Bentonite as sealing material results from the RESEAL project

1
Bentonite as sealing materialresults from the
RESEAL project

• SCK?CEN Van Geet, M., Bastiaens, W., Maes, N.,
  Weetjens, E., Sillen, X., Volckaert, G.
• UPC Gens, A.
• CIEMAT Villar, M.V.
• CEA Filippi, M., Imbert, C.
• ANDRA Plas, F.

The RESEAL project is financially supported by
the European Commission and by NIRAS/ONDRAF
June 6th, 2006
Exchange meeting on backfill material
2
overview

• Functional requirements, scope and objectives
• Material used and preparation
• Bentonite behaviour
• In-situ borehole seal experiment
• In-situ shaft seal experiment
• Conclusions

3
overview

• Functional requirements, scope and objectives
• Material used and preparation
• Bentonite behaviour
• In-situ borehole seal experiment
• In-situ shaft seal experiment
• Conclusions

4
Functional requirements (IAEA)

• restore the viability of the formation affected
  by the penetration to assure long term isolation
  of waste radionuclides.
• Idealiter
• Leave ground water circulation within and in the
  vicinity of the disposal formation exactly as it
  was before site exploration and development ?seal
  should have same hydraulic conductivity as
  geological formation
• Performance objectives
• K of seal efficiently low not to compromise the
  geological barrier
• K not influenced by the bonding of seal and host
  rock (interface)
• K of DZ must not compromise the geological
  barrier functions
• Properties of sealing materials should not change
  significantly with time

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Functional requirements international literature
review

• Presence/absence of seals has little or no effect
  on the overall performance of the system, BUT
  place anyway as
• Cautious and conservative approach appropriate
• Element of robustness protection of other EBS
  components
• Consistent with multiple barrier concept
• Sealing will add to confidence in the long-term
  isolation
• Will reduce public concern regarding long-term
  hazards
• Mainly qualitative requirements
• Prevent excavations and DZ from becoming
  preferred pathways
• Minimize escape of RN
• Minimize inflow/circulation of water in NF
  (mainly if aquifers overlie)
• Minimize connections between compartments
• Form mulitple barriers to gas and liquid flow at
  strategic locations
• Redundancy (increased reliability)
• Seal must be stable (THMC) and function
  acceptably during required lifetime
• Make unauthorised intrusions into repository
  difficult

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Scope and objectives of the RESEAL project

• RESEAL aimed to
• Evaluate HM characteristics of materials
• Test feasibility to fabricate and install
  borehole and shaft seal
• Check efficiency of seals (water, gas and RN
  transport)
• Model the HM behaviour
• Evaluate the experiments in the frame of PA

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overview

• Scope and objectives
• Material used and preparation
• Bentonite behaviour
• In-situ borehole seal experiment
• In-situ shaft seal experiment
• Conclusions

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Materials and preparation

• Materials studied
• FoCa clay interstratified Ca-beidellite(50)/kaol
  inite(50)
• kaolinite, quartz, goethite, hematite, calcite,
  gypsum
• Serrata clay interstratified smectite (90)/
  illite(10)
• quartz, plagioclase, K-feldspar, calcite, opal
• Preparation of the material
• Pre-compaction to obtain certain dry density
• Pre-compacted blocks
• Fixed shape, difficult to fill irregularities
• Mixture (50/50) of powder and high density
  pellets
• Adapt to shape to be filled
• Pellets are 25x25x15 mm with rdry2.0g/cm3

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overview

• Scope and objectives
• Material used and preparation
• Bentonite behaviour
• Homogenisation
• Hydro-mechanical properties
• Migration parameters
• In-situ borehole seal experiment
• In-situ shaft seal experiment
• Conclusions

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Will the pellet/powder mixture homogenise?
dry
2 weeks suction
6 weeks suction
Suction 1 month injection
Suction 4 months injection
g/cm3
2.5
1.5
1
2
0
0.5
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Bentonite behaviour hydraulic conductivity
50/50 pellet/powder mixture shown in slide before
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Bentonite behaviour swelling pressure
y0.013 x 11.79
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Bentonite behaviour swelling pressure evolution

• Typical saddle
• Variations due to effects of initial height and
  dry density
• ?plot dimensionless

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Bentonite behaviourswelling pressure evolution
rdry 1.6 g/cm3

