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The (Li,Be)Fx fluoride salt is an ionic liquid with complex non-ideal thermodynamic behaviour due to the formation of short-range order. In this work, we explore the relationship between local structure, thermophysical and thermodynamic properties in this system using a multidisciplinary approach that couples molecular dynamics simulations using the Polarizable Ion Model (PIM) and thermodynamic modelling assessment using the CALPHAD method. The density, thermal expansion, viscosity, thermal conductivity, molar and mixing enthalpies and heat capacity of the (Li,Be)Fₓ melt are extracted from the polarizable ionic interaction potentials and investigated across a wide range of compositions and temperatures. The agreement with the available experimental data is generally very good. The local structure is also examined in detail, in particular the transition between a molecular liquid with Li⁺, BeF²₄⁻ and F⁻ predominant species at low BeF₂ content, and a polymeric liquid at high BeF₂ content, with the formation of polymers (Be₂F³₇⁻, Be₃F⁴₁₀⁻, Be₄F⁵₁₃⁻, etc.), and finally of a three-dimensional network of corner-sharing tetrahedrally coordinated Be²⁺ cations for pure BeF². Based on the available experimental information and the output of the MD simulations, we moreover develop for the first time a coupled structural-thermodynamic model for the LiF-BeF² system based on the quasi-chemical formalism in the quadruplet approximation, that provides a physical description of the melt and reproduces (in addition to the thermodynamic data) the chemical speciation of beryllium polymeric species predicted from the simulations.

The (Li,Be)Fx fluoride salt is an ionic liquid with complex non-ideal thermodynamic behaviour due to the formation of short-range order. In this work, we explore the relationship between local structure, thermo-physical and thermodynamic properties in this system using a multidisciplinary approach that couples molecular dynamics simulations using the Polarizable Ion Model (PIM) and thermodynamic modelling assessment using the CALPHAD method. The density, thermal expansion, viscosity, thermal conductivity, molar and mixing enthalpies and heat capacity of the (Li,Be)Fx melt are extracted from the polarizable ionic interaction potentials and investigated across a wide range of compositions and temperatures. The agreement with the available experimental data is generally very good. The local structure is also examined in detail, in particular the transition between a molecular liquid with Li+, BeF4 2− and F− predominant species at low BeF2 content, and a polymeric liquid at high BeF2 content, with the formation of polymers (Be2F7 3−, Be3F10 4−, Be4F13 5−, etc.), and finally of a three-dimensional network of corner-sharing tetrahedrally coordinated Be2+ cations for pure BeF2. Based on the available experimental information and the output of the MD simulations, we moreover develop for the first time a coupled structural-thermodynamic model for the LiF-BeF2 system based on the quasi-chemical formalism in the quadruplet approximation, that provides a physical description of the melt and reproduces (in addition to the thermodynamic data) the chemical speciation of beryllium polymeric species predicted from the simulations., RST/Reactor Physics and Nuclear Materials

In the context of a comprehensive campaign for the characterisation of transmutation fuels for next generation nuclear reactors, the melting behaviour of mixed uranium-americium dioxides has been experimentally studied for the first time by laser heating, for Am concentrations up to 70 mol. % under different types of atmospheres. Extensive post-melting material characterisations were then performed by X-ray absorption spectroscopy and electron microscopy. The melting temperatures observed for the various compositions follow a markedly different trend depending on the experimental atmosphere. Uranium-rich samples melt at temperatures significantly lower (around 2700 K) when they are laser-heated in a strongly oxidizing atmosphere compressed air at (0.300 ± 0.005) MPa, compared to the melting points (beyond 3000 K) registered for the same compositions in an inert environment (pressurised Ar). This behaviour has been interpreted on the basis of the strong oxidation of such samples in air, leading to lower-melting temperatures. Thus, the melting temperature trend observed in air is characterized, in the purely pseudo-binary dioxide plane, by an apparent maximum melting temperature around 2850 K for 0.3 < x(AmO2) < 0.5. The melting points measured under inert atmosphere uniformly decrease with increasing americium content, displaying an approximately ideal solution behaviour if a melting point around 2386 K is assumed for pure AmO2. In reality, it will be shown that the (U, Am)-oxide system can only be rigorously described in the ternary U-Am-O phase diagram, rather than the UO2-AmO2 pseudo-binary, due to the aforementioned over-oxidation effect in air. Indeed, general departures from the oxygen stoichiometry (Oxygen/Metal ratios ≠ 2.0) have been highlighted by the X-ray Absorption Spectroscopy (XAS). Finally, to help interpret the experimental results, thermodynamic computations based on the CALPHAD method will be pre, RST/Reactor Physics and Nuclear Materials

