Impact of (Na,Ca)NO3 concentration on the deviatoric mechanical behaviour of Boom Clay

The disposal of long-lived intermediate- and high-level radioactive waste is a major environmental concern. Deep geological disposal is widely regarded as the safest long-term solution. In Belgium, Boom Clay (BC) has been selected as the reference host formation due to its low permeability, self-sealing properties, and strong capacity to retain radionuclides. One specific waste type, Eurobitum—an intermediate-level bituminized waste—is stored in steel canisters within the repository. Over time, groundwater is expected to infiltrate the system and come into contact with these canisters. The bituminized waste swells upon water contact, releasing large amounts of (Na,Ca)NO₃ and generating a saline plume that diffuses into the Boom Clay. This plume, rich in sodium ions (Na⁺), can significantly impact the clay’s physicochemical and hydro-mechanical behaviour. In this study, undrained triaxial tests under isotropic consolidation were conducted on intact BC samples pre-equilibrated with (Na,Ca)NO₃ solutions of varying concentrations. Solutions corresponded to sodium occupancies of 60% (1.0 M) and 90% (2.0 M), along with a reference Boom Clay synthetic water solution (0.015 M NaHCO₃). Results showed an increase in shear strength and friction angle with sodium concentration, attributed to clay particle aggregation, shrinkage, and diffuse double layer contraction induced by salinity.