Nuclear graphite has been used in nuclear reactors for almost 80 years, its material behavior after oxidation and irradiation is still not fully understood. In this paper, an old question of why the oxidation has a considerable effect on Young’s modulus but not on the coefficient of thermal expansion (CTE) is tried to be answered. The multi-scale finite-element method was used to analyze CTE dependence on various porosities, pore shape ratios and pore arrangements. The numerical results suggested that these factors have no significant influence on the CTE. Three different types of nuclear graphite samples were thermally oxidized at 525 °C. The microstructures of the graphite samples before and after oxidation were observed by using a laser scanning confocal microscope. The changes in sample volume, weight loss and CTE were measured after each oxidation period. The experimental results suggested that thermal oxidation at 525 °C was controlled by the in-pore diffusion-controlled regime and weight losses of up to 30% had no significant influence on CTE.

中文翻译：

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热氧化对核石墨热膨胀系数的影响

核石墨已在核反应堆中使用了近 80 年，其氧化和辐照后的材料行为仍未完全了解。在本文中，试图回答一个古老的问题，即为什么氧化对杨氏模量有相当大的影响，但对热膨胀系数 (CTE) 没有影响。多尺度有限元方法用于分析 CTE 对各种孔隙率、孔隙形状比和孔隙排列的依赖性。数值结果表明这些因素对 CTE 没有显着影响。三种不同类型的核石墨样品在 525 °C 下进行热氧化。采用激光扫描共聚焦显微镜观察氧化前后石墨样品的微观结构。样本量的变化，在每个氧化期后测量重量损失和 CTE。实验结果表明，525 °C 的热氧化受孔内扩散控制机制的控制，重量损失高达 30% 对 CTE 没有显着影响。