1 X-ray microfocus computed tomography in Materials research, doctoraatsseminarie HP15, KULeuven...
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Transcript of 1 X-ray microfocus computed tomography in Materials research, doctoraatsseminarie HP15, KULeuven...
1X-ray microfocus computed tomography in Materials research, doctoraatsseminarie HP15, KULeuven
February 20th 2003
Characterization of the hydration of a clay pellet/powder mixture
by means of µCTVan Geet1, Roels2, Swennen2, Dereeper1, Maes1 and Put1
E-mail: [email protected]
1SCK•CEN 2KULeuven
Part of RESEAL II project co-funded by the European Commission and performed as part of the fifth EURATOM framework programme, key action
Nuclear Fission (1998-2002) and co-funded by NIRAS/ONDRAF
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Overview
• Introduction• Principles of µCT• Engineered clay sample: hydration of
a clay pellet/powder mixture Experimental set-up Hydration Swelling
• Conclusions
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Introduction: RESEAL project
• SCK studies the possibilities of underground storage of high-level nuclear waste
• RESEAL aims to demonstrate sealing techniques for shafts and boreholes
• To demonstrate that a low permeability seal of bentonite avoids preferential migration of water, gas and radionuclides along the seal/host rock interface and through the excavation disturbed zone
• To demonstrate that it is possible to predict the hydro-mechanical behaviour of the seals
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12 tons of pellet/powder mixture used for the seal construction
S a n d
F oC aclay
C o n c re teR esin
R esin
B oom c lay
H o stro c k in s tru m en ta tio n
-1 6 .3 5 m
-1 3 .9 5 m
-1 2 .9 0 m
-1 4 .11 m
C on crete sea lObjective :Hydraulic conductivity of the seal hydraulic
conductivity of the host rock
Seal dimensions : 2.24 m height / 2.20 m diameter
50/50 pellet/powder mixture of FoCa clay
Compaction of the mixture on the first 60 cm- density 1.55 g/cm³
Manual filling
- density 1.40 g/cm³
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Visualisation of linear attenuation coefficient
2.3
8.3
0
0
log
E
Zba
sI
I
eII s
Beer’s Law:
Measured: intensity of X-rays passing through the object
During reconstruction:
Final visualisation
Linear attenuation coefficient:
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Engineered clay sample
• Plexiglass hydration cell with FoCa-Clay mixture
Length 7 cm
Width 3.8 cm
Mixture 50% pellets / 50% powder
Dry density
1.4 g/cm3
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Hydration scheme
1/2 month 1 month 1 month 3 months
Suction Suction Injection injection
Low pressure
Low pressure
5 bar 5 bar
After hydration a permeability test was performed, showing a constant permeability
complete hydration can be assumed
hydraulic conductivity = 2.0E-12 m/s
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Visualisation of the dry cell
• Pellets/powder can be discriminated
• Fractures and microfractures within the pellets
• High density features (Fe-oxy/hydroxides)
• Heterogeneous porosity and macroporosity within powder
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Time evolution
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dry ½ month of suction
1 ½ months of suction
1 ½ months of suction
and 1 month of injection
1 ½ months of suction
and 4 months of injection
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Difference images in time
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Dry - ½ month of suction
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Density calculation
•As 3 densities(pellet ,total dry density,total wet density) are known•As atomic number is assumed constant
attenuation can be converted to density
•Adapted reconstruction was performed to correct for X-ray variations inbetween measurements•Air can not be used as known density as we zoomed in on plexiglass cell to enhance resolution
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Swelling of pellets
• At the bottom of the sample “new developed”?? fractures were found before final hydration
• Questions : Is this the outline of an
original pellet? If so, can we measure the
swelling?
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Swelling of pellets (2)
• Centroid in every slice is correlated with centroid of original pellet
Centroid position in several sequential slices
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Swelling of pellets (3)
• Outline is quite similar to the original pellet
• For this part of the pellet a volume increase of 54% is measured
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Dismantling and drying
• After dismantling water content in bottom pellet and total sample was measured (destructively) Sample: 27.54% Pellet: 29.1% Complete hydration can be assumed
• After drying, new scan was made Original position of pellets can not be found Sample shows several fractures in random
position
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Conclusions
• µCT is a complementary tool for clay and fluid flow characterisation
• Within Foca clay mixture, pellets are affected first, probably due to higher suction
• Some fractures were observed after several weeks of injection, origin unknown, outlined along original pellet. These fractures disappeared after total saturation.
• Apart from these fractures, homogenisation does occur between pellets and powder
• No evidence was found for a ‘memory’ of the pellets
• The pellet/powder mixture seems to fulfill the demands for resealing a repository, although hydration takes very long.