Christine Guéneau

Thermodynamic assessment of the uranium-oxygen system

Abstract A thermodynamic assessment of the uranium–oxygen system is presented. A consistent set of experimental data is selected among the numerous data in the literature on the phase diagram and oxygen chemical potential. The thermodynamic properties of the phases are described using the compound energy model with ionic constituents for the solids and an ionic two-sublattice model for the liquid. For the uranium dioxide, the structure is described using three sublattices, one for the cations U3+, U4+ and U6+, one for the normal site of oxygen ions, and one for the interstitial oxygen ions. Vacancies are included in both oxygen sublattices. In this first approach, the homogeneity ranges of the U4O9−y and U3O8−y compounds are not represented. A set of consistent model parameters that describes both the phase diagram and the oxygen chemical potential data in the whole composition range is thus obtained. The description of this basic binary system will be used to calculate higher order systems such as O–U–Zr and Fe–O–U which are important for simulating severe nuclear accidents.

Mass spectrometric study of UO2–ZrO2 pseudo-binary system

Abstract The thermodynamic activity of UO2 in the UO2–ZrO2 pseudo-binary system, for fractional concentrations of ZrO2 from 0 to 1 in the 2000–2400 K temperature range was investigated by partial pressure measurements of UO2 (g) using the multiple Knudsen cell-mass spectrometric method. This system exhibits a positive deviation from the ideal behavior at 2200 K and becomes close to ideal near 2500 K.

Mass spectrometric study of the oxygen potential at the UO2−x–U(l) phase limit

Abstract Activity measurements of UO2 at the phase boundary U(l)–UO2−x have been performed by high temperature mass spectrometry in order to solve discrepancies observed in earlier works. The direct measurement of the UO2 activity was performed using the multiple Knudsen cell method and a special collimation device in order to (i) circumvent any usual calibration methods (ii) discard any earlier observed parasitic contributions in this system. The oxygen potential is then directly calculated on the basis of pure UO2(s) accurate thermodynamic data for the diphasic U(l)–UO2−x domain in the 2000–2250 K range.

Insight into the Am–O Phase Equilibria: A Thermodynamic Study Coupling High-Temperature XRD and CALPHAD Modeling

In the frame of minor actinide transmutation, americium can be diluted in UO2 and (U, Pu)O2 fuels burned in fast neutron reactors. The first mandatory step to foresee the influence of Am on the in-reactor behavior of transmutation targets or fuel is to have fundamental knowledge of the Am-O binary system and, in particular, of the AmO2-x phase. In this study, we coupled HT-XRD (high-temperature X-ray diffraction) experiments with CALPHAD thermodynamic modeling to provide new insights into the structural properties and phase equilibria in the AmO2-x-AmO1.61+x-Am2O3 domain. Because of this approach, we were able for the first time to assess the relationships between temperature, lattice parameter, and hypostoichiometry for fcc AmO2-x. We showed the presence of a hyperstoichiometric existence domain for the bcc AmO1.61+x phase and the absence of a miscibility gap in the fcc AmO2-x phase, contrary to previous representations of the phase diagram. Finally, with the new experimental data, a new CALPHAD thermodynamic model of the Am-O system was developed, and an improved version of the phase diagram is presented.

Direct measurements of the chromium activity in complex nickel base alloys by high temperature mass spectrometry

Chromium rich, nickel based alloys Haynes 230 and Inconel 617 are candidate materials for the primary circuit and intermediate heat exchangers (IHX) of (Very)-High Temperature Reactors. The corrosion resistance of these alloys is strongly related to the reactivity of chromium in the reactor specific environment (high temperature, impure helium). At intermediate temperature – 900°C for Haynes 230 and 850°C for Inconel 617 – the alloys under investigation are likely to develop a chromium-rich surface oxide scale. This layer protects from the exchanges with the surrounding medium and thus prevents against intensive corrosion processes. However at higher temperatures, it was shown that the surface chromia can be reduced by reaction with the carbon from the alloy [1] and the bare material can quickly corrode. Chromium appears to be a key element in this surface scale reactivity. Then, quantitative assessment of the surface requires an accurate knowledge of the chromium activity in the temperature range close to the operating conditions (T ≈ 1273 K). High temperature mass spectrometry (HTMS) coupled to multiple effusion Knudsen cells was successfully used to measure the chromium activity in Inconel 617 and Haynes 230 in the 1423- 1548 K temperature range. Appropriate adjustments of the experimental parameters and in-situ calibration toward pure chromium allow to reach accuracy better than ± 5%. For both alloys, the chromium activities are determined. Our experimental results on Inconel 617 are in disagreement with the data published by Hilpert [2]. Possible explanations for the significant discrepancy are discussed.

