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Cesium and iodine, which are formed during a fission process in a nuclear reactor, are considered as major fission products responsible for the environmental burden in case of a nuclear accident. From the safety point of view, it is thus important to understand their release mechanism when overheating of the reactor core occurs. This work presents an experimental investigation of the behaviour of caesium iodide and caesium fluoride in fluoride based molten salt reactor fuel during high temperature events. It has been demonstrated that CsF will be retained in the fuel salt and thus its volatility will be significantly reduced, while CsI will not dissolve in the fluoride-based fuel matrix and will thus remain more volatile. The influence of the presence of CsI and CsF on the melting behaviour of the fuel has been investigated using calorimetry, revealing their negligible effects.

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.

The Raman features of neptunium dioxide (NpO2) and uranium–neptunium mixed oxides were investigated. Raman spectra of 16O and 17O-NpO2 enriched sample were measured with excitation energy range of Ei = 1.91–2.54 eV. Raman spectra of both NpO2 samples and the uranium–neptunium rich mixed oxides show an asymmetry at low energies of the Γ25′ (T2g) mode. The asymmetry was evidenced to originate from an electronic band at 431 cm–1 which originates from crystal field splitting. This assignment is in good agreement with theoretical models for NpO2 with the hypothesis that the electron–phonon bound state is decoupled at high temperatures. The asymmetry of the Γ25′ mode can be considered as a fingerprint of the NpO2 lattice.

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.

A detailed physico-chemical investigation of molten system (LiF-NaF)eut-LaF3 up to 15 mol% of LaF3 has been done. The phase equilibria were measured by two independent methods (TA and DSC) and consequently Calphad modelling and calculation was performed in order to interpret obtained experimental data. Additional physico-chemical properties like density, electrical conductivity and surface tension were measured. It was found that all investigated properties reflect the ionic changes in the melt providing consistent data, i.e. the most significant changes in these properties are observed at ca. 10 mol% LaF3. The volume properties were calculated, as well, providing information about volume expansion. Also temperature anomalous behaviour was observed for the electrical conductivity and surface tension.

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 (xCaF2 = 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.

Knudsen effusion mass spectrometry (KEMS) has been used to investigate the vapour pressure over the molten LiF–ThF4 salt and determine the thermodynamic activity of LiF and ThF4 in the liquid solution. As part of the study, the vaporization of pure LiF and pure ThF4 was examined and the results were compared with the literature values finding a good agreement. Next, the vapour pressure of the LixTh1−xF4−3x liquid solution was investigated by measuring four samples having different compositions (XLiF ∼ 0.2, 0.4, 0.6, 0.8 mol%). In order to determine the thermodynamic activities, the vapour pressure of LiF and ThF4 species over the liquid solution, as calculated from our results, were compared with the vapour pressure over the pure LiF(l) and pure ThF4(l) systems. A strong deviation from the Raoult's law was observed, more evident in case of LiF species, in agreement with the predictions by our thermodynamic model.

The enthalpy increments of the (Th1-y, Puy)O2 solid solution with y= 0.03, 0.08, 0.30, 0.54 and 0.85 and the PuO2 and ThO2 end-members were measured using drop calorimetry in the temperature range 476-1790 K. The heat capacity was obtained by derivation of the obtained enthalpy data with respect to temperature. The presented results for PuO2 and ThO2 are compared with the available literature data whereas the results obtained for (Th1-y, Puy)O2 solid solutions were compared with the Neumann-Kopp's molar additivity rule to search for possible non-ideal behaviour. This paper presents the first heat capacity data obtained for this system., JRC.E.3-Materials research

