Evaluating ellipsometry as a method to quantitatively measure anisotropy in graphite matrix fuel forms

The Xe-100 is a high temperature gas-cooled reactor design proposed by X-energy that uses spherical graphite matrix pebbles embedded with tristructural isotropic (TRISO) particles as the fuel element. The fuel quality control plan is extensive and includes measurements on the fuel kernel, TRISO particle, and pebble fuel form. One specification listed in the quality control plan restricts the level of anisotropy within the graphite matrix of the pebble. The matrix is a mixture of natural graphite, synthetic graphite, and resin. Graphite is inherently anisotropic; therefore, the pebble manufacturing process uses isostatic pressing to produce random orientation of the graphite flake and near-isotropic thermal properties in the final fuel form. The anisotropy specification is stated as a maximum value of the ratio of the coefficient of thermal expansion (CTE) in the parallel to perpendicular directions, with respect to the pebble mold loading direction. The matrix anisotropy is measured by machining samples from a graphite matrix-only pebble in both directions and testing the CTE’s in a push-rod dilatometer. Ellipsometry is currently used in the TRISO particle quality control plan to measure the anisotropy of the pyrolytic carbon layers. Ellipsometry has also been used in recent years at Oak Ridge National Laboratory to observe the anisotropy of graphite matrix samples and has successfully provided qualitative texture analysis in compacts and pebbles. The objective of this work was to evaluate the use of ellipsometry as an alternative method to quantitatively measure the graphite matrix anisotropy specification for pebble fuel. Graphite matrix-only samples were fabricated using uniaxial and isostatic pressing to target varying levels of anisotropy. The samples were measured with a Two Modular Generalized Ellipsometer Microscope and a dilatometer in the parallel and perpendicular directions. The results provide a relationship between the CTE (dilatometry) and the diattenuation (ellipsometry) values with increasing graphite matrix anisotropy. Measurements were also made on a Xe-100 prototypical graphite matrix-only pebble and an Advanced Gas Reactor Fuel Development and Qualification Program compact and benchmarked against thermal conductivity results to further demonstrate feasibility of this method for future use.