Bituminized Waste materials (BW) were produced by an industrial reprocessing of radioactive waste qualified low or medium activity and long life (LA-LL and MA-LL). BW is composed of precipitation sludge from the chemical reprocessing of spent nuclear fuel, immobilised in bitumen matrix. Geological storage is the reference solution for this kind of wastes. Under geological disposal conditions, and after a period of hundred thousand years, BW will undergo water re-saturation from host rock. Water up-tacking by BW will first induced free swelling in order to fill all different types of void existing in the storage disposal (void in primary canister, void in concrete container and void in rock vault). Then eventually swelling in contact with host rock and under special stress conditions. That's why the study of the behaviour of this type of material is very-important. Bituminized waste can be considered as a very-low permeable material containing one or several salt crystals. In order to describe the behaviour of such a material in contact of water, several mechanisms has to be coupled. The aim of this work is to study theses coupling during water up-tacking. Swelling behaviour in contact of water is govern by two principal mechanisms. First mechanism is the solvent transport leading to the dissolution of salt crystals. During dissolution, salt crystals volume increases, leading to global swelling of the bitumen matrix. Second mechanism is osmotic flow, which is leading directly to an overpressure in pore water due to chemical gradient (osmotic pressure) A model based on classical poromechanical approach has been developed in order to evaluate which is the leading mechanism and to study all the coupling. The chemical part of this model manage the precipitation/dissolution of salt crystals present in the bitumen matrix. It is the principal driving force of water up-taking, leading to salt saturation in pore water and increasing the porosity. That create a chemical gradient (salt concentration gradient) between pore water and host rock's water. Which initiate osmotic phenomenon: the bitumen matrix play the role of semi-permeable membrane allowing increasing of pore water pressure in the bituminized waste (osmotic overpressure). Over wise the porosity created by the salt crystals dissolution allow advectif and diffusive transport of water and salt through the bitumen matrix. The mechanical behaviour is strongly dominated by creep-deformation needing viscoplastic deformation management. A chemo-hydro mechanical numerical model in one dimension has been implemented (finite volume) in order to evaluate all mechanisms and coupling. This numerical model has shown that osmosis is the principal mechanism of water up-taking and that other mechanisms are not negligible. Moreover the difference of behaviour and coupling importance has been studied during both free swelling and water up-taking under constant volume. This work permitted us to find which parameters are needed to be identified experimentally
