Revealing Abnormal Micro- and Meso-Structure Evolution Mechanism of Porous Pyrolytic Carbon in TRISO Coated Fuel Particles under High-Temperature Treatment

• • Critical transition at 1400 °C: Below this temperature, structural ordering occurs (ID/IG decreases, sp2 content increases), while above it, disorder increases (ID/IG increases, diffraction peak broadens). This demarcation is essential for predicting PPyC behavior in reactor operation. • • Lattice parameter shifts: At 1400 °C, a=2.4193 Å and c=3.4911 Å (most ordered); at 1600 °C, a=2.4347 Å (more disordered in a-axis) and c=3.4853 Å (more ordered in c-axis). These anisotropic changes indicate differential atomic migration along crystallographic axes, affecting dimensional stability. • • Porosity evolution: Below 1400 °C, closed porosity increases while mesopore volume decreases, indicating open-to-closed pore conversion. Above 1400 °C, closed porosity decreases and mesopore surface area increases, suggesting partial reopening of pores. This directly impacts fission gas retention and mechanical integrity. • • Onion-like carbon (OLC) with amorphous-core/crystalline-shell growth mode discovered, with three transformation stages. This nanostructural feature explains the anomalous volume expansion vs. thickness shrinkage contradiction, providing a mechanistic basis for optimizing TRISO fuel performance.