Report from the Crystalline rocks & buffer workshop Mattias Åkesson, Clay Technology AB

Workshop divided in four blocks 1.Buffer/backfill hydration and influence of rock 2.THM process understanding 3.Homogenization, pellets, piping and erosion 4.Chemical and mineralogical interactions A number of specific issues were suggested for each block Each block started with an introduction Possible form of output: ProcessesRelevance for PANeed for experimental data Need for code development Need for development of conceptual models

1. Buffer/backfill hydration and influence of rock Introduction: M. Åkesson Specific issues: To what degree can we confidently predict buffer saturation times? Is the degree of uncertainty important from PA point of view? Interaction rock buffer /crystalline host rock: i) Effect of localized water entry (individual fractures inflow); ii) Is it necessary to consider especial properties for the rock/buffer interface? Should we work more intensively towards routine performance of 3D THM numerical analysis or 2D computations are sufficient for our purposes?

Modeling approaches 2D – axisymmetric buffer model

Modeling approaches 2D-axisymmetric backfill model Example of influence of fracture distance

1. Buffer/backfill hydration and influence of rock Discussion PA aspects: –Moderate relevance of time of hydration in itself –Critical that it gets hydrated eventually –Less crucial how long it takes –If hydration is slow, it may have influence on the chemical state of the barrier Rock interaction and interface properties –Planned Äspö project ”BRIE”. Study effects of intersecting fracture. Joint task for EBS and groundwater modelers. –No general need for special interface properties, except perhaps friction elements 3D or 2D modeling –Bounding calculations in 2D, with sensitivity analysis, is sufficient if it can be justified –3D without sensitivity analysis in dangerous - Order of complexity: T, TH, THC, THM

2. THM process understanding Introduction: H.R. Thomas Specific issues: Are there additional phenomena that should be included in our understanding/modelling of the buffer? For instance: i)effect of microstructure evolution; ii)existence of different states of water in the bentonite; iii)existence of a threshold gradient in water flow; iv)thermo-osmosis Can we obtain all required buffer parameters from (small scale) laboratory tests?

2. THM process understanding Discussion –In some experiments the models tend to over predict the rate of hydration –This may be caused by the uncertainty in parameter data –Or by limits in the conceptual models: A number of phenomena has been proposed which may explain these: i) effect of microstructure evolution; ii) existence of different states of water in the bentonite; iii) existence of a threshold gradient in water flow; iv) thermo-osmosis –Confidence building. Scientific approach has a central role,: –Characterize uncertainties; –Extrapolation to very long time scales – thermodynamically grounding & use natural analogs

3. Homogenization, pellets, piping and erosion Introduction: M. Åkesson Specific issues: Do we require better understanding of the THM behaviour of special materials such as pellets or pellet-powder mixtures? Is microstructure evolution especially important in this case? Significance of the processes associated with Homogenization / gaps / erosion / piping of the bentonite buffer. Are potential irreversible effects in large swelling/compression cycles understood?

Comparison with experimental results Maximum fallout Theoretical section Saturated oedometer test CRT void ratio profile Data indicate that hysteretic effects are one cause of remaining heterogeneities

Piping and erosion A model has been derived from erosion tests results, assuming a linear relation in a double logarithmic diagram: where m s = accumulated mass of eroded bentonite (g) m w = accumulated mass of eroding water (g)  = α = 0.65 The current limit that only 1 % of the inflow into one tunnel is allowed from one deposition hole is supported by this model.

3. Homogenization, pellets, piping and erosion Pellets and erosion –Difficulties to predict the water flow through pellets fillings. Influence how fast we must backfill. But, this is possible to test and even to train. –Question about if we should expect homogenization: Yes if it is the same material, but perhaps not if swelling/compression cycles are irreversible –Erosion of pellets depending on salinity, water flow and granularity –Empirical model imply a maximum allowed inflow of 1% of tunnel available pore space into the deposition hole –Grouting is the method to limit the inflow to 1 % of total inflow. –The plug is a therefore an important system component Homogenization –Definition of homogenization: Convergence of densities or keeping the difference in permissible bounds –Irreversible swelling/compression cycles should be expected. –Same phenomena observed in free swelling retention curves The mechanical constitutive laws have a key role, in contrast to conventional THM modeling

4. Chemical and mineralogical interactions Introduction: O. Karnland Specific issues: What are the effects of the characteristics and phenomena of the transient period on the long term state of the buffer? Possible presence of chemical phenomena interacting with THM behaviour and leading to irreversible effects: i) dissolution/precipitation phenomena; ii) cation exchange processes; iii: others?

4. Chemical and mineralogical interactions Introduction Possible mineral reactions Enrichment of substances originating from the saturating groundwater Accessory mineral dissolution/precipitation Montmorillonite surface reactions, Ion - exchange, Na → Ca Colloid release from Na-montmorillonite Montmorillonite alteration, illitization/dissolution Geochemical modeling Microstructure view – Multipore systems The chemical view – Ion equilibrium

0.5 g Wyoming material after 43 days in 100 mL H 2 O Ca- montmorillonite Na- montmorillonite No colloid formation in Ca-montmorillonite

Conceptual view of montmorillonite (bentonite) C1C1 C 2 <<C 1 flux of anions C 3 =0 Chemical view: ion equilibrium with interlayer space Ion equilibrium between montmorillonite interlayer space and an external solution - Consequences for diffusional transport. Birgersson and Karnland, 2008, in press, Geochimica et Cosmochimica Acta, DOI: /j.gca Bentonite solution 1solution 2 C1C1 C 2 =0 flux of anions C 3 =0 Micro-structural view: no anions in interlayer space microstructure Bentonite solution 1solution 2

4. Chemical and mineralogical interactions Discussion Good if different groups have different conceptual understanding Multi-component reactive transport models, not pushed to the same level as THM models

Advances since 2003 Empirical erosion model “Chemical view” of bentonite – conceptual breakthrough Thermodynamic approach Molecular dynamics Significant advances of conceptual understanding of bentonite behavior The large-scale experiments Industrial aspects

Need for further research Bentonite hydration: Identification of processes not included in present formulation THM behavior of pellets and pellets mixtures Piping and erosion Irreversible swelling/compression in large gap filling Chemical issues: implementation of “chemical view” in geochemical codes. Greater integration of THM & C