Masatoshi Yamaji

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

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.

Buckling Fracture of Two-Dimensional Carbon-Fiber Reinforced Carbon-Matrix Composite

Due to excellent high temperature resistivity on the strength, carbon fiber reinforced carbon matrix composites are thought to be one of attractive materials in a nuclear engineering field, e.g. the application to control rod sheath and so on in high temperature gas-cooled reactors as well as plasma facing components in fusion reactors. Prevention of the buckling fracture is placed as one of key subjects in the component design of C/C composites. The buckling fracture test was, therefore, carried out using 2-dimensional C/C composite (2D-C/C composite), and fracture surfaces were observed by SEM. From the buckling fracture test, it was found that there are three kinds of fracture modes; i.e. delamination fracture mode of fiber bundles at low aspect-ratio region, shear fracture mode of fiber bundles at middle aspect-ratio region and shear fracture mode of fiber bundles accompanying by fiber buckling at high aspect-ratio region.

Correlation of Microstructure and Compressive Strength of C/C Composite Using X-ray Tomography

For the control rod element of a Very High Temperature Reactor, carbon fiber reinforced carbon matrix composite (C/C composite) is one of the major candidate materials for its high strength and thermal stability. In this study, in order to correlate the microstructure of the C/C composite to its compressive strength, the X-ray tomography was applied to visualize the internal microstructure of the C/C composite. The relationship between change in the compressive strength and that in the microstructure was also investigated. This study showed that the pore distribution in the C/C composite could be confirmed visually and the volume and shape of the pores could be evaluated by the X-ray tomography in three-dimension. Moreover, since the matrix was gradually lost and transverse cracks became large with increasing the oxidation, the bonding strength between fiber bundles became weak and the compressive strength of parallel to lamina decreased.

Basic Concept on Structural Design Criteria for Zirconia Ceramics Applying to Nuclear Components

As an advanced in-core material in high temperature gas-cooled reactors (HTGRs), superplastic ceramics is attractive due to the possibility of the plastic working. For the application to the nuclear fields, the basic concept of design criteria was studied for typical superplastic ceramics, tetragonal zirconia polycrystals containing 3mol% yttria (3Y-TZP). The experimental results on 3Y-TZP showed that it is possible to apply the Weibull weakest-link theory to decide the stress limits in the criteria. The Weibull parameter m was evaluated as 9.5 for the bending and as 26.5 for the compressive. The applicability of the Weibull theory was also verified by the bending test results with different span. Based on the graphite structural design guidelines for the High Temperature Engineering Test Reactor (HTTR), the design stress limits for 3Y-TZP was proposed. It was shown that the proposed stress limits have appropriate safety margin and thought to be effective to evaluate the integrity of in-core structure made of 3Y-TZP.