9.2 A High Strength and High Fracture Toughness C/C Composite Control Rod for High Temperature Gas Cooled Reactors

Fig. 9-4 Conceptual structure of the HTTR and the C/C composite control rod and its control position in the reactor

The C/C composite control rod can be inserted into the high temperature core of the reactor directly under control conditions, which contributes to easy control of the reactor and higher system efficiency.

Fig. 9-5 Load-deformation curve of the C/C composite control rod tube under compressive load

The C/C composite control rod tube shows a quasi plastic deformation under the load, similar to ductile metal materials, whereas graphite materials show brittle fracture.

A refractory metallic control rod is normally used for the HTTR. This control rod is first inserted in the surrounding core, then after into the cooled core, to avoid being damaged by high temperature (>800°C ) and heavy irradiation damage in the core of HTTR.
Carbon/Carbon (C/C) composite material is a very attractive material for high temperature (~1,000°C ) use and it allows easy control of the reactor as the control rod in future HTGRs (High Temperature Gas cooled Reactors).
We succeeded in making C/C composite control rods for the HTTR and the concept is shown in Fig. 9-4. This control rod consists of an outer and inner sheath tube, bolts and joints, which are all made of three-dimensional C/C composite material. New three-dimensional carbon fiber weaving techniques were applied in the manufacture to enforce the control rod structure. In particular, the rod was strengthened in circumferential, radial and axial directions with a high content of carbon fiber, and matrix materials were filled by the chemical vapor impregnation and a partial SiC conversion technique to achieve a high density and strength of the C/C materials. Figure 9-5 shows the typical deformation behavior of the C/C composite control sheath tube under compressive load. Whereas graphite exhibits brittle fracture under the load, C/C composite sheath tubes show a higher strength and stable fracture, similar to ductile metals.
The deformation behavior is compatible with the high safety design of the control rod for the HTGR.

S. Ishiyama et al., Strength Test of C/C Composite High Performance Control Rod for HTGRs, Nippon Genshiryoku Gakkai-Shi, 41(10), 1092 (1999).

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