Fig. 5-1 Relationship between load and displacement of C/C composite pellet holder (load along the axial direction of PAN- and pitch-fiber holder) The figure indicates that deformation progresses by almost constant load after the maximum load. It is understood that the carbon fiber in composite material, like reinforced concrete, prevents rapid crack propagation; specifically, the holder dose not break suddenly.

The materials for the control rods of the High Temperature Engineering Test Reactor (HTTR) are required to be stable at very high temperatures. Heat resisting alloy, Alloy 800H, was selected for this application. The graphite core of the HTTR provides structural stability at very high temperatures. If all materials in the control rod were heat resisting ceramic materials, the high temperature gas cooled reactor core would be provided with an additional margin of safety. Research and development on carbon/carbon (C/C) composite material, which would show better performance at very high temperatures, was performed. The composite material consists of carbon and carbon fiber that is made by a carbonization process of pitch or plastic (polyacrylonitrile: PAN) fibers at 3000 degrees cent. The control rod contains boroncarbide pellets, the neutron absorber, in a holder. As the pellet holder is required to have good structural integrity for reactor safety, material strength tests were performed. Figure 5-1 shows load-displacement curves of a pellet holder made of PAN- and pitch-based composite materials. It is clear that stable fracture with large deformation is observed for both materials after the peak load. These results indicate that the carbon fiber distributed in the composite material prevents rapid crack propagation and that the properties of composite material are suitable for the pellet holder.

Reference E. Eto et al., Development of Carbon/Carbon Composite Control Rod for HTTR (1) - Preparation of Elements and Their Fracture Tests, JAERI-Research 96-043 (1996).

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