A Thermodynamic Approach to Predict the Metallic and Oxide Phases Precipitations in Nuclear Waste Glass Melts

AbstractAmong the large number of matrixes explored as hosts for high-level nuclear wastes, conditioning of fission products and minor actinides into a homogeneous borosilicate glass is the most promising technique already implemented at the industrial scale. The advantage of this vitrification process is the volume reduction of the high level waste coming from the spent fuel reprocessing and its stability for the long-term storage. Nevertheless, some fission products are poorly soluble in molten glasses:•Platinoids (Pd, Ru, Rh) which precipitate as (Pd-Te, Ru-Rh) metallic particles and (Rh,Ru)O2 oxide phases with acicular or polyhedral shapes during the vitrification process.•Molybdenum oxide (MoO3) which can form complex molybdates.In order to point out the chemical interactions between the glass and these precipitated phases issuing from the calcinated waste, a thermodynamic approach is developed using the Calphad method. The objective of this work is to calculate thermodynamic properties for complex fission product systems in order to predict the precipitation of platinoids or molybdate phases.This thermodynamic database is being developed on the Mo-Pd-Rh-Ru-Se-Te-O complex system. This flexible tool enables to predict phase diagrams, composition and relative stability of the metallic or oxide precipitated phases as a function of both temperature and oxygen potential in the glass melt

Glass, crystallization and phase separation an insight into glass enriched in MoO3_3

International audienceHighly focused research programs are currently underway in CEA Marcoule to develop and optimize vitrification processes for the production of waste containment glasses. The main issues facing us today are to accommodate new types of waste and higher waste loading, while enhancing the glass quality and increasing the production capacity and robustness of the plants. This requires extensive knowledge of the physical and chemical properties of glassy materials. Academic research on glass melting is conducted to understand the phenomenology behind the formation of the glass and its evolution after cooling from atomic to macroscopic scale. Understanding the physical and chemical properties of glass in the liquid and solid state is a major challenge to allow increased waste loading and the development of new glass formulations. In this paper we illustrate the contribution of this research considering the phases separation and crystallisation process in glass enriched in MoO$_3$. We show in particular how the thermodynamic and stuctural aspects can elucidate the mechanism of incorporating molybdenum oxide in alkali borosilicate glass

High temperature corrosion phenomena during a nuclear severe accident - The corium-concrete interaction

International audienceDuring a severe accident in a nuclear reactor, extreme temperatures may be reached (T>2500 K). In these conditions, the nuclear fuel may react with the Zircaloy cladding and then with the steel vessel, forming a mixture of solid-liquid phases called in-vessel corium. In the worst scenario, this mixture may pierce the vessel and reach the concrete underneath the reactor. Many phenomena take place during MCCI (Molten Core Concrete Interaction) high temperature concrete ablation/decomposition, heat transfer due to gas bubbles agitation, formation of several phases, oxidation of metals, etc. Several studies on the interaction between corium and concrete were published starting from the 1980s; in particular numerous large scale experiments were performed on this subject. Although large scale experiments answer to macro-scale phenomena, such as ablation profile, corium flooding behaviour, coolability of the molten core, density and viscosity evolution, etc., the interpretation of the microstructure of the post-experiments sample is rather challenging. In this framework a campaign of small-scale experiments is ongoing on prototypic corium+concrete system U-Zr-Ca-Si-Al-O. These tests provide useful data for the comprehension of the phenomena occurring during a severe accident, when the molten corium reaches the concrete underneath the damaged steel vessel. It has been observed that depending on the composition of the concrete (more of less rich in SiO$_2$), the final configuration of the ex-vessel corium (i.e., after the in-vessel corium/concrete interaction) can be significantly differen

Thermodynamic assessment of the palladium–selenium (Pd–Se) system

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High Temperature Interaction Between UO2 and Carbon: Application to TRISO Particles for Very High Temperature Reactors

