Modeling diffusion of an alkaline plume in a clay barrier
Résumé
The design of clay plugs used for sealing access galleries to a radioactive waste repository built with concrete structures in a deep clayey formation must take into consideration their chemical evolution over time. Diffusion of an alkaline plume from concrete into bentonite was therefore modeled over a 100 ka period with the PHREEQC geochemical code in order to determine, as a function of time, modifications to mineral surfaces, dissolution of existing minerals and precipitation of new mineral phases. The modeled system consisted of an OPC (Ordinary Portland Cement) barrier, a MX80 bentonite clay barrier and the corresponding equilibrated pore waters. A specific database including aqueous complexes, mineral-phase solubilities and ion-exchange parameters for Na+, K+, Ca2+, Mg2+ and H+ for an MX80 bentonite was created. Only the mineral phases capable of precipitating in this system were considered in the model. The width of the clay barrier was taken as 8 m. Simulations were carried out at 25 degreesC and 1 bar. Transport modeling was based on a one-dimensional diffusion model, assuming thermodynamic equilibrium. Constant dissolved concentrations were assumed for the concrete pore fluid. The clay barrier was modeled as a semi-infinite medium with a single diffusion coefficient of 10(-11) m(2)/S. The simulation revealed a sequence of mineralogical transformations after 100 ka. From the host clayey rock to the concrete, the transformations begin with ion exchange reactions changing Na-montmorillonite into a more potassic and calcic phase. Then illitization of the montmorillonite occurs. Between the illitized zone and the concrete interface, zeolite phases are precipitated. Finally, cement phases replace zeolites at the concrete interface. The cementation of the concrete interface leads to a large increase in the total clay volume whereas illitization of montmorillonite produces a decrease in this volume. The sensitivity of the calculations to exchange reactions and the diffusion coefficient was tested. Calculations were done linking and not linking the cation exchange capacity to the amount of montmorillonite in order to evaluate the influence of the exchange reactions. This produced only very minor differences, indicating a limited influence of exchange-reactions in the mineralogical evolution. Simulations for sensitivity to the value of the diffusion coefficient enabled the authors to develop a phenomenological law indicating that the extent of the mineral transformations is proportional to the square root of the diffusion time and the diffusion coefficient. The simulations also demonstrate the efficiency of pH buffering by a mineralogical assemblage that controls the CO2 partial pressure.
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