In the context of a comprehensive campaign for the characterisation of transmutation fuels for next generation nuclear reactors, the melting behaviour of mixed uranium-americium dioxides has been experimentally studied for the first time by laser heating, for Am concentrations up to 70 mol. % under different types of atmospheres. Extensive post-melting material characterisations were then performed by X-ray absorption spectroscopy and electron microscopy. The melting temperatures observed for the various compositions follow a markedly different trend depending on the experimental atmosphere. Uranium-rich samples melt at temperatures significantly lower (around 2700 K) when they are laser-heated in a strongly oxidizing atmosphere compressed air at (0.300 ± 0.005) MPa, compared to the melting points (beyond 3000 K) registered for the same compositions in an inert environment (pressurised Ar). This behaviour has been interpreted on the basis of the strong oxidation of such samples in air, leading to lower-melting temperatures. Thus, the melting temperature trend observed in air is characterized, in the purely pseudo-binary dioxide plane, by an apparent maximum melting temperature around 2850 K for 0.3 < x(AmO2) < 0.5. The melting points measured under inert atmosphere uniformly decrease with increasing americium content, displaying an approximately ideal solution behaviour if a melting point around 2386 K is assumed for pure AmO2. In reality, it will be shown that the (U, Am)-oxide system can only be rigorously described in the ternary U-Am-O phase diagram, rather than the UO2-AmO2 pseudo-binary, due to the aforementioned over-oxidation effect in air. Indeed, general departures from the oxygen stoichiometry (Oxygen/Metal ratios ≠ 2.0) have been highlighted by the X-ray Absorption Spectroscopy (XAS). Finally, to help interpret the experimental results, thermodynamic computations based on the CALPHAD method will be presented.

The low-temperature heat capacity of (U1−yThy)O2 and 238Pu-doped UO2 samples were determined using hybrid adiabatic relaxation calorimetry. Results of the investigated systems revealed the presence of the magnetic transition specific for UO2 in all three intermediate compositions of the uranium-thorium dioxide (y = 0.05, 0.09 and 0.12) and in the 238Pu-doped UO2 around 25 K. The magnetic behaviour of UO2 exposed to the high alpha dose from the 238Pu isotope was studied over time and it was found that 1.6% 238Pu affects the magnetic transition substantially, even after short period of time after annealing. In both systems the antiferromagnetic transition changes intensity, shape and Néel temperature with increasing Th-content and radiation dose, respectively, related to the increasing disorder on the crystal lattice resulting from substitution and defect creation., RST/Applied Radiation & Isotopes

The mixture LiF-ThF4-UF4-PuF3 (77.5–6.6-12.3–3.6 mol%), a fuel option for the Molten Salt Fast Reactor (MSFR), has been measured by differential scanning calorimetry (DSC) for determination of phase transitions, and by Knudsen effusion mass spectrometry (KEMS) for measurement of partial vapour pressures. The boiling point of the mixture was determined by extrapolation of total vapour pressure to 1 bar. Thermodynamic calculations were performed and compared with the experimental results. Novel experimental data on pure plutonium trifluoride are presented: melting point, vaporization enthalpy, vapour pressure and ionization energies by electron impact., RST/Reactor Physics and Nuclear Materials

