Abstract：In Belgium，geological disposal in clay is the primary option for the final disposal of high-level radioactive waste(HLW). Repositories are designed on the basis of a multiple barrier system between the HLW and the biosphere. The Boom clay has been investigated for more than 20 years as the reference host rock. The most important decision was taken in 1980 to build an called high activity disposal experimental facility(HADES) in Boom clay formation at a depth of about 223 m at Mol site to investigate and demonstrate the feasibility of the disposal concepts and to provide reliable data on the performance of repository barriers(natural and engineering). Among numerous in-situ experiments carried out in HADES，several tests were designed to study the thermo-hydro-mechanical(THM) behaviour of the host rock as well as the engineered barrier system(EBS) including sealing and backfilling feasibility，such as CACTUS，ATLAS，BACCHUS，and RESEAL，etc.. Since 1995，the R&D(research and design) program has been oriented towards the large-scale feasibility and demonstration tests. Main achievements are the demonstration of the construction of the underground research facility(URF，shaft and gallery) using the industrial technique which gave an opportunity to extend the knowledge on the hydro-mechanical(HM) behaviour of host rock clay(CLIPEX project)，more specifically on the excavation damaged zone(EDZ) due to tunnelling works(SELFRAC project)，the realization of a large-scale heating and hydration mock-up test of a prefabricated bentonite called“OPHELIE”. Future work consists in realizing a large-scale in-situ heater test(PRACLAY experiments) that will start in 2006 for a period about 10 years. This paper will firstly give a brief description on the Belgian repository design for HLW，then a review on the performed tests regarding to the THM behaviour of the Boom clay and some EBS(in-situ and on surface mock-up)，finally a general presentation on the future large-scale in-situ heater test(PRACLAY experiments).
