â»During the long-term evolution of deep geological repository, the alkali-leachates from cementitious materials will induce changes in compacted bentonite.
►During the long-term evolution of deep geological repository, the alkali-leachates from cementitious materials will induce changes in compacted bentonite affecting radionuclides migration [1, 2]. ►For the purpose of SÚRAO project "Research Support for Safety Evaluation of Deep Geological Repository" diffusion module DifNelinV1 in the GoldSim environment was developed to model and evaluate diffusion of radionuclides on interfaces (bentonite/cement, bentonite/corrosion products and bentonite/granite). ►The module was applied to evaluate the pilot HTO through diffusion experiment on interface of compacted bentonite and hardened cement paste.
►Diffusion module, used in the modeling and evaluation of diffusion experiments on single material with/without filters on both edges, has been improved with addition of the second material: possibility to monitor diffusion through up to four materials (corrosion products, bentonite, host rock, cement construction materials). ►Parameters of materials: length L, diameter d, dry and specific density ρd and ρs, porosity ε, geometrical factor G, and sorption characteristics (linear-Kd, Langmuir, or Freundlich). ►Output: 3 datasets: concentration profile for each material, time evolution of the concentration in the inlet/outlet reservoirs. ►Evaluation of through diffusion experiments: for reference diffusivity Dw [m2/s] multiparametric fitting (G [-], εeff [-], sorption parameters) of datasets (HTO – expectations: sorption not expected, εeff = εtot). ►Modeling: - successive HTO saturation of the layer consisting of hardened cement paste, bentonite and filter, - sorption with variable parameters (ε; G; Kd) of cement paste.
Solid phase: ►Compacted Bentonite and montmorillonite "BaM": Černý vrch deposit (CZ), CEC = (57 ± 5) meq/100g; dry density ρd ≈ 1.60 g/cm3. ►Hardened cement paste "CEM": CEM II/A-S 42.5R + H2O (w = 3:2, hydration 28 days). Liquid phase: ►Synthetic granitic water "SGW-UOS": Ca-Cl type, IS = 3 mmol/dm3 .
►HTO saturation of the layer consisting of cement paste, bentonite and filter in time:
►Diffusion experiments on CEM (2×) and BaM (2×):
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Fig. 1-3 Model of time evolution of HTO activity in both reservoirs (V = 60 cm3) and in the layer (L [mm],ε [-],G [-]), where CEM: (10; 0.4; 0.1), BaM: (15; 0.4; 0.2), filter: (0.75; 0.25; 0.04), d = 30 mm, ρs = 2,9 g/cm3, Dw = 2,3·10-9 m2/s.
►Various parameters of cement paste (ε, G, Kd1), for other parameters see Fig. 1-3:
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Fig. 4-6 Model of HTO relative activity in the layers CEM/BaM/filter a filter/BaM/CEM after 20 days with various parameters of cement paste (ε [-], G [-], Kd [cm3/g]), for other parameters see Fig. 1-3.
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0.460 0.460 0.429 0.487 0.496 0.424
De·1011 [m2/s] 2.33 10.7 1.09 17.4 20.7 1.73
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(0.4; 0.02; 0) (0.4; 0.02; 0.1) (0.4; 0.02; 0.5) (0.4; 0.1; 0) (0.4; 0.1; 0.1) (0.4; 0.1; 0.5)
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►Diffusion experiment on BaM/CEM and CEM/BaM:
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Fig. 7-8 HTO relative activity time evolution in the outlet reservoirs and corresponding fits by DifNelinV1.
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εtot [-] G [-] Vin,out = 60 cm3
Vin,out = 160 cm3
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Tab. 2 Results of HTO diffusion on layers CEM, BaM, CEM/BaM and BaM/CEM.
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Through diffusion experiment in four configurations: - CEM (inlet/CEM/outlet) ► d ≈ 46 mm; L ≈ 11 mm, ► Tab. 1 Size of materials - BaM (inlet/filter/BaM/filter/outlet), L [mm] ► CEM d [mm] - BaM/CEM (inlet/filter/BaM/CEM/outlet), 36 11 ► BaM - CEM/BaM (inlet/CEM/BaM/filter/outlet), 30 15 - non-constant concentration in the inlet reservoir, - monitoring HTO activity in reservoirs and in the layers, - evaluation: Dw = 2.3·10-9 m2/s, DifNelinV1, - population of exchangeable cations in the BaM after diff. experiment.
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Fig. 9-11 HTO relative activity time evolution in the outlet reservoirs and in the layers BaM/CEM and CEM/BaM after 28 days of through diffusion experiment and corresponding fits by DifNelinV1.
►No significant change in cation population in BaM after experiment was found.
►Similar parameters were obtained from HTO diffusion experiments on individual and coupled materials, differences are caused by unachieved same value of εtot in all configurations and by non optimal methodology of sample cutting (time vs. evaporation) for acquiring relative activity in CEM material. ►For future experiments, we are improving the methodology of obtaining the concentration profile in cementitious materials by means of gradual grinding. ►Diffusion module upgrade with additional barrier material can be considered as successful. It represents an effective tool for modelling and evaluating diffusion experiments on the different type of interface, especially for materials on which the diffusion experiment cannot be performed separately (e.g. such as a layer of corrosion products on contact with bentonite) .
[1] T. Melkior, et al., App. Clay. Sci. 26, 99 (2004). [2] M. Birgersson, et al., SKB Technical Report 09-34 (2009).
This contribution is the result of Radioactive Waste Repository Authority (SÚRAO), Czech Republic, project “Research Support for Safety Evaluation of Deep Geological Repository”.
