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.

Keywords

diffusion / nuclear graphite / oxidation / pore structure / reaction rate

References

[1] J. Gao, S. Lv, Y. Zhao, D. Chen, and Z. Li, “Simulation of Primary Knock-On Atom Distribution in HTGR Graphite under Neutron and Ion Irradiation,” Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 492 (2021): 15–22.

[2] A. A. Campbell, A. Selby, N. Cetiner, et al., “Materials Property Changes in ETU-10 Graphite Due to Neutron Irradiation at Elevated Temperatures,” Carbon 238 (2025): 120178.

[3] Y. J. Cho, D. Garcia, H. Z. Yu, Z. Deng, L. Li, and K. Lu, “Oxidation Behaviors of Matrix-Grade Graphite During Water Vapor Ingress Accidents for High Temperature Gas-Cooled Reactors,” Carbon 185 (2021): 161–176.

[4] B. A. Brigham, K. I. Montoya, T. J. Gerczak, and E. S. Sooby, “Determination of Oxidation Rates and Volatile Oxidation Products for HTGR Graphite Matrix Material Exposed to Steam Atmospheres,” Journal of Nuclear Materials 557 (2021): 153256.

[5] Z. Hu, Z. Li, D. Chen, W. Miao, and Z. Zhang, “CO₂ Corrosion of IG-110 Nuclear Graphite Studied by Gas Chromatography,” Journal of Nuclear Science and Technology 51, no. 4 (2014): 487–492.

[6] G. Zheng, P. Xu, K. Sridharan, and T. Allen, “Characterization of Structural Defects in Nuclear Graphite IG-110 and NBG-18,” Journal of Nuclear Materials 446, no. 1–3 (2014): 193–199.

[7] C. I. Contescu, T. Guldan, P. Wang, and T. D. Burchell, “The Effect of Microstructure on Air Oxidation Resistance of Nuclear Graphite,” Carbon 50, no. 9 (2012): 3354–3366.

[8] K. L. Jones, G. M. Laudone, and G. P. Matthews, “A Multi-Technique Experimental and Modelling Study of the Porous Structure of IG-110 and IG-430 Nuclear Graphite,” Carbon 128 (2018): 1–11.

[9] N. Zhu, Y. Yang, Y. Li, Y. Bai, J. Rong, and C. Wu, “Carbon-Based Interface Engineering and Architecture Design for High-Performance Lithium Metal Anodes,” Carbon Energy 6, no. 1 (2024): e423.

[10] H. Wu, R. Gakhar, A. Chen, S. Lam, C. P. Marshall, and R. O. Scarlat, “Comparative Analysis of Microstructure and Reactive Sites for Nuclear Graphite IG-110 and Graphite Matrix A3,” Journal of Nuclear Materials 528 (2020): 151802.

[11] B. Lin, L. Chen, G. Liu, and Y. Shen, “Experimental Study on the Oxidation Behavior of Nuclear Graphite IG-110,” Journal of Nuclear Materials 608 (2025): 155712.

[12] Z. Cheng, Z. Luo, H. Zhang, et al., “Targeted Regeneration and Upcycling of Spent Graphite by Defect-Driven Tin Nucleation,” Carbon Energy 6, no. 4 (2024): e395.

[13] J. J. Kane, A. C. Matthews, C. J. Orme, C. I. Contescu, W. D. Swank, and W. E. Windes, “Effective Gaseous Diffusion Coefficients of Select Ultra-Fine, Super-Fine and Medium Grain Nuclear Graphite,” Carbon 136 (2018): 369–379.

[14] M. Jiang, J. Ell, H. Barnard, et al., “On the Reduced Damage Tolerance of Fine-Grained Nuclear Graphite at Elevated Temperatures Using In Situ 4D Tomographic Imaging,” Carbon 222 (2024): 118924.

[15] Z. He, J. Song, Z. Wang, X. Guo, Z. Liu, and T. J. Marrow, “Comparison of Ultrafine-Grain Isotropic Graphite Prepared From Microcrystalline Graphite and Pitch Coke,” Fuel 290 (2021): 120055.

[16] R.-C. Li, Q. Huang, X.-L. Yao, H. Zhou, and X.-T. Zhou, “Pore Structure Development of Oxidized Nuclear Graphite,” Journal of Nuclear Materials 601 (2024): 155342.

[17] S.-H. Chi and G.-C. Kim, “Comparison of the Oxidation Rate and Degree of Graphitization of Selected IG and NBG Nuclear Graphite Grades,” Journal of Nuclear Materials 381, no. 1–2 (2008): 9–14.

[18] W.-H. Huang, S.-C. Tsai, C.-W. Yang, and J.-J. Kai, “The Relationship Between Microstructure and Oxidation Effects of Selected IG- and NBG-Grade Nuclear Graphites,” Journal of Nuclear Materials 454, no. 1–3 (2014): 149–158.

[19] J. J. Lee, T. K. Ghosh, and S. K. Loyalka, “Comparison of NBG-18, NBG-17, IG-110 and IG-11 Oxidation Kinetics in Air,” Journal of Nuclear Materials 500 (2018): 64–71.

[20] R. E. Smith, J. J. Kane, and W. E. Windes, “Determining the Acute Oxidation Behavior of Several Nuclear Graphite Grades,” Journal of Nuclear Materials 545 (2021): 152648.

