The present study describes the thermodynamic assessment of three pseudo-binary systems relevant to CsI solubility in molten iodide salts: KI-CsI, NaI-CsI, and NaF-CsI. The motivation for this study was to corroborate a single previously reported data set of the NaI-CsI system, resolve inconsistencies reported by two different data-sets of the KI-CsI system, and generate new experimental data on the NaF-CsI system. Equilibrium data for all systems were obtained using Differential Scanning Calorimetry. Thermodynamic treatments of the three pseudo-binary systems were revised using the CALPHAD method with the thermodynamic software FactSage and Thermochimica. Both experimental and computational investigations provide increased confidence in the thermochemical behaviour of CsI in Molten Salt Reactor nuclear systems.

The heat capacity of technetium metal has been measured from 2.1 K to 293 K using relaxation calorimetry and the enthalpy increment up to 1700 K using drop calorimetry. The low-temperature calorimetry measurements revealed a superconducting transition temperature of T_C = (7.76 ± 0.08) K. The zero-degree Debye temperature(θ_E) and the electronic heat capacity coefficient (γ_e) of the normal state were derived as (307 ± 5) K and (4.22 ± 0.20) mJ·K⁻²·mol⁻¹, respectively. The standard entropy of the superconducting standard state was derived as S_m^° (298.15) = (36.8 ± 1.3) J·K⁻¹·mol⁻¹. The fitting of enthalpy-increment data together with high-temperature heat capacity data reported in literature yielded a heat capacity equation up to 1700 K.

This work examines the thermochemistry of the chromium difluoride CrF₂ corrosion product in the molten [Formula presented] fuel salt system. Through a combination of experimental investigations and thermodynamic modeling assessment, the study elucidates the thermodynamic properties, phase diagram equilibria, and overall thermodynamic behavior of CrF₂ corrosion product, following dissolution from a structural material to the molten salt fuel environment. In this work, two different synthesis methods were developed for pure CrF₂, further allowing to experimentally measure the phase equilibria in the [Formula presented], [Formula presented], and [Formula presented] systems. Then, thermodynamic models were developed using the CALPHAD method based on the quasichemical model in the quadruplet approximation.

Phase equilibria in the ThF₄-PuF₃ system were measured by differential scanning calorimetry. Samples were encapsulated to prevent leakage during the measurements and to prevent fluoride contamination of the instrument. Formation of intermediate compounds and solid solutions was investigated by X-ray diffraction. A thermodynamic model based on the CALPHAD approach was developed for the measured system. Good fit between measurements and phase equilibria calculated using the model was achieved.

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 structural, thermochemical, and thermophysical properties of the NaF-ThF4 fuel system were studied with experimental methods and molecular dynamics (MD) simulations. Equilibrium MD (EMD) simulations using the polarizable ion model were performed to calculate the density, molar volume, thermal expansion, mixing enthalpy, heat capacity, and distribution of [ThFn]m- complexes in the (Na,Th)Fx melt over the full concentration range at various temperatures. The phase equilibria in the 10-50 mol % ThF4 and 85-95 mol % ThF4 regions of the NaF-ThF4 phase diagram were measured using differential scanning calorimetry, as were the mixing enthalpies at 1266 K of (NaF/ThF4) = (0.8:0.2), (0.7:0.3) mixtures. Furthermore, the β-Na2ThF6 and NaTh2F9 compounds were synthesized and subsequently analyzed with the use of X-ray diffraction. The heat capacities of both compounds were measured in the temperature ranges (2-271 K) and (2-294 K), respectively, by thermal relaxation calorimetry. Finally, a CALPHAD model coupling the structural and thermodynamic data was developed using both EMD and experimental data as input and a quasichemical formalism in the quadruplet approximation. Here, 7- and 8-coordinated Th4+ cations were introduced on the cationic sublattice alongside a 13-coordinated dimeric species to reproduce the chemical speciation, as calculated by EMD simulations and to provide a physical description of the melt.

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]

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.

