Modelling the interaction of hyperalkaline fluids with simplified rock mineral assemblages
Introduction
There are a range of options for the long-term management of radioactive waste. One of these is to place the wastes in a repository excavated in stable rock formations, deep underground (deep geological disposal). Current concepts for the geological disposal of radioactive wastes envisage the use of multiple barriers (i.e., both engineered and geological barriers) to contain the radioactive materials in the waste. The barriers include the waste form itself, a container (or canister) surrounded by a backfill and finally the geosphere. Cementitious materials, for example concretes and grouts, may be used extensively in the construction of the disposal facility and as a backfill. After closure, the repository will saturate with groundwater and become part of a modified regional groundwater flow system. One possible scenario then is that groundwater equilibrated with the cement porewater will migrate into the geosphere and as a result form an alkaline plume (initially with a pH of 12.5–13). The alkaline plume will react with the components of the rock and that may lead to changes in the mineralogical, chemical and physical characteristics of the surrounding rock. The interactions of the alkaline plume with the surrounding rock will be complex, and therefore the use of coupled geochemistry and hydrogeological transport computer models to scope the changes is indicated. The extent and timescale of leaching of alkali from the cementitious materials will depend on the relative importance of different groundwater transport processes that could operate in the repository and on the groundwater chemistry. For example, self-sealing of a repository could occur as a result of calcite precipitation under some conditions [e.g., Pfingsten (2002)], reducing the formation of an alkaline plume. Nevertheless, it is important to understand the consequences that could result for scenarios in which a significant alkaline plume is formed downstream from a cementitious repository.
In the recent past, a series of laboratory column experiments were carried out by the British Geological Survey in order to test the capabilities of coupled models to predict the evolution of outflow fluid compositions and product solids. These experiments reacted single minerals (i.e., quartz, albite, calcite and muscovite/quartz) (Bateman et al., 2001a) and potential host rock lithologies (i.e., Borrowdale Volcanic Group fault rock, Äspö granite and Wellenberg marl) (Bateman et al., 2001b) with simplified cement pore fluids.
An overview of the experimental procedures and preliminary predictive modelling have been given in Bateman et al. (1999). The objectives of the present study were to develop a better understanding of these experiments and to improve the correspondence between the data and the predictions of the computer models.
Section snippets
Description of experiments
A schematic diagram of the experimental set-up is shown in Fig. 1. The equipment consisted of two reservoirs, which were kept at room temperature and which contained the reactant fluids under a protective N2 atmosphere (1 bar) to prevent ingress of CO2. The reservoirs were connected to a series of heated conditioning vessels, held at the experimental temperature (70 °C), which in turn were connected via peristaltic pumps to the columns. Each column (internal diameter 7.5 mm and length 0.3 m)
Modelling codes
A number of reactive transport codes have been developed over recent years by various authors for a variety of purposes. The rigour with which these computer programs address each of the coupled processes relevant to the near-field environment of an underground repository varies depending on the application envisioned by the developer. That is, there are significant differences in the conceptual and mathematical models, the numerical implementations and the solution methods. The simulations
Models of single minerals
Coupled geochemistry and hydrological transport models of the experiments on columns packed with a single mineral (i.e., quartz, albite, calcite or a mixture of muscovite and quartz) are developed systematically in this section.
The features of the experimental results that can be used to assess the quality of a model include:
- (a)
The chemistry of the outflow fluid.
- (b)
The extent of primary mineral dissolution.
- (c)
The type and location of the secondary minerals formed.
The first of these is the most
Models of synthetic host rock lithologies
Coupled geochemistry and hydrological transport models of the experiments on columns packed with a synthetic representation of a potential host rock (i.e., Borrowdale Volcanic Group fault rock, Äspö granite or Wellenberg marl) are developed systematically in this section. Synthetic representations of the potential host rocks are considered (rather than the actual rocks) because it should be simpler to relate these to the experiments on columns packed with a single mineral (Section 4). In
Discussion
In this work, an attempt has been made to develop systematically reactive transport models of the column experiments carried out by the British Geological Survey. The results of this study are summarised below.
Conclusions
The following general conclusions derive from this study:
- (a)
Some care is required in the employment of “standard” thermodynamic databases (e.g., see Section 3.2.1) to ensure that the data they contain is appropriate for the intended application (e.g., the data is satisfactorily complete).
- (b)
In simulating the percolation, at rather high flow rates, of highly alkaline fluids through columns packed with either single minerals or simple mixtures, the model parameters that have the most significant effect
Acknowledgements
The work described here was carried out as part of the ECOCLAY II project, which was undertaken within the framework of the European Commission’s R&D programme on Nuclear Fission Safety (1999–2002), with funding by the European Commission and United Kingdom Nirex Limited.