[21] M. R. Ammar, N. Galy, J. N. Rouzaud, et al., “Characterizing Various Types of Defects in Nuclear Graphite Using Raman Scattering: Heat Treatment, Ion Irradiation and Polishing,” Carbon 95 (2015): 364–373.

[22] M. Jiang, K. Ammigan, G. Lolov, F. Pellemoine, and D. Liu, “A Novel Method for Quantifying Irradiation Damage in Nuclear Graphite Using Raman Spectroscopy,” Carbon 213 (2023): 118181.

[23] Z. Zhou, W. G. Bouwman, H. Schut, T. O. van Staveren, M. C. R. Heijna, and C. Pappas, “Influence of Neutron Irradiation on the Microstructure of Nuclear Graphite: An X-Ray Diffraction Study,” Journal of Nuclear Materials 487 (2017): 323–330.

[24] S. Lv, J. Gao, Y. Jin, et al., “The Structure Evolution in Neutron-Irradiated Nuclear Graphite and Post-Annealing,” Radiation Physics and Chemistry 197 (2022): 110156.

[25] T. Grebennikova, I. Ipatova, D. N. T. Barton, et al., “Evolution of Irradiated Pile Grade A Graphite Microstructure under Novel Molten Salt Decontamination Conditions,” Journal of Nuclear Materials 615 (2025): 155935.

[26] M. A. Ban̄ares-Mun̄oz, L. V. F. González, and J. M. M. Llorenta, “Adsorption Isotherms of Nitrogen and Argon on an ‘Agot’ Grade Artificial Nuclear Graphite at 77 and 90 K,” Carbon 25, no. 5 (1987): 603–608.

[27] N. C. Gallego, J. D. Arregui-Mena, and C. I. Contescu, “Probing Basal Planes and Edge Sites in Polygranular Nuclear Graphite by Gas Adsorption: Estimation of Active Surface Area,” Carbon 179 (2021): 633–645.

[28] Y. Mu, C. Huang, Y. Chu, et al., “Revolutionizing High-Areal-Capacity Silicon Anodes With a Multi-Level Carbon Construction Strategy for Practical Li-Ion Batteries,” Carbon Energy 7, no. 6 (2025): e702.

[29] D. Ren, J. Xu, S. Su, et al., “Characterization of Internal Pore Size Distribution and Interconnectivity for Asphalt Concrete With Various Porosity Using 3D CT Scanning Images,” Construction and Building Materials 400 (2023): 132751.

[30] J. J. Kane, A. C. Matthews, W. D. Swank, and W. E. Windes, “Effects of Air Oxidation on the Evolution of Surface Area Within Nuclear Graphite and the Contribution of Macropores,” Carbon 166 (2020): 291–306.

[31] J.-L. Su and D. D. Perlmutter, “Effect of Pore Structure on Char Oxidation Kinetics,” AIChE Journal 31, no. 6 (1985): 973–981.

[32] S. K. Bhatia and D. D. Perlmutter, “A Random Pore Model for Fluid-Solid Reactions: I. Isothermal, Kinetic Control,” AIChE Journal 26, no. 3 (1980): 379–386.

[33] C. Fouard, G. Malandain, S. Prohaska, and M. Westerhoff, “Blockwise Processing Applied to Brain Microvascular Network Study,” IEEE Transactions on Medical Imaging 25, no. 10 (2006): 1319–1328.

[34] R. M. Paul, J. D. Arregui-Mena, C. I. Contescu, and N. C. Gallego, “Effect of Microstructure and Temperature on Nuclear Graphite Oxidation Using the 3D Random Pore Model,” Carbon 191 (2022): 132–145.

[35] E. Loren Fuller and J. M. Okoh, “Kinetics and Mechanisms of the Reaction of Air With Nuclear Grade Graphites: IG-110,” Journal of Nuclear Materials 240, no. 3 (1997): 241–250.

[36] M. S. El-Genk and J.-M. P. Tournier, “Comparison of Oxidation Model Predictions With Gasification Data of IG-110, IG-430 and NBG-25 Nuclear Graphite,” Journal of Nuclear Materials 420, no. 1 (2012): 141–158.

[37] K. Shen, K. Xu, S. Yu, and F. Kang, “The Optical Texture of PGA, Gilsocarbon, NBG-18, and IG-110 Nuclear Graphite,” Journal of Nuclear Materials 552 (2021): 153013.

[38] K. Shen, S. Yu, and F. Kang, “The Microstructure and Texture of Gilsocarbon Graphite,” Carbon 153 (2019): 428–437.

[39] M. S. Jouini, F. Bouchaala, M. K. Riahi, M. Sassi, H. Abderrahmane, and F. Hjouj, “Multifractal Analysis of Reservoir Rock Samples Using 3D X-Ray Micro Computed Tomography Images,” IEEE Access 10 (2022): 67898–67909.

[40] T. Zeng, J. Fu, P. Cong, X. Liu, G. Xu, and Y. Sun, “Research on Ring Artifact Reduction Method for CT Images of Nuclear Graphite Components,” Journal of X-Ray Science and Technology: Clinical Applications of Diagnosis and Therapeutics 33, no. 2 (2025): 317–324.

RIGHTS & PERMISSIONS

2026 The Author(s). Carbon Energy published by Wenzhou University and John Wiley & Sons Australia, Ltd.