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Research Papers: Nuclear Power

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

[+] Author and Article Information
Stéphane Gossé, Christine Guéneau, Thierry Alpettaz, Sylvie Chatain

DEN/DANS/DPC/SCP/LM2T, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France

Christian Chatillon

SIMAP, CNRS, ENSEEG BP75 Grenoble, 38402 Saint-Martin d’Hères Cedex, France

J. Eng. Gas Turbines Power 132(1), 012903 (Oct 01, 2009) (9 pages) doi:10.1115/1.3098430 History: Received November 30, 2008; Revised December 12, 2008; Published October 01, 2009

## Abstract

For 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 (Guéneau, Chatain, Gossé, 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.

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## Figures

Figure 1

Variation in CO pressure as a function of the O/U ratio near the stoichiometry of UO2

Figure 2

Calculated UCO isothermal section at 1473 K

Figure 3

Calculated UCO isothermal section at 1873 K

Figure 4

Critical review of CO(g) pressure from in the UO2-UC2-C three phase domain

Figure 5

Critical review of CO(g) pressure in the UO2-UC2-UC three phase domain

Figure 6

Cell block with multiple effusion Knudsen cells

Figure 7

Measurement of CO(g) and CO2(g) pressures versus time for the UO2+C pellets at 1573 K and 1623 K

Figure 8

CO(g) and CO2(g) pressure measurements versus time. Illustration of the latency time before the pressure increase due to the interfacial reaction (T=1473 K).

Figure 9

Application of Spencer’s model (Eq. 3) to the experimental degree of conversion (ξ) of the UO2+C reaction at 1473 K, 1573 K, 1606 K, and 1623 K. The bold straight lines represent the domain of validity of the interface model.

Figure 10

Application of Zhuravlev’s model (Eq. 6) to the experimental degree of conversion (ξ) of the UO2+C reaction at 1573 K, 1606 K, and 1623 K. The bold straight lines represent the domain of validity of the diffusion model.

Figure 11

CO(g) and CO2(g) releases from ionic intensities measurements by high temperature mass spectrometry at 1673 K

Figure 12

Measurement of CO(g) and CO2(g) pressures versus time for the UO2 kernels in a carbon black bed at 1523 K

Figure 13

Measurement of CO(g) and CO2(g) pressures versus time for the UO2 kernels in a carbon bed 1673 K

Figure 14

Application of Valensi’s model (Eq. 7) to the experimental degree of conversion (ξ) of the reaction between UO2 kernels and carbon black at 1573 K. The right scale represents the degree of conversion (ξ).

Figure 15

Application of Valensi’s model (Eq. 7) to the experimental degree of conversion (ξ) of the reaction between UO2 kernels and carbon black at 1673 K

Figure 16

Application of Zhuravlev’s model (Eq. 6) to the experimental degree of conversion (ξ) of the reaction between UO2 kernels and carbon black at 1623 K. The right scale represents the degree of conversion (ξ).

Figure 17

View of the UO2 kernels by scanning electron microscopy after heat treatments at 1623 K

Figure 18

X-ray diffractogram of UO2 kernels in a carbon bed after the 1623 K experiment (the UC phase and the UO2 phase are shown)

Figure 19

View of a nonuniform profile of a UO2 kernels by SEM after heat treatments at 1673 K (UO2 is the dark phase and UC is the light phase)

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