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: mvgeet@sckcen.be

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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HADES-PRACLAYunderground research facility

Borehole sealing test

Shaft sealing test

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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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Principles of µCT

Acquisition

Reconstruction

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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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and 4 months of injection

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Difference images in time

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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.