Processes Related to the Water Uptake by EUROBITUM Bituminised Radioactive Waste: Theoretical Considerations and First Experimental Results

According to the present Belgian radioactive waste management program, EUROBITUM bituminised radioactive waste will be disposed of in a geologically stable underground clay formation. The Boom Clay is studied as a potential host formation because of its low diffusion and high retention properties towards radionuclides. The presence of the radioactive waste should not disturb these properties. Due to the presence of hygroscopic salts (25-30 weight% NaNO3), EUROBITUM will take up pore water which will result in a swelling and possibly in a very high swelling pressure. First scoping calculations suggest that the swelling pressure exerted to Boom Clay should remain below 7-8 MPa to avoid the formation of fractures. To better understand the interaction between the swelling EUROBITUM and the host formation, coupled hydro-chemical-mechanical constitutive laws for EUROBITUM have to be developed. Therefore, water uptake tests under constant volume and constant stress conditions are being performed. After ~2 years of hydrating small inactive EUROBITUM samples in constant volume conditions, the swelling pressure has raised to ~12MPa. The volume of samples that can swell against constant stresses of 2.2-3.3-4.4MPa increased with ~5-11 volume%, independently of the applied stresses. About 10 weight% of the initial NaNO3 content has been leached.