Understanding of fission products transport in SiC layer of TRISO fuels by nanoscale characterization and modeling

Abstract This paper details recent advances in the understanding of both intergranular and intragranular fission product transport in the silicon carbide (SiC) layer of neutron-irradiated tristructural isotropic (TRISO) coated-fuel particles from Advanced Gas Reactor (AGR)-1 and AGR-2 experiments. The SiC layer is the primary barrier responsible for retaining metallic fission products in the fuel particle. Chemical reactions between the SiC layer and fission products have been observed. Previously, intragranular precipitates rich in fission product Pd were identified as nanosized nodular and columnar features, distributed through the irradiated SiC grains. More recent studies reported hexagonal-shaped Pd silicide at Frank loops and stacking faults, where α-SiC (amid the bulk β-SiC) acts as a precursor for Pd silicide formation. While the Pd silicide precipitates were identified as having L12-Pd3Si stoichiometry from electron diffraction study, ab initio phonon calculations performed in this study suggest that the L12-Pd3Si structure remains dynamically unstable from zero pressure up to at least 100 GPa and will spontaneously transform into other lower-symmetry structures via different unstable phonon modes. Apart from the nanoscale hexagonal morphology of the Pd3Si precipitate, a new form of elongated Pd silicide, confined between a pair of stacking faults, has been observed in a high 110mAg-retention TRISO-coated particle. The peculiar discontinuity of such precipitates along the stacking faults raises questions regarding the necessity of α-SiC precursors for their nucleation, as well as the nature of the intragranular-transport pathway of Pd along the radial direction of SiC. The large-scale fission product precipitates in the inner-pyrolytic carbon (IPyC) and grain boundaries of SiC layers were characterized, and different chemically segregated regions were observed within single fission product precipitates.