The Thermodynamics of Advanced Fuels – International Database (TAF-ID) was developed using the Calphad method to provide a computational tool to perform thermodynamic calculations on nuclear fuel materials under normal and off-normal conditions. Different kinds of fuels are considered: oxide, metallic, carbide and nitride fuels. Many fission products are introduced as well as structural materials (e.g., zirconium, steel, concrete, SiC) and absorbers (e.g., B₄C), in order to investigate the thermochemistry of irradiated fuels and to predict their chemical interaction with the surrounding materials. The approach to develop the database and the models implemented in the database are described. Examples of models for key chemical systems are presented. Finally, a few examples of application calculations on severe accidents with UO₂ fuels, irradiated fuel chemistry of MOX and metallic fuels and metallic fuel/cladding interaction show how this tool can be used. To validate the database, the calculations are compared to the available experimental data. A good agreement is obtained which gives confidence in the maturity degree and quality of the TAF-ID database. The working version is only accessible to the participants of the TAF-ID project (Canada, France, Japan, the Netherlands, Republic of Korea, United Kingdom, USA). A public version is accessible by all the NEA countries. The current version contains models on the Am–Fe, Am–Np, Am-O-Pu, Am–U, Am–Zr, C–O–U-Pu, Cr–U, Np–U, Np–Zr, O–U–Zr, Re–U, Ru–U, Si–U, Ti–U, U-Pu-Zr, U–W systems. It is progressively extended with our published assessments. Information on how to join the project is available on the website: https://www.oecd-nea.org/science/taf-id/.

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The existence of a novel double-molybdate phase with a palmierite-type structure, Cs2Ba(MoO4)2, is revealed in this work, and its structural properties at room temperature have been characterized in detail using X-ray and neutron diffraction measurements. In addition, its thermal stability and thermal expansion are investigated in the temperature range 298-673 K using high-temperature X-ray diffraction, leading to the volumetric thermal expansion coefficient αV ≈ 43.0 × 10-6 K-1. The compound's standard enthalpy of formation at 298.15 K has been obtained using solution calorimetry, which yielded ΔfHm°(Cs2Ba(MoO4)2, cr, 298.15 K) = -3066.6 ± 3.1 kJ· mol-1, and its standard entropy at 298.15 K has been derived from low-temperature (2.1-294.3 K) thermal-relaxation calorimetry as Sm°(Cs2Ba(MoO4)2, cr, 298.15 K) = 381.2 ± 11.8 J K-1 mol-1.

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Neutron diffraction, X-ray absorption spectroscopy (XAS), and Raman spectroscopy measurements of the quaternary perovskite phase Ba2NaMoO5.5 have been performed in this work. The cubic crystal structure in space group Fm3¯ m has been refined using the Rietveld method. X-ray absorption near-edge structure spectroscopy (XANES) measurements at the Mo K-edge have confirmed the hexavalent state of molybdenum. The local structure of the molybdenum octahedra has been studied in detail using extended X-ray absorption fine structure (EXAFS) spectroscopy. The Mo-O and Mo-Ba distances have been compared to the neutron diffraction data with good agreement. The coefficient of thermal expansion measured in the temperature range of 303-923 K, using high temperature X-ray diffraction (HT-XRD) (αV = 55.8 × 10-6 K), has been determined to be ∼2 times higher than that of the barium molybdates BaMoO3 and BaMoO4. Moreover, no phase transition nor melting have been observed, neither by HT-XRD nor Raman spectroscopy nor differential scanning calorimetry, up to 1473 K. Furthermore, the standard enthalpy of formation (ΔfHm°) for Ba2NaMoO5.5(cr) has been determined to be -(2524.75 ± 4.15) kJ mol-1 at 298.15 K, using solution calorimetry. Finally, the margin for safe operation of sodium-cooled fast reactors (SFRs) has been assessed by calculating the threshold oxygen potential needed, in liquid sodium, to form the quaternary compound, following an interaction between irradiated mixed oxide (U,Pu)O2 fuel and sodium coolant.@en

