Optical characterization of tristructural isotropic fuel particle cross-sections using generalized ellipsometry

One particularly important measure of quality of tristructural isotropic (TRISO) fuel particles is the degree of preferred orientation of crystallites in the polycrystalline pyrolytic carbon coatings. Excessive crystallographic anisotropy leads to unwanted anisotropic dimensional changes during irradiation that can cause the TRISO coatings to fail. Early optical methods were developed in the 60s and 70s to measure this anisotropy by taking advantage of the large optical anisotropy of graphite. Since that time, there have been significant improvements in both the theoretical understanding and experimental techniques in the understanding of optical anisotropy. Here we discuss a new method, based on the two-modulator generalized ellipsometer (2-MGE) to measure the optical anisotropy. This technique has been demonstrated to measure the optical diattenuation to an accuracy from ±0.001 to ±0.005 and the preferred direction of the crystallites to an accuracy of better than ±2° with a spatial resolution of better than 5 μm. Diattenuation 'pictures' of the nuclear fuel cross-sections reveal that the inner pyrocarbon layer (IPyC) is far from uniform both in the degree of diattenuation and in the direction of the principal axis. The 2-MGE technique is faster, more accurate, and collects considerably more data than previous optical anisotropy measurements of TRISO fuel particles.