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

Qingrong Xiong,Andrey P Jivkov,Claudia Joseph,Katja Schmeide

doi:10.1039/c5cp05243b

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.

Highlights

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
Representative distribution of pore sizes was obtained with relatively small number of cells, e.g., it was sufficient that N 4 5 for the distributed pores to follow the experimental pore size distribution closely
The anisotropy and heterogeneity of the clay can be simulated via different length parameters and percentage of pores in different directions

Summary

Introduction

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. 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. 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. Pores were related to the sites and pore throats were related to the bonds of regular PNM.[11,12,13] If such correspondence is to be statistically representative of the material modelled, sufficiently rich experimental information is required: shape and size distribution of pores and throats, as well as the pore coordination spectrum, i.e. percentages of pores coordinated by different numbers of throats.[13,14] These can be obtained from structures with distinguishable pores and pore throats. One established way is analysing 3D images from synchrotron X-ray tomography.[15,16]

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