Thermal Oxidation of Nuclear Graphite and Pyrolytic Carbon Coatings

Abstract

The oxidation of pyrolytic carbon (PyC) deposited via fluidized bed chemical vapor deposition was characterized and compared with that of standard nuclear-grade graphite. The materials were heated at 700 to 1,000 °C in a thermogravimetric analysis system under 20% v/v O2 flow, allowing for direct comparison of dynamic oxidative mass change in each material. Three different PyC samples fabricated under different conditions exhibited variation in total mass loss and mass loss rate, varying by as much as 709 mg/cm2 in total mass loss and 14.2 (mg/cm2)/min in mass loss rate at a single temperature. These variations highlight the correlation between PyC microstructure/defect density and oxidation susceptibility. Additionally, changes in the microstructure and composition between PyC and graphite were characterized via scanning electron microscopy and correlated to the mass loss results. The results of this work have implications toward the safety of tristructural isotropic (TRISO) and other coated particle fuels, especially under off-normal conditions, given the limited information that exists about the oxidation behavior of PyC.

Keywords: Oxidation, microstructure, carbon, thermogravimetric analysis, scanning electron microscopy