• No scale effect on kinetics
• BUT Difference between low and high rdry
• ?loosely compacted powder
• higher initial K
• preferential hydration
• faster hydration
• faster swelling

rdry lt 1.6 g/cm3
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Bentonite behaviourswelling pressure evolution
winit 5

• Similar for powder and mixture
• BUT if initial water content higher ? first peak
  disappears
• Collapse of microstructure on saturation more
  important when hydration starts from high
  suction, whereas compensated by overall swelling
  of microstructure when initial suction is lower

winit 12
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Bentonite behavioursuction
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Bentonite behaviour modelling the swelling
pressure evolution

• CODE_BRIGHT including a dual porosity concept in
  the model allows to simulate very well the
  observed data
• Underbuilds the theory of the preferential
  hydration along the macropores in the powder

experiment
model
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Bentonite behaviour migration parameters
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overview

• Scope and objectives
• Material used and preparation
• Bentonite behaviour
• In-situ borehole seal experiment
• Lay-out
• Hydration
• Hydro-mechanical evolution
• Water gas transport
• RN migration
• In-situ shaft seal experiment
• Conclusions

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Borehole seal lay out

• Piezometer
• 250 mm diameter
• 2 seal compartments of 55 cm length with internal
  tube (56 mm)
• Several filters and total stress sensors
• Blocks 1.8g/cm3 rdry
• compartments 1.6g/cm3 rdry

• Horizontal borehole
• 275 mm diameter
• 14.7 m deep
• Sealing from 12.7 to 14.7 m depth in the borehole

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Borehole seal hydration
Start Artificial hydration
End Artificial hydration

• ?5 months till full saturation
• gt90 from natural hydration
• Swelling pressure less than foreseen due to
  disturbance of surrounding host rock

installation
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Borehole sealmodel prediction of hydration
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Borehole sealmodel simulation of hydration
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Borehole sealhydro-mechanical evolution
At installation
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Borehole sealhydro-mechanical evolution
At saturation
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Borehole sealhydro-mechanical evolution
After 7 years
Large variation in current pressure measurements,
but this is the current best estimate
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Borehole seal water gas transport

• Hydraulic conductivity as predicted from lab
  measurements 10-13 m/s
• Gas breakthrough not through seal, but at
  interface or through EDZ

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Borehole sealradionuclide migration

• 3.5 ml of NaI solution labelled with 3.28108 Bq
  of 125I
• Circulated over injection filter to allow pure
  diffusion

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Borehole sealevaluation of radionuclide migration

• First modellings Diffusion Advection
• Deff as determined by SCK?CEN on samples of
  1.6g/cm3 has been used
• Optimisation of modellings needed to correct for
  slight changes
• geometry
• transport parameters
• storativity of materials

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overview

• Scope and objectives
• Material used and preparation
• Bentonite behaviour
• In-situ borehole seal experiment
• In-situ shaft seal experiment
• Lay-out
• Hydration
• Hydro-mechanical evolution
• EDZ evolution
• Water gas transport
• Ongoing RN migration
• Conclusions

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Shaft seal lay out

• Filling of experimental drift with concrete
• Host rock instrumentation
• Removal of lining
• Installation of seal
• Start artificial hydration

Dry density
1.4 g/cm3
1.6 g/cm3
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Shaft sealhydration
2000 L of water artificially injected 50 of
expected volume
2 years after hydration
gt75 of RH reached after 2 y of hydration, full
saturation only reached after 5y
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Hydro-mechanical evolutioncontact pressure
seal-hostrock

• Gradual increase
• Top lt bottom
• /- homogeneous
• Still increasing

15th May 06
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Hydro-mechanical evolutionTotal stress in the
seal
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Hydro-mechanical evolutionPore water pressure
in the seal

• Middle of seal
• PW build-up 2004-2005
• Effective stress
• levels off

PT
PW
Nov04
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Displacements around the seal

• Middle of seal
• Displacements
• in the host rock
• 35cm into host
• 51cm into host
• Change of
• orientation as time goes by
• Swelling seal vs. host rock
• Towards equilibrium

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Shaft sealEDZ evolution in host rock
Time needed for pressure increase depends on
hydration of seal
Pressure increase from Furthest to nearest filter
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Shaft sealEDZ Numerical modelling
Point inside clay at 0.5m from interface with seal
Start artificial hydration
Start artificial hydration
Circumferential Total stress
Water pressure

