A deep tertiary clay formation, "Boom Clay", present under the Mol-Dessel nuclear site, was selected as a potential host formation for the disposal of high-level and long-lived radioactive waste (HLW). An underground laboratory (HADES) has now been operational for more than 15 years. An extension of the facility is necessary to implement full-scale demonstration tests. Its construction started in 1997 and consists of the construction of a second shaft (1997–1999), and the excavation of a gallery 84 m long to connect the new shaft with the existing facility. The knowledge acquired since the beginning of the construction of the underground laboratory has led to the use of improved techniques, which have significantly reduced both the disturbance of the host rock and the cost. The paper describes the changes made due to the new techniques and the construction of the second shaft. Stress and convergence measurements on the lining are discussed. Finally conclusions are drawn and the future underground construction work is outlined.
The R&D programme for finding appropriate geological sites for the disposal of high-level and long-lived waste was initiated in Belgium at the Belgian nuclear research centre SCK•CEN in 1974. For this purpose an underground laboratory was constructed in Boom Clay at Mol at a depth of 223 m. As at that time Boom Clay was expected to creep quickly, the first part of the laboratory was constructed using the ground freezing technique. Later the feasibility of digging in unfrozen clay was demonstrated. The facility currently consists of one shaft and one horizontal gallery 110 m in length. An extension consisting of a second shaft and a connecting gallery is necessary to implement full-scale demonstration tests. This work is managed by EIG PRACLAY (Economic Interest Grouping between the Belgian radioactive waste agency NIRAS/ONDRAF and the SCK•CEN).