The work herein presents the first high temperature Knudsen effusion mass spectrometric measurements carried out on the sodium neptunate α-Na2NpO4. The material’s vaporization behaviour was investigated under vacuum conditions, in a tungsten metal cell up to T = 2700 K, and in an alumina cell up to T = 1900 K. Gaseous and condensed phases are in equilibrium under Knudsen conditions. The equilibrium decomposition reaction of α-Na2NpO4 to neptunium oxide, sodium oxide, and oxygen was established in the temperature range 1030 K to 1206 K. The enthalpy of formation of α-Na2NpO4 at 298.15 K was furthermore estimated at (−1761.0 ± 7.7) kJ·mol−1 using a second law analysis in the interval 1030 K to 1115 K, in good agreement with the value reported by Goudiakas et al. [25] (−1763.9 ± 7) kJ·mol−1, who used solution calorimetry. In the interval 1115 K to 1206 K, where the vaporization of sodium oxide formed by decomposition of α-Na2NpO4 is considered, the analysis yielded at 298.15 K (−1761.9 ± 20.4) kJ·mol−1 and (−1758.8 ± 22.3) kJ·mol−1 taking into account two different sodium oxide sublimation mechanisms suggested by Hildenbrand and Lau [56] and Steinberg and Schofield [37] respectively. Finally, the detection of the masses 253 and 292 in the temperature range 1350 to 1620 K suggests a probable sublimation of the high temperature tetragonal phase of the Na2NpO4 compound in parallel with the decomposition. The method developed in the present paper holds promise for the thermodynamic investigation of other sodium neptunates and plutonates phases.

A thermodynamic investigation of the LiF–ThF 4 system is presented in this study. The enthalpy of mixing of the (Li,Th)F x liquid solution was measured for the first time using a method designed for conventional DSC technique. To verify the possibility to measure the mixing enthalpy with the used calorimeter, the known LiF–KF system was first investigated and compared to the literature data. After the successful test, this technique was applied to investigate the LiF–ThF 4 system and obtained novel results are presented in this study. Furthermore, using the DSC technique, new equilibrium data of the LiF–ThF 4 phase diagram were measured, confirming the stability of the LiThF 5 phase. The intermediate compound Li 3 ThF 7 was synthesized and its enthalpy of fusion was determined. Considering the new experimental data, the LiF–ThF 4 system was re-optimized using a quasi chemical model for the description of excess Gibbs parameters of the liquid solution.

Basic thermodynamic and electrochemical data of pure actinide fluorides and their mixtures are required for the design and safety assessment of any presently studied molten salt reactor concept based on molten fluoride salt fuel. Since the actinide fluorides are usually not produced commercially, they have to be prepared from the available input materials, typically oxides. In this work, a specially designed facility for synthesis of pure actinide fluorides using pure HF gas is described, as well as a complete procedure of synthesis and characterisation of pure UF 4 and ThF 4 . The fluorination installation consists of a glove box kept under a purified argon atmosphere, a high temperature horizontal fluorination reactor and a HF supply gas line connected to the glove box. The fluorides were synthesised from high specific surface oxides prepared from the respective oxalates by low temperature calcination. The fluorination was partly stationary and partly in a HF gas flow, based on a heterogeneous powder-gas reaction at high temperatures. The products were characterised by X-ray diffraction and differential scanning calorimetry, which confirmed high purity products obtained by this method. Moreover, the melting point of UF 4 was revised using a pure sample and a new value is suggested.

Using drop calorimetry we measured enthalpy increments of the LiF-KF, LiF-RbF and LiFCsF binary systems at temperatures above melting. 10 samples with different compositions (four compositions for LiF-KF, one composition for LiF-RbF and five compositions for LiFCsF) were prepared and measured between 884K and 1382 K. To protect the calorimeter from corrosive fluoride vapor at high temperature an encapsulating technique developed for this purpose was used. The samples were filled in nickel containers which were sealed by laser welding and afterwards used for the measurements. From the obtained results we derived the molar heat capacity functions of the respective samples. The heat capacities of the samples, having different compositions of the same binary system, were compared with the values for ideal behavior and the excess heat capacity function was determined for the whole composition range of the liquid solution. It was found that the excess heat capacities clearly depend on the cation radius and increases in the order LiF-NaF < LiF-KF < LiF-RbF < LiF-CsF., JRC.E.3-Materials research

In this study, we present new experimental heat capacity data of solid and liquid phase of CsF covering the temperature range from 5 K to 1400 K. The low temperature data were obtained by adiabatic calorimetry and compared with the theoretical heat capacity computed from ab initio harmonic crystal approximation. The absolute entropy at T = 298.15 K determined based on the presented results is 93.60 J · K - 1 · mol - 1 . The high temperature heat capacity was measured using a drop calorimeter and the data for the solid phase revealed significant excess properties. The data for the liquid phase were correlated with the molecular dynamic simulation obtaining a good agreement. The recommended molar heat capacity of the CsF solid phase is: C p , m ( J · K - 1 · mol - 1 ) = 24.291 + 6.4607 · 10 - 2 · ( T / K ) + 589981 · ( T / K ) - 2 J · K - 1 · mol - 1 and the one of the liquid phase was determined as: C p , m = 70.56 J · K - 1 · mol - 1 . From the calorimetric results the enthalpy of fusion of CsF was determined as: △ fus H = 22.1 ± 2.0 kJ · mol - 1 .