International audienceFor very high temperature reactors, 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 tristructural isotropic particle. The partial pressures of the gaseous carbon oxides formed at the fuel (UO2)-buffer (C) interface leading to the build up of the internal pressure in the particle have to be predicted. A good knowledge of the phase diagram and thermodynamic properties of the uranium-carbon-oxygen (UCO) system is also required to optimize the fabrication process of "UCO" kernels made of a mixture of UO2 and UC2. Thermodynamic calculations using the FUELBASE database dedicated to Generation IV fuels (Gueneau, Chatain, Gosse, Rado, Rapaud, Lechelle, Dumas, and Chatillon, 2005, "A Thermodynamic Approach for Advanced Fuels of Gas Cooled Reactors," J. Nucl. Mater., 344, pp. 191-197) allow predicting the phase equilibria involving carbide and/or oxycarbide phases at high temperature. Very high levels of CO(g) and CO2(g) equilibrium pressures are obtained above the UO2 +/- x fuel in equilibrium with carbon that could lead to the failure of the particle in case of high oxygen stoichiometry of the uranium dioxide. To determine the deviation from thermodynamic equilibrium, measurements of the partial pressures of CO(g) and CO2(g) resulting from the UO2/C interaction have been performed by high temperature mass spectrometry on two types of samples: (i) pellets made of a mixture of UO2 and C powders or (ii) UO2 kernels embedded in carbon powder. Kinetics of the CO(g) and CO2(g) as a function of time and temperature was determined. The measured pressures are significantly lower than the equilibrium ones predicted by thermodynamic calculations. The major gaseous product is always CO(g), which starts to be released at 1473 K. From the analysis of the partial pressure profiles as a function of time and temperature, rates of CO(g) formation have been assessed. The influence of the different geometries of the samples is shown. The factors that limit the gas release can be related to interface or diffusion processes as a function of the type of sample. The present results show the utmost importance of kinetic factors that govern the UO2/C interaction

Experimental and calculation investigation on severe accidents in PWR reactors

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Paper 58148 HIGH TEMPERATURE INTERACTION BETWEEN UO 2 AND CARBON: APPLICATION TO TRISO PARTICLES FOR VERY HIGH TEMPERATURE REACTORS CEA Marcoule -DEN/MAR/DTEC/SGCS/LMAC 30207 Bagnols-sur-Cèze, France

ABSTRACT 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 UO 2 fuel kernel and the surrounding structural materials (C, SiC) that could damage the particle. The formation of the gaseous carbon oxides at the fuel (UO 2 )-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 UO 2 and UC 2 . 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 CO 2(g) pressures, phase diagrams) were performed using the thermodynamic FUELBASE database dedicated to generation IV fuels In a second step, the partial pressures of CO (g) and CO 2(g) resulting from the UO 2 /C interaction have been measured by high temperature mass spectrometry. Two types of samples were used (i) pellets made of a mixture of UO 2 and C powders or (ii) UO 2 kernels disseminated in a carbon bed. The kinetic measurements of the release of CO (g) and CO 2(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 UO 2 / C interaction to determine realistic CO (g) pressure levels inside a TRISO particle or to improve the fabrication process of the "UCO" kernels

Experimental investigation of the radiative properties of the project reactor astrid vessels

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Thermodynamic Assessment of the Fe-Te System. Part II Thermodynamic modelling

International audienceA thermodynamic description of the Fe-Te system modeled via the Calphad method is proposed, based on data published in a preceding publication $Part\ I:\ Experimental\ study$, and that available in literature. End-member formation energies for the phases $\beta$; $\beta'$; $\delta$, $\delta'$ and $\epsilon$; as well as lattice stabilities of FCC and BCC tellurium, have been evaluated via DFT and used in the numerical optimization. The final Gibbs energy models fit thermodynamic and phase diagram data well, and inconsistencies are discussed. The thermodynamic description is then used to evaluate Gibbs energy of formation for selected Fe-Te compounds of interest for the modeling of internal corrosion of stainless steel fuel pin cladding during operation of Liquid Metal-cooled Fast Reactors (LMFR)