Characterization of the porosity in TRISO coated fuel particles and its effect on the relative thermal diffusivity

A heat transfer model for a Tri-structural Isotropic (TRISO) coated fuel particle was developed using Image Based Modelling (IBM). Computed X-ray tomography at a resolution of 0.7 μm was used to quantify the porosity of each layer. In order to study the thermal diffusivity of these coatings, an internal heat pulse was simulated in the kernel and the temperature, as a function of time, was measured from the surface of the Outer Pyrolitic Carbon (OPyC). Consequently, the half rise time of the temperature increase was found. The novel idea behind this technique is that once a heat pulse propagates through the particle layers, the half rise time of the temperature can be obtained from different spots on the OPyC surface. The article presents a correlation between the pore size distribution (measured by X-ray tomography) and the relative thermal diffusivity. The average porosities in OPyC (relative to the volume of the ceramic coating) measured using helium pycnometry, mercury intrusion porosimetry and X-ray tomography were 12.3 vol%, 9.0 vol% and 11.1 vol%, respectively.