In this article new thermodynamic descriptions of the (LiF + UF3) and the (NaF + UF3) systems based on new calorimetric data are presented. The experimental investigations were carried out using differential scanning calorimetry (DSC) to obtain phase transition temperatures between (0 and 60) mol% of UF3. On the basis of these measurements a thermodynamic assessment of the ( Li x U 1 - x ) F 3 - 2 x and ( Na x U 1 - x ) F 3 - 2 x liquid solutions, using the quasi-chemical model was achieved. UF3 undergoes a disproportionation at higher temperatures to UF4 and U metal which had a significant impact on the transition temperatures in the (LiF + UF3) system. For this reason the influence of the presence of up to 15 mol% of UF4 in this system was calculated and compared to the experimental results. The final thermodynamic description was corrected for the pure binary system.

Knudsen effusion mass spectrometry measurements on neptunium dioxide are reported in this work, which have allowed to improve the understanding of its vapourization behaviour and solved discrepancies noticed in the literature: the enthalpy of formation of NpO2(g) has been re-assessed and the composition of neptunia at congruent vapourization has been determined at 2262 K. In addition, a thermodynamic model for the neptunium-oxygen system has been developed using the CALPHAD method. The non stoichiometric NpO2−x phase is described herein using the compound energy formalism with ionic constituents ( Np 3 + , Np 4 + ) 1 ( O 2 - ,Va ) 2 , while the liquid phase is represented with the ionic two-sublattice model ( Np 4 + ) P ( O 2 - , Va Q - ,O ) Q . The reliability and consistency of all optimized Gibbs energies have been verified by calculating the phase equilibria, thermodynamic data, oxygen chemical potential and equilibrium partial pressures. Finally, a number of ill-defined data in the Np–O system have been identified after critical review of the literature and comparison with the present experimental results and CALPHAD model.

In this work a thermodynamic assessment of the (LiF + NaF + CaF 2 + LaF 3 ) system is reported. For the thermodynamic modeling of the liquid phase, the classical polynomial model, and the modified quasi-chemical model were used in parallel and compared. The extrapolation to higher order systems was done according to the Toop mathematical formalism. Furthermore, differential-scanning calorimetry data of the ternary (LiF + CaF 2 + LaF 3 ), (NaF + CaF 2 + LaF 3 ), and the quaternary (LiF + NaF + CaF 2 + LaF 3 ) mixtures are presented. Good agreement between the experimental data and the thermodynamic assessment was obtained.

The enthalpy increments of pure NpO2 have been measured using drop calorimetry in the temperature range from (376 to 1770) K. The heat capacity was derived using the obtained data constrained to 66.20 J · K−1 · mol−1 at 298.15 K taken from published low temperature measurements. The heat capacity of NpO2 determined in this study is given by the following equation: C p ,m 0 / ( J · K - 1 · mol - 1 ) = 71.608 + 0.015845 · ( T / K ) - 900648 · ( T / K ) - 2 .

An innovative encapsulation technique designed for the DSC high temperature measurements of the volatile samples is presented in this study. Stainless steel crucibles with a screwed bolt were developed for this purpose. The technique was demonstrated on the phase diagram measurement of the NaNO3–KNO3 system and the obtained results are presented in this study. Up to date two thermodynamic assessments of the NaNO3–KNO3 phase diagram have been made, one characterized by a single eutectic point with limited solubilities in the solid state in both NaNO3 and KNO3 rich corners, whereas the other version prefers a continuous solid solubility with minimum on the liquidus line. Based on our results we confirm limited solubilities in the solid state.

