Silicon carbide grain boundary distributions, irradiation conditions, and silver retention in irradiated AGR-1 TRISO fuel particles

Distributions of silicon carbide grain boundary types (random high angle, low angle, and coincident site lattice-related boundaries), were compared in irradiated tristructural isotropic-coated fuel particles from the Advanced Gas Reactor-1 experiment exhibiting high (>80%) and low (<19%) Ag-110m retention. Grain orientation from transmission electron microscope-based precession electron diffraction data, and, ultimately, grain boundary distributions, indicate irradiated particles with high Ag-110m retention correlate with lower relative fractions of random, high-angle grain boundaries. An inverse relationship between the random, high-angle grain boundary fraction and Ag-110m retention was found and is consistent with grain boundary percolation theory. Also, the SiC grain boundary distribution in an irradiated, low Ag-110m retention, Variant 1 particle was virtually identical to that of a previously reported as-fabricated (unirradiated) Variant 1 TRISO particle. Thus, SiC layers with grain boundary distributions associated with low Ag-110m retention may have developed during fabrication and were present prior to irradiation, assuming significant microstructural evolution did not occur during irradiation. Finally, irradiation levels up to 3.6 × 1025 n/m2 and 16.7% fissions per initial metal atom were found to have little effect on association of fission product precipitates with specific grain boundary types in particles exhibiting between 19% and 80% Ag-110m retention.