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The ZrC coating layer is a candidate to replace the SiC coating layer of the Triso-coated fuel particles for high-temperature gas-cooled reactors. Post-irradiation heating tests of the ZrC-Triso coated UO 2 particles were performed at 1800°C for 3000 h and at 2000°C for 100 h to study the release behavior of fission products. The fission gas release monitoring and the X-ray microradiography revealed that no through-coating failure occurred during the heating tests. The high cesium retention of the ZrC-Triso coated fuel particles was confirmed up to 1800°C. The diffusion coefficient for cesium in the ZrC layer was more than two orders smaller than that in the SiC layer at 1800°C. The diffusion coefficient for ruthenium in the ZrC layer was almost the same as that for cesium in the SiC layer.

For fuel particles of the high-temperature gas-cooled reactors, three types of new combinations of the coating layers were invented to prevent the corrosion of the SiC layer by fission products. The idea was that a layer to trap fission products was added inside the SiC layer of the Triso-coating. Two kinds of materials of SiC and SiC with free carbon were selected as additional layers. The fuel particles with these advanced coatings were fabricated to test the effectiveness of the coatings in out-of-reactor and irradiation experiments. These experiments showed that the advanced coatings had good irradiation performance and functioned effectively against corrosion of the SiC layer, although further irradiation tests are needed to completely demonstrate the effectiveness of the advanced coatings.