The enthalpy increments of four liquid solutions Li 0.8 Na 0.2 F, Li 0.6 Na 0.4 F , Li 0.4 Na 0.6 F , and Li 0.2 Na 0.8 F were measured in the temperature range (1230 to 1470) K by drop calorimetry. The liquid phases of the LiF and NaF end-members were also measured, in the temperature range (1230 to 1470) K and (1320 to 1540) K, respectively. From the obtained results the molar heat capacity was derived and the data for pure LiF and NaF were correlated with the literature. In order to overcome problems related with the corrosive nature of alkali metal fluoride vapours at high temperature, a novel encapsulation technique is presented in this study. Small nickel crucibles were developed for this purpose which are sealed by laser welding.

The enthalpy increments of UPd3 and ThPd3 have been measured using the drop calorimetry for temperature range (480 to 1290) K. The heat capacity curves have been derived using the simultaneous linear regression of our data and the low temperature heat capacity data from previous studies. Furthermore the Schottky contributions for UPd3 were determined by comparing the ThPd3 and UPd3 heat capacity functions and correlated with the theoretical values calculated from the crystal field energy levels.

This work is a critical review of the chemical–physical properties of the fluoride salts of interest in the Molten Salt Reactor project. In total five salt compositions are discussed. Two of them are choices for coolant applications (based on LiF–BeF 2 and NaF–NaBF 4 systems) one is considered as a heat transfer salt (LiF–NaF–KF system) whereas the other two are the main candidates for the fuels in non-moderated breeder and thermal breeder reactors, respectively (LiF–ThF 4 and LiF–BeF 2 –ThF 4 systems). For all the systems the phase diagrams are presented with the emphasis on the melting behaviour and the vapour pressure. Heat capacity, density, viscosity and thermal conductivity, as well as the solubility for actinides in case of the fuels are presented also.

WOS: 000345253800025, A laser heating approach combined with fast pyrometry in a thermal arrest method was used to provide new data for the melting/solidification phase transition in mixed (PuO2 + ThO2) at high temperature. At low concentration of ThO2 in PuO2 a minimum in the solidification temperature in the pseudo binary (PuO2 + ThO2) was observed. The minimum was found around a composition with 5 mol% ThO2. Phase transition temperatures of other compositions are closer to an ideal solution behaviour. To detect changes in the material a complete investigation with electron microscopy, Raman spectroscopy and powder X-ray diffraction was done. Raman vibration modes were found, that are characteristic for materials containing PuO2, and high temperature segregation effects during solidification were described. The results obtained in the present work are compared to other mixed actinide dioxides and compared to the ideal solution case for this system. The presented results show the importance of the high-temperature oxygen chemistry in this actinide oxide phase. (C) 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license., European CommissionEuropean Commission Joint Research Centre; E.ON Kernkraft GmbH [32920], The Authors are indebted to the JRC - ITU members G. Pagliosa, D. Bouexire, B. Cremer, A. Janssen and O.S. Valu for their precious help in the sample characterisation and P. Raison and A.L. Smith for the helpful discussions and valuable suggestions. This work is funded by the European Commission and partly supported by E.ON Kernkraft GmbH via contract No. 32920.

The heat capacity of gadolinium pyrochlore ( Gd 2 Ti 2 O 7 ) was measured by enthalpy increment measurements in the range of (585 to 1485) K utilizing drop calorimetry on a synthetic Gd-pyrochlore and the high-temperature dependence of the heat capacity has been determined.

The low temperature heat capacity of UF(3) has been measured using an adiabatic low temperature calorimeter in the temperature range from 10 to 350 K. These data are complemented at the lowest temperature region with data obtained with a Quantum Design PPMS-14 device in the temperature range from 0.5 to 20 K. Good agreement between both techniques has been found, and from these experimental results the absolute entropy of UF(3) at 298.15 K has been determined as 126.8 ± 2.5 J K(-1) mol(-1). On the basis of the specific heat data and the magnetization measurements performed on a SQUID device, a transition at 1.59 K attributed to Curie temperature of a ferromagnetic transition has been found in this study. This observation makes UF(3) a unique compound with an unusually low ferromagnetic ordering temperature.

International audience; We have studied the thermal properties of BaHf(PO4)2 during the heating from the room temperature up to 1600K by thermal analysis and drop calorimetry and up to 835K by Raman spectroscopy. A phase transition appears at about 791K during heating and at 744K during cooling. For the high-temperature polymorph we determined the crystal class and the most likely space groups based on Raman spectroscopic data. The enthalpy of the transition was derived from the high-temperature heat capacity measurements.