Neutron diffraction measurements of the double molybdate Cs₃Na(MoO₄)₂ have been performed for the first time in this work and the crystal structure refined using the Rietveld method. The thermal expansion of this trigonal phase, in space group P3¯m1, measured using high temperature X-ray diffraction (XRD), remains moderate: α_a=31·10⁻⁶K⁻¹ and α_c=24·10⁻⁶K⁻¹ in the temperature range T = (298−723) K. The melting temperature of this compound has been determined at T_fus= (777 ± 5) K using Differential Scanning Calorimetry (DSC). No phase transition was detected, neither by DSC, nor by high temperature XRD or high temperature Raman spectroscopy, which disagrees with the literature data of Zolotova et al. (2016), who reported a reversible phase transition around 663 K. Finally, thermodynamic equilibrium calculations have been performed to assess the probability of formation of Cs₃Na(MoO₄)₂ inside the fuel pin of a Sodium cooled Fast Reactor by reaction between the cesium molybdate phase Cs₂MoO₄, which forms at the pellet rim at high burnup, the fission product molybdenum (either as metallic or oxide phase), and the liquid sodium coolant in the accidental event of a breach of the stainless steel cladding and sodium ingress in the failed pin.

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The enthalpy of formation at 298.15 K and low temperature heat capacity of Cs3Na(MoO4)2 have been measured for the first time in this work using solution calorimetry and thermal-relaxation calorimetry in the temperature range T = (1.9–299.6) K, respectively. The solution calorimetry measurements, performed in 2 M HNO3 solution, have yielded an enthalpy equal to ΔrHmΔrHm(298.15 K) = (6.79 ±± 1.72)  kJ··mol−1 for the reaction:
3/2Cs2MoO4(cr)+1/2Na2MoO4(cr)=Cs3Na(MoO4)2(cr)3/2Cs2MoO4(cr)+1/2Na2MoO4(cr)=Cs3Na(MoO4)2(cr)
Combining with the enthalpies of formation of Cs2MoO4(cr) and Na2MoO4(cr), also determined in this work in 0.1 M CsOH and 0.1 M NaOH solutions, respectively, the standard enthalpy of formation of Cs3Na(MoO4)2 at 298.15 K has been determined as View the MathML sourceΔfHmo(Cs3Na(MoO4)2, cr, 298.15 K) = −(2998.5 ±± 3.0) kJ··mol−1. The heat capacity and entropy values of Cs3Na(MoO4)2 at 298.15 K have been derived as View the MathML sourceCp,mo(Cs3Na(MoO4)2,cr,298.15K)=(296.3±3.3) J··K−1··mol−1 and View the MathML sourceSmo(Cs3Na(MoO4)2,cr,298.15K) (467.2±6.8) J··K−1··mol−1. Combining the newly determined thermodynamic functions, the Gibbs energy of formation of Cs3Na(MoO4)2 at 298.15 K has been derived as View the MathML sourceΔfGmo(Cs3Na(MoO4)2,cr,298.15K)=-(2784.6±3.4) kJ··mol−1. Finally, the enthalpies, entropies and Gibbs energies of formation of Cs3Na(MoO4)2 from its constituting binary and ternary oxides have been calculated.@en

ASTEC-Na is a computer code system which evaluates protected and unprotected accidents in Sodium-cooled Fast Reactors throughout the Initiation Phase. The fuel pin behavior models implemented in ASTEC-Na simulate the essential aspects of the thermal and mechanical behavior of SFR fuel pins in nominal and accidental conditions. Besides the improvement of existing ASTEC-Na models, a specific cladding mechanical model and an in-pin fuel relocation model have been developed during the JASMIN project, where the latter has been already implemented in the code. ASTEC-Na fuel pin behavior models are described in this paper and their transient predictions for four CABRI transient tests are compared to available experimental data as well as to SAS-SFR and SIMMER-III code calculations. Conclusions on the overall performance of ASTEC-Na fuel pin models are also presented.

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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.

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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.

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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.

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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.

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The physicochemical properties of the potassium neptunate K₂NpO₄ have been investigated in this work using X-ray diffraction, X-ray absorption near edge structure (XANES) spectroscopy at the Np-L₃ edge, and low-temperature heat capacity measurements. A Rietveld refinement of the crystal structure is reported for the first time. The Np(VI) valence state has been confirmed by the XANES data, and the absorption edge threshold of the XANES spectrum has been correlated to the Mössbauer isomer shift value reported in the literature. The standard entropy and heat capacity of K₂NpO₄ have been derived at 298.15 K from the low-temperature heat capacity data. The latter suggest the existence of a magnetic ordering transition around 25.9 K, most probably of the ferromagnetic type.

