Contribution à l'étude de l’interaction chimique entre le combustible MOX et le sodium pour la sûreté des Réacteurs à Neutrons Rapides à caloporteur au sodium (RNR)

In case of a severe accident in Sodium-cooled Fast Reactors, interactions between partly molten fuel and sodium could happen at high temperature. Therefore, to predict the degradation evolution of fuel pins and phase formation in the different systems existing in the irradiated fuel, a thorough study of the Na-FP-Pu-U-O with FP= Ba, Cs, I, Mo, Te has to be performed. For such multicomponent system and large temperature and composition range, the Calphad method is a suitable way for developing a thermodynamic database to predict the phase formation depending on the temperature, pressure and composition of the system. Compositions with four Pu/(U+Pu) ratio in the Na-O-Pu-U system were synthesised by solid state synthesis method using nanoparticules of MOX fuel and characterised by XRD, ²³Na-NMR and HR-XANES techniques. The oxidation state of plutonium and uranium in the different structures was systematiquely investigated. When the measured oxidation state of actinides was different from the theoretical one, charge compensation mechanisms were suggested either by adding sodium in the structure or oxygen vacancies. Then, the structure of quaternary compounds in the Ba-Mo-Na-O and Cs-Mo-Na-O systems were also investigated by several structural techniques (XRD, neutron diffraction, HT-XRD, HT-Raman spectroscopy, XAS). Thermodynamic properties such as standard enthalpy of formation or enthalpy of decomposition were also determined. Finally, the Cs₂MoO₄-Na₂MoO₄ pseudo-binary section was re-investigated experimentally by DSC and a Calphad model for this system was developed.Dans le cas d’un accident grave dans un réacteur refroidi au sodium, des interactions entre le combustible irradié et le sodium pourraient se produire à très haute température. Pour pouvoir prédire la dégradation des aiguilles combustibles et les phases produites en tenant compte de tous les systèmes présents dans le combustible irradié, une étude des systèmes Na-PF-Pu-U-O avec PF= Ba, Cs, I, Mo, Te est primordiale. Pour un système avec de nombreux éléments à étudier sur une large échelle de température et de composition, la méthode Calphad est particulièrement adaptée. Celle-ci permet de prédire les phases formées en fonction de la température, pression et composition du système. Les composés dans le système Na-O-Pu-U ont été synthétisé par synthèse solide avec quatre ratios Pu/(U+Pu) différents dans les nanoparticules de MOX utilisés comme précurseurs. Les structures obtenues ont été caractérisés par DRX, ²³Na-RMN et HR-XANES. Le degré d’oxydation des échantillons a systématiquement été déterminé pour le plutonium et l’uranium et des mécanismes de compensations de charge avec insertion de sodium ou de lacunes d’oxygène ont été suggéré lorsque le degré d’oxydation du plutonium mesuré était différent du théorique. Les structures des composés quaternaires dans les systèmes Ba-Mo-Na-O et Cs-Mo-Na-O ont ensuite ont été étudiés par DRX, diffraction neutronique, DRX-HT, spectroscopie Raman-HT, XAS. L’enthalpie standard de formation et l’enthalpie de décomposition ont aussi été déterminées. Finalement, la section Cs₂MoO₄-Na₂MoO₄ a été ré-étudiée par DSC et un model Calphad pour ce système a été developpé

Structural and Thermodynamic Investigation of the Perovskite Ba₂NaMoO_5.5

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&amp;macr; 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.</p

Structural and Thermodynamic Investigation of the Perovskite Ba2NaMoO 5.5

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Synthesis and characterization of nanocrystalline U1-xPuxO2(+y) mixed oxides

We report here the first synthesis of mixed oxide U1-xPuxO2(þy) nanoparticles. The obtained nanopowders were characterized by X-ray diffraction, thermal ionization mass spectrometry, transmission electron microscopy, Raman spectroscopy, and U M4 edge high-energy-resolution X-ray absorption near edge structure (HR-XANES). The HR-XANES spectra give evidence for the partial oxidation of UIV to UV. This novel route toward the formation of actinide-actinide solid solution opens research opportunities that are not accessible using bulk materials. We give details on the X-ray diffraction study on plutonium oxalate hexahydrate, as a reagent for the synthesis of such nanoparticles.JRC.G.I.3-Nuclear Fuel Safet

A New Look at the Structural and Magnetic Properties of Potassium Neptunate K2NpO4 Combining XRD, XANES Spectroscopy, and Low-Temperature Heat Capacity

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

A New Look at the Structural and Magnetic Properties of Potassium Neptunate K₂NpO₄ Combining XRD, XANES Spectroscopy, and Low-Temperature Heat Capacity

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.RST/Reactor Physics and Nuclear Material