References (25)
- et al.
Kinetics of quartz dissolution at low temperature
Chem. Geol.
(1990) - et al.
Dependence of albite dissolution kinetics on pH and time at 25 °C and 70 °C
Geochim. Cosmochim. Acta
(1986) - et al.
The dissolution kinetics of quartz as a function of pH and time at 70 °C
Geochim. Cosmochim. Acta
(1988) - et al.
Muscovite dissolution kinetics as a function of pH and time at 70 °C
Geochim. Cosmochim. Acta
(1989) - et al.
Rate and mechanism of the reaction of silicates with cement pore fluids
Appl. Clay Sci.
(1992) - Allison, J.D., Brown, D.S., Novo-Gradac, K.J., 1990. MINTEQA2/PRODEFA2 – Geochemical Assessment Model for Environmental...
- Atkinson, A., 1985. The time dependence of pH within a repository for radioactive waste disposal. UKAEA Report...
- Ball, J.W., Nordstrom, D.K., 1991. WATEQ4F – User’s Manual with Revised Thermodynamic Data Base and Test Cases for...
- Bateman, K., Coombs, P., Noy, D.J., Pearce, J.M., Wetton, P., Haworth, A., Linklater, C.M., 1999. Experimental...
- Bateman, K., Coombs, P., Pearce, J.M., Wetton, P.D., 2001a. Nagra/Nirex/SKB column experiments: fluid chemical and...
Cited by (11)
Reactive transport modelling of cement-groundwater-rock interaction at the Grimsel Test Site
2017, Physics and Chemistry of the EarthTwo-dimensional reactive transport modeling of the alteration of a fractured limestone by hyperalkaline solutions at Maqarin (Jordan)
2016, Applied GeochemistryCitation Excerpt :The interaction between groundwater and cement causes the generation of hyperalkaline solutions (pH 12.5–13.5), which may react with the rocks hosting the repositories and change their physical and chemical properties. Experimental and modeling studies of such interactions have been common in the last years (e.g. Adler, 2001; Read et al., 2001; Savage et al., 2002, 2011; Soler, 2003, 2013; Gaucher et al., 2004; Hoch et al., 2004; Mäder et al., 2005; Soler and Mäder, 2005, 2007, 2010; Sánchez et al., 2006; Marty et al., 2009, 2014; Honty et al., 2010; Soler et al., 2011; Kosakowski and Berner, 2013; Moyce et al., 2014). A common finding of these studies has been a reduction of porosity near the cement–rock interface due to the precipitation of secondary phases.
High-pH plume from low-alkali-cement fracture grouting: Reactive transport modeling and comparison with pH monitoring at ONKALO (Finland)
2012, Applied GeochemistryCitation Excerpt :In the context of the safety assessment of deep geological repositories for radioactive waste, it has been common practice to assume long-term interaction between host-rocks or engineered-barrier materials with hyperalkaline solutions (pH > 12) originating from the degradation of hydrated cement and concrete. Experimental and modeling studies of such interactions have been common in recent years (e.g. Adler, 2001; Savage et al., 2002; Soler, 2003; Gaucher et al., 2004; Hoch et al., 2004; Mäder et al., 2005; Sánchez et al., 2006; Soler and Mäder, 2005, 2007, 2010; Honty et al., 2010). Additionally, low-alkali cements have been recently developed to minimize the potential impact of hyperalkaline solutions (e.g. Arenius et al., 2008; Lothenbach and Wieland, 2010).
A comparative study of the modelling of cement hydration and cement-rock laboratory experiments
2011, Applied GeochemistryCitation Excerpt :Many of these studies have highlighted difficulties in scaling reactive surface areas, not only as applied to mineral dissolution (Hoch et al., 2004; Soler et al., 2006; Soler and Mäder, 2007), but also to mineral growth (Watson et al., 2009). The incorporation of geometric or BET surface areas in simulations to model mineral dissolution has led to the overestimation of reaction rates by up to two orders of magnitude (Hoch et al., 2004; Soler et al., 2006; Soler and Mäder, 2007). The issues associated with the up-scaling of laboratory-derived kinetic data to model at the natural system scale has previously been encountered during the evaluation of weathering in groundwater catchments in granitic rocks (e.g. White and Brantley, 1995, 2003).
The effect of Al(OH)<inf>4</inf><sup>-</sup> on the dissolution rate of quartz
2006, Geochimica et Cosmochimica Acta
- 1
Present address: ANSTO Environment, PMB 1 Menai, NSW 2234, Australia.