Report from the
Crystalline rocks & buffer workshop
Mattias Åkesson, Clay Technology AB
T
C LAY
ECHNOLOGY AB
Workshop divided in four blocks
1.
2.
3.
4.
Buffer/backfill hydration and influence of rock
THM process understanding
Homogenization, pellets, piping and erosion
Chemical and mineralogical interactions
A number of specific issues were suggested for each block
Each block started with an introduction
Possible form of output:
Processes
Relevance for PA
Need for
experimental
data
Need for code
development
Need for development
of conceptual models
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ECHNOLOGY AB
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?
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Modeling approaches
2D – axisymmetric buffer model
2000
1E+04
T(t) = From thermal model of entire repository
pl = 4 MPa
Initial
Initial
Homogenized
Average
Saturation time [years]
Saturation time [years]
Homogenized
1E+03
1500
Average
Symmetry boundary
1E+02
Heat load
1E+01
1000
qh = 0
ql = 0
500
0
1E+00
1E-13
1E-12
1E-11
1E-10
Rock hydraulic conductivity [m/s]
1E-13
1E-12
1E-11
T(t) = From thermal model of entire repository
ql = 0
1E-10
Rock hydraulic conductivity [m/s]
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Modeling approaches
2D-axisymmetric backfill model
Symmetry axis
Example of influence of fracture distance
Symmetry
axis
10 m
3m
Symmetry plane
2.31 m
2.86 m
Maximum fallout
Saturation time (years)
1 10
Symmetry axis
3
10 m
80 m
100
12 m
T=5E-9
T=5E-10
Numerical models
10
1
10
100
Fracture distance (m)
Theoretical section
(years)
3
1 10
2.31 m
Symmetry plane
2.86 m
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1. Buffer/backfill hydration and influence of rock
Discussion
•
PA aspects:
–
–
–
–
•
Rock interaction and interface properties
–
–
•
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
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
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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?
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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
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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?
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Comparison with experimental results
CRT void ratio profile
Saturated oedometer test
1,90
1,70
initial void ratio
homogenized initial values
1,50
void ratio [-]
Experimental data CRT
1,30
1,10
0,90
0,70
0,50
525
575
625
675
725
775
825
875
radial distance [mm]
2
1.2
TheorSect
MaxFallout
p_swell(e)
KMXAR4
1
Void ratio (-)
Void ratio (-)
1.1
Maximum fallout
0.9
0.8
1.5
1
0.7
Theoretical section
0.6
0
0.5
1
1.5
2
2.5
0.5
0.1
1
10
100
3
Net mean stress (MPa)
Radius (m)
Data indicate that hysteretic effects are one cause of remaining heterogeneities
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Piping and erosion
400
Displacement
measurements
Thin slots where
water will pour out
Erosion
measurements
600
Ditch collecting
water and eroded
material
Bentonite blocks
Bentonite pellets
Copper tube
Water inflow
A model has been derived from erosion tests
results, assuming a linear relation in a double
logarithmic diagram:
ms    ( mw )
where
ms= accumulated mass of eroded bentonite (g)
mw= accumulated mass of eroding water (g)
= 0.02-2
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.
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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
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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?
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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
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0.5 g Wyoming material after 43 days in 100 mL H2O
Camontmorillonite
Namontmorillonite
No colloid formation in Ca-montmorillonite
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Conceptual view of montmorillonite (bentonite)
Micro-structural view:
no anions in interlayer space
microstructure
solution 1
C1
Bentonite
C2=0
solution 2
flux of anions
C3=0
Chemical view:
ion equilibrium with interlayer space
solution 1
C1
Bentonite
C2<<C1
solution 2
flux of anions
C3=0
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: 10.1016/j.gca.2008.11.027
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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
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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
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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
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C LAY
ECHNOLOGY AB