The effect of thermal oxidation on the coefficient of thermal expansion of nuclear graphite

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.