To assure the safety of oxide-fuel based nuclear reactors, the knowledge of the atomic-scale properties of U1−yMyO2±x materials is essential. These compounds show complex chemical properties, originating from the fact that actinides and rare earths may occur with different oxidation states. In these mostly ionic materials, aliovalent cationic configurations can induce changes in the oxygen stoichiometry, with dramatic effects on the properties of the fuel. First studies on U1−yAmyO2±x indicated that these materials exhibit particularly complex electronic and local-structure configurations. Here we present an in-depth study of these compounds, over a wide compositional domain, by combining XRD, XAS and Raman spectroscopy. We provide evidences of the co-existence of four different cations (U4+, U5+, Am3+, Am4+) in U1−yMyO2±x compounds, which nevertheless maintain the fluorite structure. Indeed, we show that the cationic sublattice is basically unaffected by the extreme multi-valence states, whereas complex defects are present in the oxygen sublattice., RST/Reactor Physics and Nuclear Materials

The complete thermodynamic description of the niobium-fluorine system is presented for the first time in this work. It results from a critical evaluation of the available experimental data and new thermodynamic calculations. In total, three niobium fluoride solid phases (NbF3, NbF4 and NbF5) and seven gaseous species (NbF, NbF2, NbF3, NbF4, NbF5, Nb2F10 and Nb3F15) have been considered during the assessment. Novel data for all the gaseous species were calculated combining density functional theory (DFT), for the prediction of the molecular parameters, and statistical mechanical calculations, for the determination of the thermal functions (i.e. standard entropy and heat capacity). The developed thermodynamic model was found to correctly reproduce all the available experimental data and was used to calculate the Nb-F phase diagram, which is presented in this work as well., RST/Reactor Physics and Nuclear Materials

The present study describes the full thermodynamic assessment of the Li,Cs,Th//F,I system. The existing database for the relevant fluoride salts considered as fuel for the Molten Salt Reactor (MSR) has been extended with two key fission products, cesium and iodine. A complete evaluation of all the common-ion binary and ternary sub-systems of the LiF-ThF4-CsF-LiI-ThI4-CsI system has been performed and the optimized parameters are presented in this work. New equilibrium data have been measured using Differential Scanning Calorimetry and were used to assess the reciprocal ternary systems and confirm the extrapolated phase diagrams. The developed database significantly contributes to the understanding of the behaviour of cesium and iodine in the MSR, which strongly depends on their concentration and chemical form. Cesium bonded with fluorine is well retained in the fuel mixture while in the form of CsI the solubility of these elements is very limited. Finally, the influence of CsI and CsF on the physico-chemical properties of the fuel mixture was calculated as function of composition., RST/Reactor Physics and Nuclear Materials

mericium 241 is a potential alternative to plutonium 238 as an energy source for missions into deep space or to the dark side of planetary bodies. In order to use the 241Am isotope for radioisotope thermoelectric generator or radioisotope heating unit (RHU) production, americium materials need to be developed. This study focuses on the stabilization of a cubic americium oxide phase using uranium as the dopant. After optimization of the material preparation, (Am0.80U0.12Np0.06Pu0.02)O1.8 has been successfully synthesized to prepare a 2.96 g pellet containing 2.13 g of 241Am for fabrication of a small scale RHU prototype. Compared to the use of pure americium oxide, the use of uranium-doped americium oxide leads to a number of improvements from a material properties and safety point of view, such as good behavior under sintering conditions or under alpha self-irradiation. The mixed oxide is a good host for neptunium (i.e., the 241Am daughter element), and it has improved safety against radioactive material dispersion in the case of accidental conditions.

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., RST/Reactor Physics and Nuclear Materials

Thin films of U1− xThxO2 (x = 0 to 1) have been deposited via reactive DC sputter technique and characterized by X-ray/Ultra-violet Photoelectron Spectroscopy (XPS/UPS), X-ray Powder Diffractometer (XRD) and Cyclic Voltammetry (CV) in order to understand the effect of Thorium on the oxidation mechanism. During the deposition, the competition between uranium and thorium for oxidation showed that thorium has a much higher affinity for oxygen. Deposition conditions, time and temperature were also the subject of this study, to look at the homogeneity and the stability of the films. While core level and valence band spectra were not altered by the time of deposition, temperature was affecting the oxidation state of uranium and the valence band due to the mobility increase of oxygen through the film. X-ray diffraction patterns, core level spectra obtained for U1 − xThxO2 versus the composition showed that lattice parameters follow the Vegard's law and together with the binding energies of U-4f and Th-4f are in good agreement with literature data obtained on bulk compounds. To study the effect of thorium on the oxidation of U1 − xThxO2 films, we used CV experiments at neutral pH of a NaCl solution in contact with air. The results indicated that thorium has an effect on the uranium oxidation as demonstrated by the decrease of the current of the oxidation peak of uranium. XPS measurements made before and after the CV, showed a relative enrichment of thorium at the extent of uranium at the surface supporting the formation at a longer term of a thorium protective layer at the surface of uranium-thorium mixed oxide., RST/Reactor Physics and Nuclear Materials

