A geological disposal facility for high-level radioactive waste (HLW) encompasses both natural (host rock) and (generally clay-based) engineered barriers. Many processes can influence, either positively or negatively, the effectiveness of the safety functions of isolation and retardation. This paper focuses on the analysis of a large-scale heating test when subjected to cooling and subsequent partial dismantling. The experiment reproduces the conditions of an HLW repository at full scale under realistic conditions. Key thermal, hydraulic, and mechanical (THM) variables, such as temperature, relative humidity, stresses, and fluid pressures, were measured in the clay barrier and surrounding rock. The experimental observations recorded during the cooling down and clay barrier excavation are analyzed in light of a fully coupled THM finite element formulation. This analysis has provided the opportunity to explore the behaviour of the clay and natural barriers under conditions very relevant for the repository performance but not analyzed previously. Overall, the model predictions are quite satisfactory when compared against experimental observations. Furthermore, model predictions for a period of 20 years, including the transient phase induced by the partial dismantling, are also presented. This additional analysis has allowed a better understanding of the effect of thermal gradient on long-term clay hydration.