The Eurobitum bituminised waste product (BWP) from the former EUROCHEMIC reprocessing plant in Mol-Dessel (Belgium) contains a lot of salts, mainly NaNO3 and CaSO4. In contact with pore water in an underground repository, the dehydrated salts in the BWP will rehydrate, resulting in swelling and, possibly, a swelling pressure build-up. A high swelling pressure might impede the integrity and the safety of the repository by creation of preferential pathways for radionuclide migration. In Belgium, the Boom Clay is studied as a reference host formation.

The interaction between the swelling BWP and the host formation is a very complicated hydromechanical process. It depends not only on the hydromechanical behaviour of the Boom Clay, but also on that of the BWP. The hydromechanical constitutive law of Eurobitum is not yet established. We have therefore realised scoping calculations to assess the stress redistribution and deformation due to the recompression of the clay around a disposal gallery caused by a swelling pressure. Sensitivity studies have evidenced some important influencing factors.

The scoping calculations showed that the swelling pressure exercised to the Boom Clay should be limited to 7 - 8 MPa. Beyond this limit, the Boom Clay host formation will be subject to important deformation, and the risk of preferential paths for radionuclide transport becomes very high. The convergence during the excavation of the disposal galleries is of primary importance for a safe waste disposal system design. Indeed, excess convergence may result in a higher damaged zone before disposal of the bituminised waste.