The impact of saline conditions on the transport of HTO and iodine in Boom Clay was investigated by successive HTO and 125I through-diffusion tests before and after equilibration with saline water. In addition, the study assessed the effect of ionic strength/salinity on the clay and pore water properties.
Boom Clay from two different depths/origins (Putte Member & Boeretang Member) and with different mineralogy (clay and silty clay) was submitted to saline conditions. Each clay was represented by three replicate clay plugs (L 30 mm x 38 mm). One replicate plug of each clay was used for conducting a reference experiment to monitor the changes in pore water chemistry during the equilibration with synthetic seawater (SSW) and to assess the impact of SSW on the clay properties. The main effect that was observed, concerned the calcium (Ca) and strontium (Sr) occupancy on the cation exchange sites. Both Ca and Sr were released from these sites into the pore water since they were replaced by the main component of the synthetic seawater, i.e. sodium (Na). Furthermore, an immediate and sharp decrease of the dissolved organic carbon (DOC) concentration was observed. It is expected that both effects have an impact on the transport behaviour of cationic radionuclides under saline conditions: the first one on the cations that sorb through ion exchange and the second one on the cations that tend to bind with dissolved organic matter (DOM).
The other two replicate plugs were each submitted twice to through-diffusion experiments with HTO and 125I (combined). First, the transport behaviour was determined in current-day conditions (Boom Clay fresh pore water from Mol site). After out-diffusion of the tracers and equilibration of the clay plugs with synthetic seawater (SSW) by percolation, the diffusion experiments were repeated with SSW in order to study the transport behaviour under saline conditions as well. As expected, the diffusion behaviour of HTO was not affected. For 125I, the effective diffusion coefficient increased by a factor of two, mainly due to the two-fold increase of the anion accessible porosity at higher ionic strength. The extent of the effect was similar for samples of different origin and composition.
|Number of pages||60|
|State||Published - 14 Sep 2020|
|Publisher||Belgian Nuclear Research Center|