In situ chemical osmosis experiment in the Boom Clay at the underground research laboratory (URL) at Mol, Belgium

Studies on Boom Clay compatibility with nitrate- bearing bituminized waste have raised interest for osmosis-induced effects. Chemical osmosis, the water flow induced by a chemical gradient across a semi-permeable membrane, can generate pressure increase. Is there a risk to create high pore pressures that could damage the near-field of MLW galleries, if osmotically driven water flows are produced by the release of 750 t NaNO3 in the formation? An in situ osmosis experiment has been conducted at the HADES URL to determine the osmotic efficiency of Boom Clay at field scale. Experimental data could be reproduced accurately by the model, and the parameter values are consistent with independent determinations for Boom Clay. A pressure increase of 2 m water column was observed. The experimental results obtained in situ confirm the occurrence of chemical osmosis in Boom Clay. The osmotic efficiency of Boom Clay is high under undisturbed conditions, but decreases when the dissolved salts concentration increases. A semi-permeable membrane behaviour of the Boom Clay may be expected for the disposal of nitrate-bearing radioactive waste. However, the presently observed osmotically induced pressure is too low to have a significant mechanical impact on the clay