Anisotropy of Pyrocarbon Layers in Irradiated TRISO Fuel

The performance of tristructural-isotropic (TRISO) particle fuel for high-temperature gas-cooled reactor (HTGR) applications is largely determined by the response and evolution of the layers during irradiation. An important property in TRISO fuel is the degree of anisotropy within the pyrocarbon layers. On the atomic scale, the structure of the pyrocarbon layers is intrinsically anisotropic. This anisotropy can manifest over larger length scales depending on material processing conditions, which during irradiation can lead to undesirable performance due to anisotropic dimensional changes inducing fracture. Thus, upper limits on the anisotropy within pyrocarbon layers are included within fuel specifications for commercial production of TRISO particles. It is well documented that pyrocarbon layers undergo microstructural evolution during irradiation; however, the change in anisotropy and its effect on performance has never been directly measured experimentally within the Advanced Gas Reactor Fuel Development and Qualification (AGR) Program. Previous modeling efforts have aimed to capture this evolution but must be benchmarked with experimental data. Here, we present the first results quantifying the anisotropy of irradiated AGR TRISO fuel using the Two-Modulated Generalized Ellipsometry Microscope (2-MGEM). Results presented further our understanding of how TRISO fuel responds under irradiation and future work involves building a database of anisotropy as a function of irradiation conditions for input into fuel performance models and tools.

This work was supported by the U.S. Department of Energy, Office of Nuclear Energy.