• Qualitative
• similarity between data and prediction
• No response of pore water pressure sensors
  unsaturated host clay
• Rapid decrease of total stress sensors, followed
  by slow increase at time of swelling
• Quantitative
• host clay remains unsaturated during a longer
  time than predicted
• And stress changes are more moderate than
  predicted

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Shaft sealHydraulic conductivity in the seal

• Steady state constant head tests
• Injection / extraction
• 2004 2 ? 3 10-12 m/s
• 2006 1 10-12 m/s
• 2006 (after gas breakthrough) no change

Filter Result Date
Sint-4 3.5 10-12 m/s 3-2004
Sint-4 2.8 10-12 m/s 3-2004
SWext-3 1.8 10-12 m/s 4-2004
Radial WEST-5 2.7 10-12 m/s 12-2005
Sint-4 0.8 10-12 m/s 1-2006
SWext-3 1.1 10-12 m/s 1-2006
SWext-3 1.1 10-12 m/s 5-2006
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Hydraulic conductivity in the host

• Same order of magnitude 2.5 ? 3.5 10-12 m/s

Filter Result D
West-1 3.1 10-12 m/s 1
West-2 3.3 10-12 m/s 0.75
West-3 3.1 10-12 m/s 0.5
West-4 3.2 10-12 m/s 0.25
West-5 2.7 10-12 m/s 0
East-2 2.8 10-12 m/s 0.75
North-3 3.0 10-12 m/s 0.5
Vert.-5 5.4 10-12 m/s 1
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Shaft sealGas breakthrough test

• PROCEDURE
• Stepwise increase of gas pressure at filter
• Breakthrough
• Wait 1 month
• Apply breakthrough pressure again
• New breakthrough

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Shaft sealGas breakthrough test

• RESULTS
• Breakthrough
• 1st 13.3 bar
• 2nd 12.8 bar
• Event only happens after some time ? perhaps also
  at lower pressures?
• Restoration of conductivity

Breakthrough
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Shaft sealongoing radionuclide migration

• Evaluate migration parameters in EDZ and seal
• Compare inner and outer zone at middle hydration
  level
• Compare middle (1.4 g/cm3) with bottom (1.6
  g/cm3) in the centre

125I injection
Continuous pore water sampling advection
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overview

• Scope and objectives
• Material used and preparation
• Bentonite behaviour
• In-situ borehole seal experiment
• In-situ shaft seal experiment
• Conclusions

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Conclusions

• Fabrication of sealing materials and installation
  of seals is technically feasible
• Lab measurements have increased knowledge on the
  behaviour of sealing material and optimised
  models
• Borehole seal
• Relatively fast hydration, mainly natural
• Efficiency towards water and gas migration fairly
  well predicted
• Efficiency towards RN migration no preferential
  pathways
• Shaft seal
• Hydration takes much longer than foreseen, mainly
  artificial
• Efficiency towards water and gas migration fairly
  well predicted
• Efficiency towards RN migration ongoing

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Questions to PA/design/ confidence building
requirement RESEAL outcome
PA (sensu stricto) Ensure diffusive transport within NF
design Feasible
design Safely installable
design Hydration time, depends on functional requirement, which depends on the position of the seal fast immediate fulfillment of function slow may smooth THM transient in host rock ?
design Hydration time, depends on functional requirement, which depends on the position of the seal fast immediate fulfillment of function slow may smooth THM transient in host rock
design Hydration time, depends on functional requirement, which depends on the position of the seal fast immediate fulfillment of function slow may smooth THM transient in host rock
Confidence building Homogenisation of mixture
Confidence building Same K as host rock
Confidence building No easy gas breakthrough and KsealltKhost
Confidence building No preferential migration of RN
Confidence building Self sealing
Confidence building Stable (THMC) during required lifetime
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Observations made in terms of Safety Functions
Phenomena RESEAL observation Relevance in terms of Safety Functions
Mechanical Hydraulic Gas Voids are effectively sealed Limited displacements (convergence, swelling) Kseal Khost formation Kafter breakthrough Kbefore b-t Seal can effectively ensure the function "limitation of access" No adverse effect on EDZ, host formation and its associated functions (delay/spread releases in time, protect EBS) Solute transport in NF is diffusive, which enhances the lifetime of EBS components and their associated functions (overpack confinement, waste form slow release) Diffusive transport and EBS safety functions not jeopardized by occasional gas breakthrough episodes
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FoCa versus Boom Clay
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Borehole seal migration modelled with pure
diffusion