[en] Highlights: • A model of gas-induced effective expansion strain rate for porous carbon materials in TRISO particles is developed. • The gas-induced effective expansion strain is mainly attributed to the irradiation creep contribution of frame materials. • The effects of irradiation temperature, irradiation conditions, initial porosity and kernel materials are investigated. • TRISO particles with UN kernels turn out to be superior to those with UO₂ kernels. • The materials easy to experience creep are good candidates for frame materials of the buffer layer. - Abstract: The application of fully ceramic microencapsulated (FCM) fuels to light water reactors has attracted increasing attention. As the main storage space for fission gases and some other gases in FCM fuels, the microstructure and thickness of porous carbon buffer layer should be optimally designed. In this study, a model of gas-induced effective expansion strain rate for porous carbon materials is developed based on meso-mechanical method and homogenization theory. By combining theoretical analysis with finite element (FE) simulation, the resulting model is verified, which considers the irradiation-induced creep deformation of dense PyC skeleton, the gas contained in the pores of the buffer layer, the accumulated effective volumetric strain and the external pressure. The effects of temperature, fission rate, kernel materials, initial porosity, external hydrostatic pressure and creep coefficient of dense PyC skeleton are investigated. This work is expected to provide a theoretical reference for optimization design of a porous carbon buffer layer, and the developed model can be used for further simulation of irradiation-induced thermo-mechanical behavior in FCM fuels.