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International audience; For the understanding of severe accidents in sodium cooled fast reactors (SFR), it is necessary to understand two prototypic accident scenarios such as ULOF (Unprotected Loss of Flow Accident) and UTOP (Unprotected Transient OverPower). As the base knowledge, it is also important to understand high temperature chemical interaction among major core materials such as MOx fuel (MOx: mixed oxide of uranium and plutonium), steel cladding and B4C neutron absorber have to be investigated. This study aims at providing experimental data on phase formation and phase-stability at various temperature and pressure conditions. A first series of samples containing a mixture of B4C and steel were prepared to obtain a homogenous metallic solid. In a second step, these metallic samples were mixed and melted with small UO2 pieces by arc melting. Then these samples underwent a heat treatment at 1900 °C for 1 hour. EDS, EBSD and EPMA analyses were performed to identify the phases formed during the solidification. In addition, thermodynamic calculations were performed for the interpretation of the results, revealing that a carbo-reduction reaction occurs: UO2 + 2 C = 2 CO + U. A significant amount of uranium from the fuel is dissolved in the metallic liquid phase, leading to the formation of mixed borides (UM3B2, UMB4, UM4B, M=Fe,Cr,Ni). In comparison with the UO2/steel interaction, the present results show that the presence of B and C in the melt improves the wetting behaviour of the metallic liquid towards UO2.

Silicon carbide-silicon carbide (SiC/SiC) composites are considered to replace the current zirconium-based cladding materials thanks to their good behavior under irradiation and their resistance under oxidative environments at high temperature. In the present work, a thermodynamic analysis of the UO 2±x /SiC system is performed. Moreover, using two different experimental methods, the chemical compatibility of SiC towards uranium dioxide, with various oxygen contents (UO 2±x ) is investigated in the 1500–1970 K temperature range. The reaction leads to the formation of mainly uranium silicides and carbides phases along with CO and SiO gas release. Knudsen Cell Mass Spectrometry is used to measure the gas release occurring during the reaction between UO 2+x and SiC powders as function of time and temperature. These experimental conditions are representative of an open system. Diffusion couple experiments with pellets are also performed to study the reaction kinetics in closed system conditions. In both cases, a limited chemical reaction is observed below 1700 K, whereas the reaction is enhanced at higher temperature due to the decomposition of SiC leading to Si vaporization. The temperature of formation of the liquid phase is found to lie between 1850

The first activity measurements of nickel, copper, chromium and iron in industrial nickel-base alloys (Monel®400, Monel®K-500, Nicorros®400, Nicorros®K-500, Hastelloy-X®, Hastelloy-C2000®, model alloy 1181) and steels (Uranus®65 and Uranus®45N) were performed using multiple Knudsen effusion Cell Mass Spectrometry in the temperature range 1483–1593 K. For each constituent, the activity is expressed as: ln(ai) = (A ± δA)/T + (B ± δB). The experimental results were compared with calculated data using the SSOL2 database from Thermo-Calc.

Temperatures of solid/liquid transitions and vanadium thermodynamic activity data are measured in the U–V system to improve the thermodynamic description of the U–V and C–U–V systems. Binary alloys are synthesized from the pure metals in a high vacuum furnace. With that apparatus, both liquidus temperatures and vanadium activities are measured for each sample. During the experiments, the temperature of the samples is monitored with an optical pyrometer. In parallel, the activity of vanadium referred to pure vanadium is measured for xV = 0.18, 0.40 and 0.62 in the (1850 to 2090) K temperature range using high temperature mass spectrometry coupled to a multiple Knudsen cell system. The quenched microstructure of the alloys is analysed by electron microscopy. These new data together with the few ones from the literature are finally used to obtain a consistent set of thermodynamic parameters for the U–V system using the Calphad method.

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.

The alloys Haynes 230 and Inconel 617 are potential candidates for the intermediate heat exchangers (IHXs) of (very) high temperature reactors ((V)-HTRs). The behavior under corrosion of these alloys by the (V)-HTR coolant (impure helium) is an important selection criterion because it defines the service life of these components. At high temperature, the Haynes 230 is likely to develop a chromium oxide on the surface. This layer protects from the exchanges with the surrounding medium and thus confers certain passivity on metal. At very high temperature, the initial microstructure made up of austenitic grains and coarse intra- and intergranular M6C carbide grains rich in W will evolve. The M6C carbides remain and some M23C6 richer in Cr appear. Then, carbon can reduce the protective oxide layer. The alloy loses its protective coating and can corrode quickly. Experimental investigations were performed on these nickel based alloys under an impure helium flow (Rouillard, F., 2007, “Mécanismes de formation et de destruction de la couche d’oxyde sur un alliage chrominoformeur en milieu HTR,” Ph.D. thesis, Ecole des Mines de Saint-Etienne, France). To predict the surface reactivity of chromium under impure helium, it is necessary to determine its chemical activity in a temperature range close to the operating conditions of the heat exchangers (T≈1273 K). For that, high temperature mass spectrometry measurements coupled to multiple effusion Knudsen cells are carried out on several samples: Haynes 230, Inconel 617, and model alloys 1178, 1181, and 1201. This coupling makes it possible for the thermodynamic equilibrium to be obtained between the vapor phase and the condensed phase of the sample. The measurement of the chromium ionic intensity (I) of the molecular beam resulting from a cell containing an alloy provides the values of partial pressure according to the temperature. This value is compared with that of the pure substance (Cr) at the same temperature. These calculations provide thermodynamic data characteristic of the chromium behavior in these alloys. These activity results call into question those previously measured by Hilpert and Ali-Khan (1978, “Mass Spectrometric Studies of Alloys Proposed for High-Temperature Reactor Systems: I. Alloy IN-643,” J. Nucl. Mater., 78, pp. 265–271; 1979, “Mass Spectrometric Studies of Alloys Proposed for High-Temperature Reactor Systems: II. Inconel Alloy 617 and Nimomic Alloy PE 13,” J. Nucl. Mater., 80, pp. 126–131), largely used in the literature.