During rapid high-temperature events, like a terrorist attack with radiological dispersal device, radiological material will be released into the environment. In these scenarios, the ratio between parent and daughter nuclides can be used for nuclear forensic investigations to determine the age of the used radioactive source. We have used fast laser heating to produce aerosols of a material often found in radioisotope thermoelectric generators. To investigate the behaviour of SrTiO3, we have recreated pure and mixed samples, mimicking several parent-to-daughter ratios. By combining scanning electron microscopy and energy-dispersive X-ray spectroscopy analysis with Raman spectroscopy, we were able to distinguish different elements and phases present in the aerosols. Two types of aerosols have been identified: individual aerosols from a few micrometre to a few tens of micrometre and agglomerates of smaller aerosols from a few hundred nanometre to a few micrometre. The bigger aerosols, formed from mechanically expelled liquefied material, showed a parent-to-daughter ratio that stays close to the value that would be anticipated by the initial composition of the material, but in the agglomerates, formed from vaporised material, the presence of the daughter elements reduces significantly due to differences in the condensation behaviour., RST/Reactor Physics and Nuclear Materials

Major and severe accidents have occurred three times in nuclear power plants (NPPs), at Three Mile Island (USA, 1979), Chernobyl (former USSR, 1986) and Fukushima (Japan, 2011). Research on the causes, dynamics, and consequences of these mishaps has been performed in a few laboratories worldwide in the last three decades. Common goals of such research activities are: the prevention of these kinds of accidents, both in existing and potential new nuclear power plants; the minimization of their eventual consequences; and ultimately, a full understanding of the real risks connected with NPPs. At the European Commission Joint Research Centre’s Institute for Transuranium Elements, a laserheating and fast radiance spectro-pyrometry facility is used for the laboratory simulation, on a small scale, of NPP core meltdown, the most common type of severe accident (SA) that can occur in a nuclear reactor as a consequence of a failure of the cooling system. This simulation tool permits fast and effective high-temperature measurements on real nuclear materials, such as plutonium and minor actinide-containing fission fuel samples. In this respect, and in its capability to produce large amount of data concerning materials under extreme conditions, the current experimental approach is certainly unique. For current and future concepts of NPP, example results are presented on the melting behavior of some different types of nuclear fuels: uranium-plutonium oxides, carbides, and nitrides. Results on the high-temperature interaction of oxide fuels with containment materials are also briefly shown., RST/Reactor Physics and Nuclear Materials

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., RST/Reactor Physics and Nuclear Materials

The charge distributions in α-Na2UO4, Na3NpO4, α-Na2NpO4, Na4NpO5, Na5NpO6, Na2PuO3, Na4PuO5, and Na5PuO6 are investigated in this work using X-ray absorption near-edge structure (XANES) spectroscopy at the U-L3, Np-L3, and Pu-L3 edges. In addition, a Rietveld refinement of monoclinic Na2PuO3, in space group C2/c, is reported for the first time, and the existence of the isostructural Na2NpO3 phase is revealed. In contrast to measurements in solution, the number of published XANES data for neptunium and plutonium solid phases with a valence state higher than IV is very limited. The present results cover a wide range of oxidation states, namely, IV to VII, and can serve as reference for future investigations. The sodium actinide series show a variety of local coordination geometries, and correlations between the shape of the XANES spectra and the local structural environments are discussed herein., RST/Reactor Physics and Nuclear Materials

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 (Np3+, Np4+)(1) (O2-, Va)(2), while the liquid phase is represented with the ionic two-sublattice model (Np4+)(P) (O2-, 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., QC 20161028

