Investigation of Microstructural Change by X-ray Tomography and Anisotropic Effect on Thermal Property of Thermally Oxidized 2D-C/C Composite for Very High Temperature Reactor

Abstract

Two-dimensional carbon fiber reinforced carbon composite (2D-C/C composite) is one of the candidate materials for reactor internals, e.g., control rod element, of Very High Temperature Reactor (VHTR) because of its high strength at high temperature and thermal stability. From the viewpoint of its application to the reactor internals of the VHTR, it is important to investigate the anisotropic effect on its properties for the design and safety analysis of the VHTR. Since the properties of the 2D-C/C composite are strongly dependent on its microstructure, it is necessary to observe its microstructural variations to correlate to the changes in its properties. This study has shown that X-ray tomography can be applied to observe the internal microstructural change of the thermally oxidized 2D-C/C composite. The relationship between the change in properties, including the thermal conductivity, coefficient of thermal expansion (CTE), and burn-off of the thermally oxidized 2D-C/C composite, can be expressed using the empirical exponential decay formula in both directions perpendicular and parallel to the lamina. The direction of the hexagonal graphite crystal structure from the carbon atoms and the microstructure of the 2D-C/C composite can explain not only the relationship between changes in the thermal conductivity, CTE, and burn-off but also the difference in the changes in the thermal conductivity and CTE between fiber directions.