Measurement and modelling of reactive transport in geological barriers for nuclear waste containment

Abstract

Compacted clays are considered as excellent candidates for barriers to radionuclide transport in future repositories for nuclear waste due to their very low hydraulic permeability. Diffusion is the dominant transport mechanism, controlled by a nano-scale pore system. Assessment of the clays' long-term containment function requires adequate modelling of such pore systems and their evolution. Existing characterisation techniques do not provide complete pore space information for effective modelling, such as pore and throat size distributions and connectivity. Special network models for reactive transport are proposed here using the complimentary character of the pore space and the solid phase. This balances the insufficient characterisation information and provides the means for future mechanical-physical-chemical coupling. The anisotropy and heterogeneity of clays is represented using different length parameters and percentage of pores in different directions. Resulting networks are described as mathematical graphs with efficient discrete calculus formulation of transport. Opalinus Clay (OPA) is chosen as an example. Experimental data for the tritiated water (HTO) and U(vi) diffusion through OPA are presented. Calculated diffusion coefficients of HTO and uranium species are within the ranges of the experimentally determined data in different clay directions. This verifies the proposed pore network model and validates that uranium complexes are diffusing as neutral species in OPA. In the case of U(vi) diffusion the method is extended to account for sorption and convection. Rather than changing pore radii by coarse grained mathematical formula, physical sorption is simulated in each pore, which is more accurate and realistic.

Permeability of porous foamy materials

“…4c shows that all data, including the ones obtained without wall, follow the same trend. Therefore, due to the peculiar pore geometry of foams, the pressure drop inside such porous materials is governed by a local mechanism which is not described by the usual Hagen-Poiseuille equation as it is done in classical porous media [6,11,27,28].…”

“…To overcome similar difficulties in simulations of flow through porous media, multi-scale approaches have been proposed [11,15,19,[27][28][29]: at the scale of a throat between two linked pores, the relationship between the flow rate passing through the throat linking pores and the difference of pressure between pores is determined by numerical simulations or analytical solutions (e.g. Hagen-Poiseuille equation); at the macro-scale, pore-network simulations are performed to determine the macroscopic permeability from local permeabilities found at the local scale [6].…”

Evolution of Hydraulic Conductivity of Unsaturated Compacted Na-Bentonite under Confined Condition—Including the Microstructure Effects

“…Bentonite is a widely considered swelling clay as an engineered barrier material in geological waste disposal because it has a large percentage of smectite (50~90%), a clayey soil swelling under water intrusion. Bentonite has lower hydraulic conductivity and diffusivity, and higher expansive capacity, specific surface area, cation-exchange potential, and thermal conductivity compared to other soils [ 1 , 2 , 3 , 4 , 5 , 6 ]; therefore, it plays a key role in various geo-environmental engineering applications, such as containment systems, carbon dioxide storage [ 7 , 8 ], bioremediation [ 9 ], and stability of petroleum reservoirs [ 10 ]. Bentonite has been attracting attention in the construction of disposal repository of high-level nuclear waste for 4 decades [ 11 ].…”

Ability of a pore network model to predict fluid flow and drag in saturated granular materials

“…Conceptually, pore network models resolve the fluid flow at length-scale that is intermediate between the fully resolved and unresolved simulations approaches mentioned above. They have been employed in a wide range of fields including petroleum engineering (Silin and Patzek, 2006;Thompson et al, 2008), nuclear waste disposal (Xiong et al, 2015), ionic diffusion (Mohajeri et al, 2010) and carbon dioxide sequestration (Li et al, 2006). Within soil mechanics and geotechnical engineering, PNM have been employed to explore single-phase permeability (Bryant et al, 1993;van der Linden et al, 2018), unsaturated soil characteristics and fluid retention curves (Held and Celia, 2001;Ferraro et al, 2017) and filtration properties (Shire and O'Sullivan, 2017).…”