Interactions entre fluides et sédiments argileux naturels : étude expérimentale dans des conditions simulant un stockage souterrain de déchets radioactifs

Abstract : The behaviour of clay rich sediments, especially an argilite from Oxfordien of Haute-Marne, a siltite from albian series of Marcoule (Gard) and a bentonite from Wyoming, were experimentally studied under physical-chemical conditions close of those of an underground radioactive waste storage. The several steps of the creation of the storage in deep formation were simulated experimentally, in particular : - the effect due to oxidation at ambient temperature and moisture degree related to the arrival of air in the gallery, was tested, especially the interaction between acid fluids generated at the micron-scale of the altered pyrite microsite and the surrounding mineraIs of the sediment, - the alteration due to weathering (damping/drying cycles) to simulate the effect of a surface storage of the sediments, - and finally, water-rock interactions at 80 and 200°C, which reproduce the thermic stress induced by the deposit of type C radioactive containers (stage ofrehydration under thermic stress). The various simulations lead to rather similar behaviour of mineraIs in the sediment and solutions. Mineralogical, geochemical and crystallographic analyses show that most minerals in sediments are preserved with no evidence of mineraI neoformation. Nevertheless, the study by Xray diffraction shows variations in the interlayer spacing in relation with modifications of the hydration states. Changes in the interlayer occupancy of the clays are due to cationic exchange of the sodium of the interlayer by the calcium issued from the dissolution of carbonate and gypsum dissolution. Ils like mineraIs crystal-chemistry generally display little changes in the tetrahedral and octahedral occupancy and a rather good stability of crystal structure. The cationic exchange capacity (CEC) of the clay sediment display unsignificant variations: after the damping/drying cycles, the argilite of Haute-Marne has lost about 15 % of their bulk CEC and the effect of acid microenvironment at pyrite microsite leads to an increase of the bulk CEC from 3 to 25%. Whatever their initial composition, the experimental solutions acquired a sulphate character in relation with the dissolution of sulphate mineraIs as gypsum and jarosite formed during the oxidation of the sediments. Equilibrium between fluids and carbonates in the rock, constitute a predominant rock forrning mineraI in the studied sediments yield to an important and rapid buffer effect on solutions, which pH ranges between 7,5 and 8,5. The liquidlrock ratio (UR) of the experience has an influence on the concentrations of the elements resulting from the dissolution of mineraIs present in the rock (sulphates from pyrites oxidation). Extrapolated ionic concentrations for low UR ratio typical of the geological formation indicate that the fluid migrating through an oxidised sediment will be enriched in SO/-,K+, Ca2+ and Mg2+ and therefore significantly distinct from that inferred in the deep sedimentary formations.
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Véronique Roubeuf. Interactions entre fluides et sédiments argileux naturels : étude expérimentale dans des conditions simulant un stockage souterrain de déchets radioactifs. Sciences de la Terre. Université Henri Poincaré - Nancy 1, 2000. Français. ⟨NNT : 2000NAN10190⟩. ⟨tel-01748367⟩

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