Thermodynamic Modelling of the Uranium-Carbon-Oxygen System in the Frame of the Uranium Oxide and Carbon Interaction in the TRISO Fuel Particle of High Temperature Reactor

A thermodynamic approach is necessary in order to predict and understand physico-chemical phenomena occurring in nuclear materials under irradiation, involving large chemical systems with a lot of elements including both initial nuclides and fission products (FP). In the frame of thermo-chemical studies of the High Temperature Reactors fuel, a first step is to assess the (U-O-C) system in order to understand the interaction between the UO2 kernel and the pyrocarbon layers constituting such a fuel particle. Our model for irradiated oxide fuel, based on Lindemer’s analysis, has been improved by introducing the (U-O-C) model developed by C. Guéneau & al into the SAGE code. Chemical compositions and related carbon oxides pressures of irradiated TRISO fuel particles have been calculated with the data published by Minato & al. We discuss our results by comparison with their thermochemical calculations and with their experimental observations. This approach can be used to predict the behaviour of complex nuclear materials, especially for the different kind of fuel materials considered in the frame of Gas Fast Reactors.

Condensation of silver–copper alloys in a solid–liquid domain of the phase diagram

Abstract An experimental study of silver–copper alloys condensation with a composition entering a liquid–solid domain of the phase diagram is presented. The vapour beams were produced by two effusion cells heated by tungsten resistors and condensed on a tilted molybdenum substrate, regulated in temperature. Condensation durations were chosen to study the successive stages of the film formation. The microstructures of the quenched condensates showed that the mechanisms were similar in the case of copper rich and silver rich domains. At the beginning, droplets of condensation formed. The surface covered with the alloy increased with time. Later on, coalescences of droplets were observed and a film was established. Both droplets and film contained solid particles whose morphology was nodular. The average nodule radius and the number density of particles in the film were determined by image analysis. It was demonstrated that the cube of the average particle radius grew with time and that the number density of crystals was inversely proportional to time. These results were in good agreement with the Ostwald ripening law.

Condensation and flowing of silver-copper alloys in a solid-liquid domain of the phase diagram

Abstract Silver–copper alloys with a composition entering a liquid–solid domain of the phase diagram are condensed on a tilted molybdenum substrate, regulated in temperature. Before the film regime, droplets of condensation contain nodular crystals, most of which are in contact with the substrate. A heterogeneous nucleation of the solid on the substrate is assumed. The film forms more quickly in the liquid–solid areas than in the liquid domains, which indicates that the crystals may constitute pinning points for the droplets. Moreover, the film/droplets boundary spreads in concordance with the isoconcentration curves, i.e. the solid fraction governs the time of the film formation. A correlation between the condensate thickness and the local solid fraction at the transition between film and droplets is given. At last, in the film areas, the gravity-dependent effect plays an important role. In case of the silver-rich condensate, the solid phase is expected to be more easily driven by the liquid flow.

Safest Roadmap for Corium Experimental Research in Europe

Severe accident facilities for European safety targets (SAFEST) is a European project networking the European experimental laboratories focused on the investigation of a nuclear power plant (NPP) s ...

SAFEST ROADMAP FOR CORIUM EXPERIMENTAL RESEARCH IN EUROPE

SAFEST (Severe Accident Facilities for European Safety Targets) is a European project networking the European corium experimental laboratories with the objective to establish coordination activitie ...