Revealing abnormal micro- and macro-structure evolution mechanism of porous pyrolytic carbon in TRISO coated fuel particles under high-temperature treatment

Porous pyrolytic carbon (PPyC) serves as a buffer layer in tri-structural isotropic (TRISO) coated particles for nuclear fuel. However, its unique low-density and porous structure leads to unclear high-temperature structure evolution behaviors, which are significantly distinct from those of dense pyrolytic carbon. Herein, PPyC was subjected to heat treatments at temperatures ranging from 1200°C to 1600°C, respectively. Results clearly identify 1400°C as the key structure-performance evolution threshold. Below this temperature, oxygen defect migration dominates structural evolution, while above it, transition zones between amorphous and crystalline domains generate abundant vacancies, and grain boundary pinning restricts atomic motion and raises structural disorder. These changes are accompanied by the reduction of PPyC particle size, reversible open-closed pore conversion mediated by particle swelling and sintering-induced coalescence, and significant layer shrinkage at the macroscale. This work systematically clarifies the high-temperature structural evolution mechanism of PPyC, effectively fills the existing gap in low-density pyrolytic carbon evolution theory, and provides valuable theoretical support for the optimal design of TRISO particles and other high-performance novel functional carbon material.