The applicability and limitations of sulphur isotope ratio as a nuclear forensic signature have been studied. The typically applied leaching methods in uranium mining processes were simulated for five uranium ore samples and the n(34S)/n(32S) ratios were measured. The sulphur isotope ratio variation during uranium ore concentrate (UOC) production was also followed using two real-life sample sets obtained from industrial UOC production facilities. Once the major source of sulphur is revealed, its appropriate application for origin assessment can be established. Our results confirm the previous assumption that process reagents have a significant effect on the n(34S)/n(32S) ratio, thus the sulphur isotope ratio is in most cases a process-related signature., RST/Reactor Physics and Nuclear Materials

Experimental tests have been performed to characterize the aerosols representative of radiological dispersion devices (RDDs, a.k.a. "dirty bombs") by applying to chosen surrogate compound rapid high temperature transients, vaporizing the sample and forming aerosols mainly by rapid cooling of the vapour. The materials, which were tested in their non-radioactive form, had been chosen from the radioactive sources widely used in industries and nuclear medicine applications, Co, CsCl, Ir and SrTiO3. Our analyses permitted the characterization of the inhalable fraction of the aerosols released, and the study of the influence of cladding materials on the aerosol release and on its characteristics., RST/Reactor Physics and Nuclear Materials

RST/Reactor Physics and Nuclear Materials

The structural properties of the sodium actinide ternary oxides are reviewed in this work, and general trends among the series U-Np-Pu are identifed. A Rietveld refnement of monoclinic Na2PuO3, in space group C2~c, is reported for the first time, and the unsuspected existence of Na2NpO3 is revealed. In addition, the charge distribution in alpha-Na2UO4, Na3NpO4, alpha-Na2NpO4, Na4NpO5, Na5NpO6, Na2PuO3, Na4PuO5, and Na5PuO6 is investigated using X-ray Absorption Near-Edge Structure (XANES) spectroscopy at the U-L3, Np-L3, and Pu-L3 edges, respectively. In contrast to measurements in solution, the number of XANES data in the literature for neptunium and plutonium solid phases with a valence state higher than (IV) is very limited. The present results cover a wide range of oxidation states, namely (IV) to (VII), and can serve as a reference databank for future investigations. Finally, the sodium actinide series show a variety of local coordination geometries, and correlations between shape of the XANES spectra and local structural environment are discussed.

In the event of a clad breach in Sodium-cooled Fast Reactors (SFRs), the metallic sodium coolant will enter the pin and react with the (U, Pu, Np)O2 fuel. The reaction products are numerous, but there is still little knowledge of their structural and thermodynamic properties. Under the oxygen potential conditions of the reactor, pentavalent Na3U1-xPuxO4 is expected to form, but its structure was the subject of controversy until now. This compound was shown in the 1980s to be of lower density and lower thermal conductivity than the fuel, leading to local swelling and creation of hot spots. Such a situation can induce further cladding failure and result in a contamination of the primary coolant with highly radioactive fission products. In this work, we have revisited the crystal structure of the Na3MO4 (M=U,Np,Pu) reaction product. Surprisingly, the structure of the sodium uranate differs from the one observed with neptunium and plutonium. In addition, this phase can accommodate excess sodium on the uranium site, with subsequent charge compensation from U(V) to U(VI), which was not previously foreseen. By contrast, the sodium neptunate and plutonate remain pentavalent. Temperature and oxygen potential are the two fundamental parameters that control the chemistry of the interaction. Assessing the margin to the safe operation of SFRs requires a thorough knowledge of the actinide cation valence state in the reaction products, and a complete thermodynamic description of the system. The valence state was determined in the Na-M-O (M=U,Np,Pu) ternary phases using XANES and Mössbauer spectroscopy, covering a wide range of oxidation states, namely (IV) to (VII). Coupling experimental thermodynamic investigations with thermodynamic modelling assessments using the CALPHAD method, we have calculated the phase equilibria in the Na-U-O system, and derived the oxygen potential threshold required within the fuel (and sodium coolant) to form the sodium uranate ternary phases.