A feature of vitrified high-level waste and spent fuel is that they generate heat over a long period of time (hundreds or even thousands of years). This waste is stored at the surface for at least 60 years, until the intensity of the heat has decreased to a manageable level for disposal. The PRACLAY Heater Experiment studies the effect of this heat on the Boom Clay on a representative scale in time and space. The combined thermal and excavationinduced disturbances of the Boom Clay are the main focus of the experiment. The performance of models that combine the hydraulic, mechanical and thermal processes will be checked. A good understanding of the coupled processes involved in changes to the Boom Clay around the repository is essential to determine the extent to which these temporary or permanent changes could affect its containment and isolation capacity. To host the Heater Experiment the PRACLAY gallery was excavated in 2007. The actual heating phase will start in 2012 and is planned to last ten years. After cooling down, the experiment will be dismantled to further investigate the behaviour and performance of the various components.