For Very High Temperature Reactors (V-HTR), the study of the Uranium-Carbon-Oxygen system is of major importance to predict the high temperature behaviour of the TRISO fuel particle. Firstly, the high level operating temperature of the fuel materials in normal and accidental conditions requires studying the possible chemical interaction between the UO2 fuel kernel and the surrounding structural materials (C, SiC) that could damage the particle. The formation of the gaseous carbon oxides at the fuel (UO2 )-buffer (C) interface that leads to the build up of the internal pressure in the particle has to be predicted. Secondly, the U-C-O ternary system is also involved in the fabrication process of “UCO” kernels made of a mixture of UO2 and UC2 . For the fabrication of such mixture of uranium oxide and carbide, the phase diagram and thermodynamic properties of the U-C-O system are necessary to investigate in order to perform adequate heat treatments. For both reasons, a new study of the U-C-O ternary system has been undertaken. Firstly, some thermodynamic calculations (equilibrium CO(g) and CO2(g) pressures, phase diagrams) were performed using the thermodynamic FUELBASE database dedicated to generation IV fuels [1]. The results allow representing the different phase equilibria involving carbide and oxicarbide phases at high temperature. They also show the high level of CO(g) and CO2(g) equilibrium pressures above the UO2±x fuel in equilibrium with carbon which could lead to the failure of the particle for high oxygen stoichiometry of the uranium dioxide. In a second step, the partial pressures of CO(g) and CO2(g) resulting from the UO2 /C interaction have been measured by high temperature mass spectrometry. Two types of samples were used (i) pellets made of a mixture of UO2 and C powders or (ii) UO2 kernels disseminated in a carbon bed. The kinetic measurements of the release of CO(g) and CO2(g) lead to measured pressures that are lower than the equilibrium pressures predicted from thermodynamic calculations. This discrepancy can be explained by limitations due to distinct kinetic mechanisms. Rates of CO(g) formation have been established taking into account the oxygen stoichiometry of uranium oxide and temperature. The major gaseous product is always CO(g) which release significantly starts at 1473 K. The influence of the different geometries is shown. The limitative kinetic step can be an interface or a diffusion process as a function of the type of sample. These results underline the up most importance of kinetic factors for studying the UO2 / C interaction to determine realistic CO(g) pressure levels inside a TRISO particle or to improve the fabrication process of the “UCO” kernels.Copyright © 2008 by ASME

Yellow cake is a commonly used name for powdered uranium concentrate, produced with the uranium ore. It is the first step in the fabrication of nuclear fuel. As it contains fissile material its circulation needs to be controlled in order to avoid proliferation. In particular there is an interest in onsite determination of the geographical origin of a sample. The yellow cake elemental composition depends on its production site and can therefore be used to identify its origin. In this work laser-induced breakdown spectroscopy (LIBS) associated with chemometrics techniques is used to discriminate yellow cake samples of different geographical origin. 11 samples, one per origin, are analyzed by a commercial equipment in laboratory experimental conditions. Spectra are then processed by multivariate techniques like Principal Components Analysis (PCA) and Soft Independent Modeling of Class Analogy (SIMCA). Successive global PCAs are first performed on the whole spectra and enable one to discriminate all samples. The method is then refined by selecting several emission lines in the spectra and by using them as input data of the chemometric treatments. With a SIMCA model applied to these data a rate of correct identification of 100% is obtained for all classes. Then to define the specifications of a future onsite LIBS system, the use of a more compact spectrometer is simulated by a numerical treatment of experimental spectra. Simultaneously the reduction of spectral data used by the model is also investigated to decrease the spectral bandwidth of the measurement. The rate of correct identification remains very high. This work shows the very good ability of SIMCA associated with LIBS to discriminate yellow cake samples with a very high rate of success, in controlled laboratory conditions.

The chemical compatibility at high temperature between the fuel kernel (U,Pu)C and SiC cladding, the reference materials for the GFR reactor, is studied. For that purpose, a thermodynamic database on the U-Pu-C-Si system was developed with the Calphad method to calculate the phase diagrams. Differential thermal analysis experiments were performed to measure phase transition temperatures in Si-U and C-Si-U systems. According to the calculated isopleth section between the hyperstoichiometric uranium carbide UC1.02 and SiC, the materials shall not react below 2056 K, the temperature at which a liquid phase shall form. These calculations are in good agreement with two chemical compatibility tests performed at 1873 K and 2073 K between the materials. Calculations were also performed to study the chemical interaction between the mixed carbide (U,Pu)C1.04 and SiC. The presence of plutonium in the fuel kernel lowers the liquid formation temperature of 167 K.