confirm claims of good fission product retention in the salt by post-irradiation Knudsen-cell effusion by fission product release measurements, in particular with respect to Cs and I which dominate the radioactive release during thermal transient and fuel failure in conventional reactor fuel, as for example occurred in the Fukushima accident, (ii) to obtain size distributions for noble metal particles using Transmission Electron Microscopy and (iii) to assess possible interactions between fuel salt and fine-grained nuclear graphite, as well as possible uptake of fission products by the graphite. The SALIENT-01 irradiation has started in August 2017 and is projected to finish in August 2019. In this contribution the design, irradiation history and status of the experiment are treated, as well as plans for post-irradiation examinations. The limitations of the experiment will also be discussed, as well as the follow-up actions taken to assess its representativeness and to improve the quality of future molten salt irradiations.

The optimized Gibbs energies for the second-nearest neighbours exchange reactions of the liquid solution were not reported correctly in Eqs. (17)–(19) in our recent work [1]. The corresponding equations should read: [Formula presented] [Formula presented] [Formula presented]

A thermodynamic assessment of the KF-ThF₄ binary system using the CALPHAD method is presented, where the liquid solution is described by the modified quasichemical formalism in the quadruplet approximation. The optimization of the phase diagram is based on experimental data reported in the literature and newly measured X-ray diffraction and differential scanning calorimetry data, which have allowed to solve discrepancies between past assessments. The low temperature heat capacity of α-K₂ThF₆ has also been measured using thermal relaxation calorimetry; from these data the heat capacity and standard entropy values have been derived at 298.15 K: C_p,m^o(K₂ThF₆,cr,298.15K)=(193.2±3.9) J·K^-1·mol^-1 and S_m^o(K₂ThF₆,cr,298.15K)=(256.9±4.8) J·K^-1·mol^-1. Taking existing assessments of the relevant binaries, the new optimization is extrapolated to the ternary systems LiF-KF-ThF₄ and NaF-KF-ThF₄ using an asymmetric Kohler/Toop formalism. The standard enthalpy of formation and standard entropy of KNaThF₆ are re-calculated from published e.m.f data, and included in the assessment of the ternary system. A calculated projection of the NaF-KF-ThF₄ system at 300 K and the optimized liquidus projections of both systems are compared to published phase equilibrium data at room temperature and along the LiF-LiThF₅ and NaF-KThF₅ pseudobinaries, with good agreement.

The isobaric heat capacity of solid and liquid thorium tetrafluoride was determined by two calorimetry methods: drop calorimetry to obtain the enthalpy increments in the range from 430 to 1600 K and DSC to obtain the high-temperature heat capacity of solid and liquid thorium tetrafluoride using the step method. Adjustments to the experimental setups were developed and implemented to increase accuracy and reduce uncertainty of the obtained data.

Using the modified quasi-chemical model in the quadruplet approximation, three new thermodynamic assessments of binary systems useful for the detailed operational design of the Molten Salt Reactor are presented: AF-NiF₂ (A = Li, Na, K). These systems are particularly relevant for the study of the molten salt-structural materials interaction, as the salt containment is made of a Ni-based alloy. Using powder X-ray Diffraction (XRD) and Differential Scanning Calorimetry (DSC), new experimental data were gathered for two of these systems, LiF-NiF₂ and KF-NiF₂, and compared to previous experimental assessments. Our data have confirmed the formation of a (Li_1-2xNi_x)F solid solution. The three thermodynamic models show a very good agreement with the experimental data. The melting point of NiF₂ was measured for the first time to be T = (1629 ± 5) K, and the thermal expansion coefficient for Li₂NiF₄ was found to be α=27.6·10^-6K^-1 in the temperature range T = (298–773) K.

The present study describes the full thermodynamic assessment of the Li,Cs,Th//F,I system. The existing database for the relevant fluoride salts considered as fuel for the Molten Salt Reactor (MSR) has been extended with two key fission products, cesium and iodine. A complete evaluation of all the common-ion binary and ternary sub-systems of the LiF-ThF₄-CsF-LiI-ThI₄-CsI system has been performed and the optimized parameters are presented in this work. New equilibrium data have been measured using Differential Scanning Calorimetry and were used to assess the reciprocal ternary systems and confirm the extrapolated phase diagrams. The developed database significantly contributes to the understanding of the behaviour of cesium and iodine in the MSR, which strongly depends on their concentration and chemical form. Cesium bonded with fluorine is well retained in the fuel mixture while in the form of CsI the solubility of these elements is very limited. Finally, the influence of CsI and CsF on the physico-chemical properties of the fuel mixture was calculated as function of composition.

The development at the Delft University of Technology (TU Delft, The Netherlands) of an experimental set-up dedicated to high-temperature in situ EXAFS measurements of radioactive, air-sensitive and corrosive fluoride salts is reported. A detailed description of the sample containment cell, of the furnace design, and of the measurement geometry allowing simultaneous transmission and fluorescence measurements is given herein. The performance of the equipment is tested with the room-temperature measurement of thorium tetrafluoride, and the Th—F and Th—Th bond distances obtained by fitting of the EXAFS data are compared with the ones extracted from a refinement of neutron diffraction data collected at the PEARL beamline at TU Delft. The adequacy of the sample confinement is checked with a mapping of the thorium concentration profile of molten salt material. Finally, a few selected salt mixtures (LiF:ThF₄) = (0.9:0.1), (0.75:0.25), (0.5:0.5) and (NaF:ThF₄) = (0.67:0.33), (0.5:0.5) are measured in the molten state. Qualitative trends along the series are discussed, and the experimental data for the (LiF:ThF₄) = (0.5:0.5) composition are compared with the EXAFS spectrum generated from molecular dynamics simulations.

The complete thermodynamic description of the niobium-fluorine system is presented for the first time in this work. It results from a critical evaluation of the available experimental data and new thermodynamic calculations. In total, three niobium fluoride solid phases (NbF₃, NbF₄ and NbF₅) and seven gaseous species (NbF, NbF₂, NbF₃, NbF₄, NbF₅, Nb₂F₁₀ and Nb₃F₁₅) have been considered during the assessment. Novel data for all the gaseous species were calculated combining density functional theory (DFT), for the prediction of the molecular parameters, and statistical mechanical calculations, for the determination of the thermal functions (i.e. standard entropy and heat capacity). The developed thermodynamic model was found to correctly reproduce all the available experimental data and was used to calculate the Nb-F phase diagram, which is presented in this work as well.

Because of its sensitivity to the atomic scale environment, solid-state NMR offers new perspectives in terms of structural characterization, especially when applied jointly with first-principles calculations. Particularly, challenging is the study of actinide-based materials because of the electronic complexity of the actinide cations and to the hazards due to their radioactivity. Consequently, very few studies have been published in this subfield. In the present paper, we report a joint experimental-theoretical analysis of thorium tetrafluoride, ThF₄, containing a closed-shell actinide (5f⁰) cation. Its crystalline structure has been revisited in the present work using powder neutron diffraction experiments. The ¹⁹F NMR parameters of the seven F crystallographic sites have been modeled using an empirical superposition model, periodic first-principles calculations, and a cluster-based all-electron approach. On the basis of the atomic position optimized structure, a complete and unambiguous assignment of the ¹⁹F NMR resonances to the F sites has been obtained.