Conductivity of UO₂ ceramics: Effect of a weakly to strongly branched pore network up to 500 ^∘C (exp) and numerical simulations beyond

We consider in this study three uranium dioxide ceramics whose porous network varies noticeably according to the manufacturing conditions. This porous network is characterized by a fine and elongated porosity which is made up of inter-granules pores but also of occluded pores which can be spotted at a much smaller scale. Thermal diffusivity measurements have been performed by a flash method on these ceramics at 50^∘C under different atmospheres but also up to 500^∘C under an argon and dihydrogen atmosphere. These measurements have shown marked differences in the thermal conductivities of the three ceramics, in particular a sharp degradation when the open porosity increases is reported. To understand the influence of these two families of porosities on the effective conductivity, a two-scale model has been developed. The effect of the occluded porosity is approximated using the Maxwell [36] model. The effect of the inter-granules porosity is evaluated by full-field numerical simulations performed on synthetic microstructures generated by the optimization process described in Moutin et al. [41]. Comparisons of model predictions to experimental results confirm the predominant role of the inter-granules porosity on the effective conductivity. Besides, it is shown that the Knudsen effect must be taken into account to accurately predict the variations in thermal conductivity of ceramics depending on the gas contained in the inter-granules pores at 50^∘C. Finally, the simulated conductivities of the ceramics are shown to predict temperatures effects up to 500^∘C.