Evolution of the microstructure of superfine grain graphites under thermal oxidation

• Published in: Nuclear engineering and design 2023-09, Vol.411, p.112421, Article 112421
• Main Authors: Tzelepi, Athanasia, McGladdery, James, Lo, I-Hsuan, Copeland, Glen
• Format: Article
• Language: English
• Subjects: Fluorescent; Image analysis; Microscopy; Polarised; Pycnometry
• ISSN: 0029-5493; 1872-759X
• DOI: 10.1016/j.nucengdes.2023.112421

Fluorescent micrographs of the cross-sections of IG-430 (left), MA (middle) and MB (right) nuclear graphite samples, oxidised at 700 °C, showing the penetration depth of the oxidation: [Display omitted] •Use of fluorescent microscopy to identify differences in the open and closed porosity of superfine grain nuclear graphites.•Use of polarised microscopy to identify differences in the texture.•Investigation into the open and closed pore evolution of thermally oxidised nuclear graphites.•Potential links between microstructure features and oxidation rates. Nuclear graphite is used as a moderator and structural material in a number of Gen-IV reactor designs, such as the (Very) High Temperature Reactors. Chronic or acute thermal oxidation affects the mechanical properties of the graphite components and the impact on the structural integrity of the core depends on the evolution of the microstructure during oxidation. This study qualitatively examined the microstructure of four superfine grain graphites using polarised and fluorescent optical microscopy. These graphites had similar physical properties but notable differences in the oxidation rate at 700 °C. Using fluorescent microscopy, it was possible to distinguish between open and closed porosity in graphite, which is useful in understanding the progression of thermal oxidation and penetration depth. Based on the samples in this work, fine open and closed porosity may be linked to increased surface oxidation and relatively high oxidation rate, whereas high penetration depth may be linked to relatively low open porosity consisting mainly of large pores. Fairly large domains of coherent crystallite orientation, perhaps due to the presence of agglomerates, may also indicate lower oxidation rates at 700 °C. This paper demonstrated that fluorescent microscopy combined with image analysis, is a useful tool in understanding the progression of thermal oxidation and the corresponding effect on the microstructure.