Fig. 5-9 Cross section of ZrC coated fuel particles held at 1600 degrees cent. for 4500 hrs after irradiation. No deterioration of the particles by the heating was seen.

Fig. 5-10 Cumulative failure fractions of the SiC and ZrC coated fuel particles in postirradiation heating. Failure of the ZrC coated fuel particles was not observed in particles heated up to 2400 degrees cent.

HTGR fuel is composed of refractory materials, such as uranium oxide, silicon carbide, and graphite, so that it safely can be used at around 1300 degrees cent. If the operating temperature of the fuel is increased, the cooling gas temperature increases, and consequently the thermal efficiency of the reactor is improved. This is the premise upon which the advanced reactor system is based. Replacing the SiC coating layer with one of ZrC greatly improved the heat-resisting capability of the coated particle fuel. Figure 5-9 is a photograph of a cross section of the ZrC coated particle that was held at 1600 degrees cent. for 4500 hrs after irradiation. As seen in the photograph, no deterioration of the particle occurred. Thus, the particle kept its integrity. Figure 5-10 shows a cumulative failure fraction of the particles during postirradiation heating. The ZrC coated fuel particles did not fail at temperatures up to 2400 degrees cent. Another experiment revealed that the ZrC coated fuel particles could retain fission products at this high temperature.

Reference K. Fukuda et al., Research and Development of HTTR Coated Particle Fuel , J. Nucl. Sci. Technol., 28(6), 570 (1991).

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