Modelling of radionuclides migration in the low pH cement / clay interface

Reactive transport modelling activities described in this work are focused on the definition of the conceptual model and the selection of the chemical and transport parameters to be applied on laboratory through diffusion experiments performed in KIT-INE (see contribution of Ait Mouheb et al., this proceedings). The model includes different coupled processes which are thought to play a role in a through diffusion experiment of HTO, 36Cl-, 129Iand Be across the interface between bentonite porewater and low pH cement. One of these processes is the impact of porosity changes due to dissolution / precipitation reactions

Characterization and sorption properties of low pH cements

Characterization of three low pH cement pastes, including the description of their sorption properties for tritiated water (HTO), 36Cl- and 129I- is described in this work. SEM-EDX and NMR analyses show that after 90 days of hydration, the main hydrated phases are C-S-H and C-A-S-H gels with a Ca:Si ratio between 0.8 - 1.0 and a Al:Si ratio of 0.05. TG-DSC and XRD indicate the presence of calcite in the mixtures where limestone filler has been used. Additional techniques were used to identify minor hydrated solid phases like ettringite (i.e., XRD and solid state NMR). Porosity and pore size distribution was characterized by MIP observing that the size of the pores in the hydrated cement phases varies from the micro to the nanoscale. Uptake studies of HTO, 36Cl- and 129Ifrom batch sorption experiments indicate very weak sorption (Kd < 0.40 ± 0.13 L/kg) for the 3 selected radionuclides. The uptake process of 36Cl- and 129I- is probably associated with surface processes in the C-S-H and C-A-S-H phases with competition for sorption sites, between them. In the case of HTO, isotopic exchange with the interlayer water of the C-S-H and the C-A-S-H seems to be the main uptake process

Experimental data: Effect of Organic Degradation Products on the Migration Behaviour of Radionuclides in Cementitious Materials

The experimental data consist of derived sorption parameters for 241Am and 152Eu under CEM V/A HCP cementitious materials as a function of phthalic acid additions. Solubility data obtained from thermodynamic modelling demonstrate the impact of phthalic acid on the speciation and solubility of 241Am and 152Eu as a function of pH. In addition of calculated Ca-speciation in the solution equilibrated with HCP CEM V/A as function of the phthalic acid concentration

EFFECT OF PHTHALATES ON THE MIGRATION BEHAVIOUR OF Am(III) AND Eu(III) IN CEMENTITIOUS MATERIALS

International audienceSmall organic molecules as degradation products from organic waste constituents or from cement additives may enhance the mobility of radionuclides in repositories for nuclear wastes. Here, we investigated the impact of phthalate ions on the migration behaviour of 241Am and 152Eu in hardened cement paste (HCP) with batch sorption experiments. In the absence of organics, 241Am/152Eu sorb strongly on HCP with Rd values of 10^5 to 10^6 L kg-1. Above a no-effect level of ~10-3 mol L-1 phthalate, a distinct decrease of the An(III)/Ln(III) sorption was observed with sorption reduction factors of 100 to 1000 at phthalate concentrations of 10-1 mol L-1. This sorption reduction is attributed to the destabilization/dissolution of C-S-H due to increasing Ca-complexation by phthalate in solution. These results and ongoing diffusion experiments indicate an increase in the mobility of 241Am/152Eu in cementitious barriers with increasing phthalate concentrations