Hydration of Eurobitum bituminized waste under free swelling conditions: osmosis-induced swelling and NaNO3 leaching

In Belgium, Eurobitum bituminized radioactive waste is an important intermediate-level long-lived waste form that contains, besides bitumen and radionuclides, large amounts of soluble salts (mainly NaNO3). Geological disposal in a water-saturated sedimentary formation will induce swelling of Eurobitum due to water uptake by the hygroscopic salts embedded in a highly efficient semi-permeable bitumen membrane. Initially, while there is still free space in the primary waste containers, free swelling will occur. To improve our understanding of the water uptake processes under free swelling conditions, water uptake and leaching tests are being performed in which inactive Eurobitum was contacted at the top with 0.1 M KOH (simplified representation of young cement water) without effective stress on the sample. Under these test conditions, the evolution of the water uptake and NaNO3 leaching processes shows several stages: (1) initial fast ingress of water into and leaching of NaNO3 from pores close to the surface; (2) slower progression of the hydration front, the swelling and the leaching of NaNO3, but behaving linearly with the square root of time, indicating that these processes are controlled by diffusion; (3) after more than 2 to 3 years, the leaching, hydration and swelling rates re-increase, attributed to the formation of interconnecting pores and (micro)cracks in the bitumen matrix in response to its increasing deformation.