Abstract: Fuel elements in nuclear reactors are subjected to complex thermomechanical conditions during service, especially under high burnup and high power operations. Traditional models, such as FuSPAC, adopt a rigid-body assumption for fuel pellets, which limit their ability to accurately describe the actual mechanical behavior under extreme conditions. To address this limitation, an improved nonlinear mechanical model, named FPESPAC, was developed based on the finite difference method. This model incorporates key physical phenomena including elastic-plastic deformation, creep, thermal expansion, and irradiation-induced swelling, thereby providing a more realistic description of stress-strain evolution in fuel pellets. The FPESPAC model was formulated under the generalized plane strain assumption and solved using an incremental finite difference approach. Material behavior was characterized by the von Mises yield criterion with ideal plasticity, and the plastic strain evolution follows the Prandtl-Reuss flow rule. Temperature- and time-dependent material properties, derived from experimental data, were employed to represent fuel performance under realistic reactor conditions. Internal pressure and appropriate boundary conditions were applied to simulate operational loads. The model was validated by comparing its predictions with both analytical solutions and the original FuSPAC code. The results demonstrate that FPESPAC significantly improves the accuracy of fuel pellet deformation and stress prediction. It successfully captures the elastic-plastic transition and reveals the influence of time-dependent effects such as creep and swelling. Furthermore, FPESPAC exhibits good numerical stability and robustness across a range of simulation scenarios. In addition, a homogenized equivalent material model was applied to describe the thermomechanical behavior of U₃Si₂-Al dispersion fuel. The model provides insights into the structural integrity and mechanical response of advanced fuel systems and supports their potential application in next-generation reactor designs. In conclusion, the FPESPAC model offers a more accurate and comprehensive framework for simulating the nonlinear thermomechanical behavior of nuclear fuel pellets. It enables improved prediction of pellet-cladding mechanical interaction (PCMI) and contributes to the design and safety assessment of advanced nuclear fuels. Future work will focus on integrating experimental validation and expanding its applicability to a broader range of fuel types.