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Na3.16(2)UV,VI 0.84(2)O4 is obtained from the reaction of sodium with uranium dioxide under oxygen potential conditions typical of a sodium-cooled fast nuclear reactor. In the event of a breach of the steel cladding, it would be the dominant reaction product forming at the rim of the mixed (U,Pu)O2 fuel pellets. High-temperature X-ray diffraction measurements show that a distortion of the uranium environment in Na3.16(2)UV,VI 0.84(2)O4 results in a strongly anisotropic thermal expansion. A comparison with several related sodium metallates Nan-2Mn+On-1 - including Na3SbO4 and Na3TaO4, whose crystal structures are reported for the first time - has allowed us to assess the role played in the lattice expansion by the Mn+ cation radius and the Na/M ratio. On this basis, the thermomechanical behavior of the title compound is discussed, along with those of several related double oxides of sodium and actinide elements, surrogate elements, or fission products.@en

The structure of α-Cs₂Mo₂O₇ (monoclinic in space group P2₁/c), which can form during irradiation in fast breeder reactors in the space between nuclear fuel and cladding, has been refined in this work at room temperature from neutron diffraction data. Furthermore, the compounds' thermal expansion and polymorphism have been investigated using high temperature X-ray diffraction combined with high temperature Raman spectroscopy. A phase transition has been observed at T_tr(α→β)=(621.9±0.8) K using Differential Scanning Calorimetry, and the structure of the β-Cs₂Mo₂O₇ phase, orthorhombic in space group Pbcm, has been solved ab initio from the high temperature X-ray diffraction data. Furthermore, the low temperature heat capacity of α-Cs₂Mo₂O₇ has been measured in the temperature range T=(1.9–313.2) K using a Quantum Design PPMS (Physical Property Measurement System) calorimeter. The heat capacity and entropy values at T=298.15 K have been derived as C_p,m ^o(Cs₂Mo₂O₇,cr,298.15K)=(211.9±2.1)JK⁻¹mol⁻¹ and S_m ^o(Cs₂Mo₂O₇,cr,298.15K)=(317.4±4.3)JK⁻¹mol⁻¹. When combined with the enthalpy of formation reported in the literature, these data yield standard entropy and Gibbs energy of formation as Δ_fS_m ^o(Cs₂Mo₂O₇,cr,298.15K)=−(628.2±4.4)JK⁻¹mol⁻¹ and Δ_fG_m ^o(Cs₂Mo₂O₇,cr,298.15K)=−(2115.1±2.5)kJmol⁻¹. Finally, the cesium partial pressure expected in the gap between fuel and cladding following the disproportionation reaction 2Cs₂MoO₄=Cs₂Mo₂O₇+2Cs(g)+ 1/2 O₂(g) has been calculated from the newly determined thermodynamic functions.

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The physical and chemical properties at low temperatures of hexavalent disodium neptunate α-Na₂NpO₄ are investigated for the first time in this work using Mössbauer spectroscopy, magnetization, magnetic susceptibility, and heat capacity measurements. The Np(VI) valence state is confirmed by the isomer shift value of the Mössbauer spectra, and the local structural environment around the neptunium cation is related to the fitted quadrupole coupling constant and asymmetry parameters. Moreover, magnetic hyperfine splitting is reported below 12.5 K, which could indicate magnetic ordering at this temperature. This interpretation is further substantiated by the existence of a λ-peak at 12.5 K in the heat capacity curve, which is shifted to lower temperatures with the application of a magnetic field, suggesting antiferromagnetic ordering. However, the absence of any anomaly in the magnetization and magnetic susceptibility data shows that the observed transition is more intricate. In addition, the heat capacity measurements suggest the existence of a Schottky-type anomaly above 15 K associated with a low-lying electronic doublet found about 60 cm^-1 above the ground state doublet. The possibility of a quadrupolar transition associated with a ground state pseudoquartet is thereafter discussed. The present results finally bring new insights into the complex magnetic and electronic peculiarities of α-Na₂NpO₄.

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