Thermodynamic characterization of salt components for Molten Salt Reactor fuel

The Molten Salt Reactor (MSR) is a promising future nuclear fission reactor technology with excellent performance in terms of safety and reliability, sustainability, proliferation resistance and economics. For the design and safety assessment of this concept, it is extremely important to have a thorough knowledge of the physico-chemical properties of molten fluorides salts, which are one of the best options for the reactor fuel. This dissertation presents the thermodynamic description of the most relevant systems for the Molten Salt Fast Reactor. A set of reference fuel options is provided and represents an important step forward in the advancing of the MSR technology.Radiation, Science and TechnologyApplied Science

Thermodynamic assessment of the niobium-fluorine system by coupling density functional theory and CALPHAD approach

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.RST/Reactor Physics and Nuclear Material

Thermodynamic assessment of the LiF–ThF4–PuF3–UF4 system

The LiF–ThF4–PuF3–UF4 system is the reference salt mixture considered for the Molten Salt Fast Reactor (MSFR) concept started with PuF3. In order to obtain the complete thermodynamic description of this quaternary system, two binary systems (ThF4–PuF3 and UF4–PuF3) and two ternary systems (LiF–ThF4–PuF3 and LiF–UF4–PuF3) have been assessed for the first time. The similarities between CeF3/PuF3 and ThF4/UF4 compounds have been taken into account for the presented optimization as well as in the experimental measurements performed, which have confirmed the temperatures predicted by the model. Moreover, the experimental results and the thermodynamic database developed have been used to identify potential compositions for the MSFR fuel and to evaluate the influence of partial substitution of ThF4 by UF4 in the salt.RST/Radiation, Science and TechnologyApplied Science

Excess heat capacity of the (Li1?xCax)F1+x liquid solution determined by differential scanning calorimetry and drop calorimetry

The work presents the measured heat capacity of the (Li1?xCax)F1+x liquid solution. Four samples with different compositions have been prepared and measured using a Differential Scanning Calorimeter. Since this technique was newly adopted for measuring encapsulated fluoride samples, some modifications were introduced in the standard configuration of the instrument and they are described in this work as well. For comparison one of the analysed composition (xCaF2xCaF2 = 0.5) was also measured using drop calorimetry, which has been previously used for similar studies. The reliability of the results obtained was confirmed by the good agreement between the two techniques. Moreover, the excess heat capacity of the (Li1?xCax)F1+x liquid solution was derived and a strong deviation from the ideal behaviour was observed.RST/Radiation, Science and TechnologyApplied Science

Thermodynamics of soluble fission products cesium and iodine in the Molten Salt Reactor

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.RST/Reactor Physics and Nuclear Material

Corrigendum to “Thermodynamics of soluble fission products cesium and iodine in the Molten Salt Reactor” [Journal of Nuclear Materials Volume 501 (2018) pages 238-252]

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]Corrigendum 10.1016./j.jnucmat.2018.01.024RST/Reactor Physics and Nuclear Material

A new approach for coupled modelling of the structural and thermo-physical properties of molten salts. Case of a polymeric liquid LiF-BeF₂

The (Li,Be)Fx 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)Fx 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.RST/Reactor Physics and Nuclear Material

Thermodynamic assessment of the LiF-NiF₂, NaF-NiF₂ and KF-NiF₂ systems

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 (Li1-2xNix)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.Accepted Author ManuscriptApplied SciencesRST/Reactor Physics and Nuclear Material

Thermodynamic assessment of the KF-ThF₄, LiF-KF-ThF₄ and NaF-KF-ThF₄ systems

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: Cp,mo(K2ThF6,cr,298.15K)=(193.2±3.9) J·K-1·mol-1 and Smo(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.RST/Reactor Physics and Nuclear Material

In situ high-temperature EXAFS measurements on radioactive and air-sensitive molten salt materials

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.RST/Reactor Physics and Nuclear MaterialsEMSD AS-south Project engineersRST/Technici PoolApplied SciencesRST/Neutron and Positron Methods in Material