This paper presents the Thermo-Hydro-Mechanical (THM) analysis of an isolation system for high level nuclear waste. A geological disposal facility of such type encompasses both: natural (host rock) and engineered barriers (generally clay based). The study deals on an ongoing large scale heating test at full scale that is being carried at the Grimsel test site under actual conditions. The experiment reproduces the conditions of a HLW repository, at full scale under actual conditions. Key thermohydro-mechanical (THM) variables such as temperature, relative humidity, stresses and fluid pressures were measured in the clay barrier and surrounding rock. The test is intensely instrumented and has provided the opportunity to study in detail the evolution of the main THM variables over a long period of time. A comprehensive laboratory investigation was carried out in the context of the FEBEX and NF-PRO European projects has allowed the identification of the model parameters to describe the THM behavior of the compacted clay and the sub-rounding rock. This paper focuses mainly on the numerical analysis of the main THM variables in the granite adjacent to the repository. Overall, the comparisons between model predictions and observations are quite satisfactory.