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 mixture LiF-ThF₄-UF₄-PuF₃ (77.5–6.6-12.3–3.6 mol%), a fuel option for the Molten Salt Fast Reactor (MSFR), has been measured by differential scanning calorimetry (DSC) for determination of phase transitions, and by Knudsen effusion mass spectrometry (KEMS) for measurement of partial vapour pressures. The boiling point of the mixture was determined by extrapolation of total vapour pressure to 1 bar. Thermodynamic calculations were performed and compared with the experimental results. Novel experimental data on pure plutonium trifluoride are presented: melting point, vaporization enthalpy, vapour pressure and ionization energies by electron impact.

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.

The low-temperature heat capacity of (U _1-y,Am_y)O _2−x solid solution with y = 0.0811 and 0.2005 and x = 0.01–0.03 was determined from a minimum of 12.52 K up to 297.1 K and from 9.77 K up to 302.3 K, respectively, using hybrid adiabatic relaxation calorimtry. The low temperature heat capacity results of the investigated system revealed the absence of the magnetic transition specific for UO₂ in the temperature region of 30 K. Since there are no experimental data available for AmO₂ in this temperature region, the results obtained for the intermediate compositions are validated based on the experimental data of UO₂ end-member and the low-temperature heat capacity computation of AmO₂. In the measured temperature interval, excess heat capacity was observed for the two investigated intermediate compositions, which is concluded to be dominated by self-radiation effects at very low temperature.

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.

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.

The enthalpy increment data for the (Th,U)O₂ and (U,Pu)O₂ solid solutions are reviewed and complemented with new experimental data (400–1773 K) and many-body potential model simulations. The results of the review show that from room temperature up to about 2000 K the enthalpy data are in agreement with the additivity rule (Neumann-Kopp) in the whole composition range. Above 2000 K the effect of Oxygen Frenkel Pair (OFP) formation leads to an excess enthalpy (heat capacity) that is modeled using the enthalpy and entropy of OFP formation from the end-members. A good agreement with existing experimental work is observed, and a reasonable agreement with the results of the many-body potential model, which indicate the presence of the diffuse Bredig (superionic) transition that is not found in the experimental enthalpy increment data.

A thermodynamic model for the Na-O system was developed for the first time using the CALPHAD method after review of the structural, thermodynamic, and phase diagram data available on this system. Differential Scanning Calorimetry measurements were moreover performed to assess the phase equilibria and liquidus temperature in the Na₂O-Na₂O₂ composition range. A CALPHAD model for the Na-U-O system was furthermore developed on the basis of both reviewed experimental data, and thermodynamic functions of the sodium uranates derived by combining ab initio calculations and a quasi-harmonic statistical model. The phase equilibria in this ternary system are particularly relevant for the safety assessment of the nuclear fuel-sodium coolant interaction in Sodium-cooled Fast reactors (SFRs). The model predicts the stability of the ternary phase field UO₂-Na₃UO₄-Na₄UO₅, which is consistent with the most recent literature data. Further optimization was moreover performed to fit the sodium partial pressures measured experimentally in the NaUO₃-Na₂U₂O₇-UO₂ and NaUO₃-Na₂UO₄-Na₂U₂O₇ phase fields, yielding an overall consistent description. Finally, the oxygen content required to form pentavalent Na₃UO₄ and hexavalent Na₄UO₅ in liquid sodium at 900 K were calculated to be 0.7 and 1.5 wppm, respectively, which are levels typically encountered in SFRs.

In this study we present a comprehensive thermodynamic description of the binary CsF-ThF₄ system. The phase equilibria of several intermediate compositions in this system have been determined using the DSC technique combined with a post-analysis using powder X-ray diffraction. Considering all the novel experimental data, a thermodynamic model for the CsF-ThF₄ system has been developed for the first time using the Calphad approach. The present model reproduces very well the measurements performed. Knudsen effusion mass spectrometry (KEMS) has further been used to investigate the vapour pressure over the molten CsF-ThF₄ salt and to determine the thermodynamic activities of CsF and ThF₄ in the liquid solution. As part of this study, the vaporization of pure CsF was examined and the results were compared with the literature showing a good agreement. Next, the vapour pressure of CsF-ThF₄ in the liquid solution was investigated by measuring three samples with compositions X ThF₄ = (0.4, 0.6, 0.8) mol/mol. A strong negative deviation from Raoult's law was observed for both species, more evident in case of CsF, and a good agreement with the predictions of our thermodynamic model was found. This article is a pre-requisite for the assessment of the ternary LiF-CsF-ThF₄ system.