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.

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.

Na_3.16(2)U^V,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 Na_3.16(2)U^V,VI _0.84(2)O₄ results in a strongly anisotropic thermal expansion. A comparison with several related sodium metallates Na_n-2Mⁿ⁺O_n-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.

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 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.