Elsevier

Engineering Geology

Volume 294, 5 December 2021, 106409
Engineering Geology

Modelling the effects of water chemistry and flowrate on clay erosion

https://doi.org/10.1016/j.enggeo.2021.106409Get rights and content

Highlights

  • A non-local coupled hydro-chemical model for the erosion of clay buffer is presented.

  • Hydro-chemical effects on the expansion and erosion of clay buffer are examined.

  • Impacts of water chemistry and hydrodynamics on the mass loss of clay are revealed.

  • A concept of tolerance time is introduced to assess the mass loss in the safety assessment.

Abstract

Clay buffer is a key component of the engineered barrier system (EBS) for geological disposal of higher activity radioactive waste. Experimental observations indicate the possibility of buffer erosion at the interface with host rock due to interactions with groundwater. Existing models for clay erosion are very limited in terms of addressing the hydro-chemical effects, while the assessment of the long-term performance of clay buffer requires robust predictive models covering expected environment conditions and multiphysics phenomena involved in the erosion process. The work presented here is a step towards such a predictive capability, which considers clay expansion, detachment of clay particles and transport of detached particles by groundwater within a single modelling framework. The effects of solution chemistry and flowrate on the penetration, extruded mass and particle release rate of clay buffer are investigated in this paper. A series of experimental data are used to validate the swelling and erosion model developed in this study. The results show that the extrusion distance, which is controlled by both clay swelling and detachment processes, is nearly linearly dependent on the water flowrate irrespective of the water chemistry. Similar linear dependence of the erosion rate on water flowrate is observed for flowrates less than 10−5 m/s. Higher water flowrates are shown to induce nonlinearly increasing erosion rates in accordance with experiments. A flowrate threshold is found above which the erosion behaviour of compacted bentonite can be significantly affected. A concept of tolerance time for clay buffer is introduced as a failure criterion. The results indicate that the coupled effect of water chemistry and velocity requires further investigation for ionic concentrations below 1 mM.

Introduction

Presently, mined geological disposal concepts are the preferred options for the long-term management and safe isolation of a high-level radioactive waste (HLW). Conceptual designs for the disposal in fractured rocks are based on constructing engineered barrier systems (EBS). A key component of EBS is the clay buffer which can be subject to erosion as the result of clay interaction with groundwater at the interface between the clay and host rock during the long-term performance of the barrier. Long term erosion can create potential pathways for radionuclides leaving the waste form to migrate into the surrounding rock with potentially significant environmental and public health impacts. Understanding the hydro-chemical effects on clay erosion and encoding them in a robust predictive model is therefore critical for the faithful assessment of EBS performance over the long-term repository function (tens of thousands of years).

Fitness-for-service assessment of the clay buffer requires a comprehensive understanding of bentonite extrusion, erosion and radionuclide release. These result from the clay interaction with groundwater, whose physical and chemical conditions may vary over time. Experimental studies of bentonite erosion have demonstrated that the erosion processes are affected strongly by both solution chemistry and fluid velocity (Schatz et al., 2013; Sane et al., 2013; Reid et al., 2015; Navarro et al., 2016; Smith et al., 2017; Missana et al., 2018; Alonso et al., 2018; Zhang et al., 2019; Bian et al., 2019; Bouby et al., 2020; Xiang et al., 2020). The experiments conducted by Baik et al. (2007) show that the flowrate of groundwater induced a significant impact on the total eluted volume of bentonite particles in the case studies. The experimental pinhole tests conducted by Sane et al. (2013) and Navarro et al. (2016) showed that the erodibility coefficients of MX-80 bentonite at high salinity (e.g. 70 g/L) were two orders magnitude larger than those at low salinity (e.g. 10 g/L). No erosion was observed for sodium montmorillonite for solution compositions from 0.5 g/L to 10 g/L NaCl (Schatz et al., 2013). The erosion rates reported for low velocity (0.0001 L/min) were more than an order of magnitude lower than those for high velocity (0.00284 L/min) (Schatz et al., 2013; Reid et al., 2015).

