*2.1. Governing Equation*

COMSOL multi-physics software o ffered the heat transfer model for the solid materials to calculate the temperature distribution in FCM and TRISO fuel. The temperature field of FCM pellet and TRISO fuel are determined by the heat conduction equation:

$$
\rho \mathbf{C}\_p \frac{\partial T}{\partial t} + \nabla \cdot \boldsymbol{q} - E\_f \dot{\mathbf{F}} = \mathbf{0} \tag{1}
$$

where *T*, ρ and *Cp* are the temperature, density (kg/m3) and heat capacity (J/(kg·K)) of the solid material, respectively. *Ef* and . *F* are the energy released in a single fission event and volumetric fission rate, respectively. The heat flux can be written as follows:

$$q = -k\nabla T.\tag{2}$$

The stress in all coated layers and the SiC matrix was caused by the accumulation of internal pressure and the deformation of other materials. The internal pressure of TRISO fuel was caused by fission gas release. The TRISO particle located in a di fferent part of the FCM pellet possessed a di fferent internal pressure caused by the temperature gradient of the FCM pellet [9]. The deformation of the coated layers and SiC matrix was caused by the internal and outer pressure, thermal expansion, irradiation and creep strain. The strain tensor of FCM was written as follows:

$$
\kappa = \varepsilon^{\varepsilon} + \varepsilon^{T} + \varepsilon^{i} + \varepsilon^{c} \tag{3}
$$

where ε is the strain, ε*e* , <sup>ε</sup>*T*, ε*i* and ε*c* represent the elastic strain, thermal expansion strain, irradiation strain and creep strain.
