The functions of the low density PyC layer (first layer) are to protect the inner high density PyC (second layer) layer by adsorbing fission fragments coming from the fuel kernel, and to provide a space for containing FP gas coming from the fuel kernel. The inner high density PyC (second layer) is provided for retention of FP gas. Short half-life of FP gas nuclei and their small diffusion coefficients in the high density PyC layer, as well as retention capability of the fuel kernel, ensure that FP gas release from the fuel is actually no problem as long as the second layer has no defects.

The SiC layer (third layer) has high retention capability for FP gas and metallic FP fragments. Due to the short half-life of these gas nuclei and their small diffusion coefficients, the SiC layer, the third layer has good retention capability of them just as the second layer does. The outer high density PyC layer (fourth layer) compresses the SiC layer externally by shrinking during irradiation. That prevents the SiC layer from experiencing a tensile load due to the internal gas pressure and hence avoids failure of the coated particle fuels.