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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.

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.

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.

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.

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.

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 .

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.

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.