Determining the Effect of Grain Size on the Microstructure and Oxidation of Nuclear Graphite

ABSTRACT Fine‐grained nuclear graphite is a key material in high‐temperature gas‐cooled reactors (HTGRs). During air ingress accidents, core graphite components undergo severe oxidation, threatening structural integrity. Therefore, understanding the oxidation behavior of nuclear graphite is essential for reactor safety. The influence of oxidation involves multiple factors, including temperature, sample size, oxidant, impurities, filler type and size, etc. The size of the filler particles plays a crucial role in this study. Five ultrafine‐ and superfine‐grained nuclear graphite samples (5.9–34.4 μm) are manufactured using identical raw materials and manufacturing processes. Isothermal oxidation tests conducted at 650°C–750°C are used to study the oxidation behavior. Additionally, comprehensive characterization is performed to analyze the crystal structure, surface morphology, and nanoscale to microscale pore structure of the samples. Results indicate that oxidation behavior cannot be predicted solely based on filler grain size. Reactive site concentration, characterized by active surface area, dominates the chemical reaction kinetics, whereas pore tortuosity, quantified by the structural parameter Ψ , plays a key role in regulating oxidant diffusion. These findings clarify the dual role of microstructure in oxidation mechanisms and establish a theoretical and experimental basis for the design of high‐performance nuclear graphite capable of long‐term service in high‐temperature gas‐cooled reactors.