Numerical Simulation of the Interaction Between Granitic Groundwater, Engineered Clay Barrier and Iron Canister
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ABSTRACT The geochemical evolution of a clay engineered barrier subjected simultaneously to granitic groundwater on its external boundary and to a steady flux of iron released by canister corrosion on the other boundary is simulated for time periods up to 1000 y. A multisite ion-exchange model combined with dissolution/precipitation of accessory minerals (calcite, gypsum, quartz, goethite, pyrite, pyrrhotite, iron
silicate, magnetite) is used to describe the geochemical evolution of the clay barrier. Diffusive transport of aqueous species and dissolved gases is simulated in a one dimension space. Retroaction of chemical reactions on the corrosion process of the canister is not included in the model. The temperature is assumed to be constant (25°C). Calculations are performed with the IMPACT geochemical coupled code. Two main scenarios are considered : (i) the reduction of sulfates (gypsum) is kinetically hindered, (ii) the reduction of sulfates is possible and leads to the precipitation of sulfides. Main results obtained are dealing with H2(gas) generation and control, pH transient and buffering effect of the clay barrier system. These simulations show that the hydrogen partial pressure in the clay barrier could possibly stay for long periods of time at a very low level due to interactions with mineral buffers or sinks. In the sulfate reduction scenario, a transient pH ,xcursion is predicted at the vicinity of the canister. Future developpements of this work include the taking into account of a temperature gradient. Improvement of the geochemical model and reaction pathway is also necessary and will be done with the guidance of on going experimental work. INTRODUCTION The understanding and prediction of materials interactions in waste storages or deep disposals are important issues for the performance assessment of such installations. Data is generally available on the behaviour of individual constituants (waste matrix, canister, etc). However some
important questions are related to the interactions of these constituants and to the physicochemical evolution of the boundaries around waste packages. The work presented here is the continuation of a previous study [I] on the dynamics of claygroundwater interactions and deals with the simulation of a clay body that interacts simultaneously with reducing groundwater and an iron canister. Questions raised by the potential interactions of such constituents include the generation and evacuation of H2 produced by iron corrosion [2,3] and the evolution of the swelling, transport and chemical properties of the clay body, due to iron reduction [4,5] or to the formation of minerals with different properties like chlorites. Depending on the corrosion rate of the canister, H 2 could be produced in excess of its solubility and generate gas overpressures in the clay pores. Chemical sinks may however exist in the vicinity of the canister and H2 partial pressures could be buffered at low levels by geochemical processes. The reduction of structural ferric iron in smectites has been s
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