This chapter presents a thermo-elasto-plastic creep damage model to predict the time-dependent THM coupled behavior of Boom Clay, essential for assessing the long-term reliability of HLW repositories. First, an elasto-plastic creep damage model based on Perzyna’s overstress theory is proposed to capture creep behavior at ambient temperature, incorporating creep hardening, damage evolution, and flow rules. The model is implemented in ABAQUS using CREEP and USDFLD subroutines, and parameters are calibrated based on experimental HM creep data. Subsequently, thermal effects are integrated by extending the model to include thermal weakening and temperature-dependent creep hardening. Strain is decomposed into elastic, plastic, and creep components, with the first two defined according to the framework established in Chap. 3 . The enhanced model is validated against THM coupled creep test results, showing good agreement between simulations and experimental data. This work provides a robust numerical tool for evaluating the long-term THM performance of clay barriers in geological disposal systems.