The initial crystalline anisotropy and density of the outer pyrocarbon coating of a two-layer Biso fuel particle have a very large effect on calculated diametral changes and coating failure probabilities during irradiation. Of the two, the anisotropy is by far the most important parameter affecting the coating failure probability. These effects arise because of variations in the irradiation-induced dimensional changes, irradiation-induced changes in crystalline anisotropy, and mechanical properties of pyrolytic carbons with these two structural parameters. In support of the stress-analysis model used for these calculations, predicted diametral changes of Biso-coated particles agree well with those observed in irradiation experiments.