These experimental observations, which highlight the importance of both physically and chemically induced erosion, show the need for predictive modelling to estimate the long-term impacts of water chemistry and water/clay interactions on erosion. The process clay buffer erosion by groundwater includes three main physical processes: (i) the initial hydration of solid clay from solid to gel and eventually discontinuously transformed into diluted sol particles; (ii) the diluted clay particles (sol) detached at the interface of clay/fluid; (iii) detached particles taken away be seeping water (Sedighi et al., 2021; Yan et al., 2021). This mechanistic understanding, however, is not fully reflected in the existing predictive models for buffer erosion (e.g., Pusch, 1999; Neretnieks et al., 2009, Neretnieks et al., 2017; Moreno et al., 2011; Navarro et al., 2017; Asensio et al., 2018). They remain largely empirical, based on experimental data for specific hydro-chemical conditions, which limits the confidence with which they could be applied to systems subjected to variable geological conditions. The limited progress in erosion modelling is in sharp contrast with the important progress made in coupling mechanical behaviour with hydro-chemical effects on clays for analyses of deformation and consolidation. The key elements missed in the existing models for erosion are the mathematical description of particle's detachment at the clay/water interface and its strong coupling with hydro-chemical processes. A model with particles detachment was firstly presented by Sedighi et al. (2021) and successfully applied to bentonite erosion.

The aim of this work is to underpin future fitness-for-purpose assessments of clay buffer by providing new insights into the effects of groundwater interaction with clay on erosion. For this, we present new developments of our mechanistic modelling approach (Sedighi et al., 2021), based on peridynamics (PD). PD is a non-local approach, particularly suitable for resolving discontinuities emerging and phase evolution (Silling, 2000; Silling and Bobaru, 2005), e.g. transport phenomena in discontinuous or highly heterogeneous systems, and stress induced corrosion (Bobaru and Duangpanya, 2012; Chen and Bobaru, 2015; Jafarzadeh et al., 2019a, Jafarzadeh et al., 2019b). Furthermore, PD has been used successfully in different areas, such as hydraulic fracturing (Ouchi et al., 2015), thermally-induced cracking (Oterkus et al., 2014; Wang et al., 2018, Wang et al., 2019), dynamic fracture (Rabczuk and Ren, 2017; Ren et al., 2017; Wang et al., 2016, Wang et al., 2017), and chemical transport in unsaturated fracture systems (Yan et al., 2020). The present model aims to incorporate the fundamental physical and chemical interactions controlling the erosion, with PD formulations of expansion, detachment of diluted particles and detached particles transport processes. It has been demonstrated that PD has the capacity to resolve the moving interface without any additional interface constraints, which is significant for simulating the moving-boundary of continuously expansion of diluted clay particles (Sedighi et al., 2021). This leads to an improved understanding of the combined impacts of the chemistry and flowrate.

The paper is arranged as follows. Section 2 presents the continuum formulation of processes and the development of Peridynamics (PD) frameworks for clay buffer erosion, including PD formulations of swelling, detachment and particle transport. Section 3 present results used for model validation and analysis of additional conditions of interest. Conclusions from the work are given in Section 4.

Section snippets

Clay-water interaction and erosion

The continuum and peridynamics formulations of the erosion in clays have been presented in the earlier work by the authorship team (i.e. Sedighi et al., 2021). This section provides a brief summary of the erosion model by Sedighi et al. (2021), which intends to support the new and further developments presented here. Sedighi et al. (2021) presented the main mechanisms and theoretical aspects involved in clay erosion and demonstrated how the erosion model can capture the key processes through a

Validations and applications

The validation of the PD model has been performed by comparisons of model predictions with experimental data for two cases reported in the literature. The data reproduced from the literature is obtained by using the function of Digitizer in Origin Pro 2016. Section 3.1 deals with bentonite expansion in a stagnant water/solution system, including bentonite extrusion and free swelling in strong sodium chloride solution and calcium chloride solution. Section 3.2 deals with bentonite erosion in a

Conclusions

A non-local model of clay erosion was presented by developing and integrating bond-based peridynamic formulations for clay swelling, particle detachment, and detached particle transport. The model was implemented within a computational framework (Pyramid; Yan et al., 2020). The model was validated by comparing its predictions to a series of experimental test, firstly under conditions prohibiting particle detachments (bentonite expansion and extrusion) and then under conditions facilitating

Author statement

Huaxiang Yan: Main developer of idea, concept, theory, implementation, validations and analysis, writing up the original draft, editing and revision.

Majid Sedighi: Major contribution to the development of idea, concept and theory, analysis of results, co-writing up the original draft, editing and revision. Supervision of the research.

Andrey P. Jivkov: Major contribution to the development of idea, concept and theory, analysis of results, co-writing up the original draft, editing and revision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Yan acknowledges the financial support through a joint PhD Scholarship by the China Scholarship Council (CSC No.201808350074) and the Department of Mechanical, Aerospace and Civil Engineering at the University of Manchester. Sedighi acknowledges gratefully the financial support provided by the Royal Society, UK, via IEC\NSFC\181466. Jivkov acknowledges the financial support from the Engineering and Physical Sciences Research Council (EPSRC), UK, via Grant EP/N026136/1.

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