Decommissioning nuclear reactor buildings poses a significant challenge due to the large volumes of radioactive concrete and the complex interplay of physical, chemical, and mechanical degradation processes. Among these, the infiltration of fission products (FPs) such as cesium can critically weaken concrete structures surrounding reactor cores, potentially compromising long-term safety. This study presents a finite element model (FEM) designed to predict cesium infiltration through reactor building concrete, linking microstructural damage induced by high-temperature exposure to macroscopic reductions in mechanical strength. The model employs the Arrhenius equation to quantify the diffusion coefficient of cesium, complemented by elastic modulus reduction data currently being obtained through experimental studies. These parameters are integrated into a Thermal, Diffusive and Degradation FEM framework to simulate cesium behavior in concrete. The ultimate goal is to predict the penetration depth of cesium and assess its long-term effects on structural integrity. This will inform the development of containment strategies, such as polymer membrane barriers, to mitigate FP leakage during decommissioning. The proposed approach not only enhances decommissioning safety and efficiency but also provides valuable insights into the future design of resilient concrete structures for nuclear applications. The findings are expected to make a substantial contribution to the field of nuclear facility decommissioning by offering practical tools for minimizing radioactive waste and ensuring structural safety.