1 X-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 µCT Van Geet 1, Roels 2, Swennen 2, Dereeper 1, Maes 1 and Put SCKCEN 2 KULeuven 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 ( ) and co-funded by NIRAS/ONDRAF

2 Overview Introduction Principles of µCT Engineered clay sample: hydration of a clay pellet/powder mixture  Experimental set-up  Hydration  Swelling Conclusions

3 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

4 HADES-PRACLAY underground research facility Borehole sealing test Shaft sealing test

5 12 tons of pellet/powder mixture used for the seal construction Objective : 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

6 Principles of µCT Acquisition Reconstruction

7 Visualisation of linear attenuation coefficient Beer’s Law: Measured: intensity of X-rays passing through the object During reconstruction: Final visualisation Linear attenuation coefficient:

8 Engineered clay sample Plexiglass hydration cell with FoCa-Clay mixture Length7 cm Width3.8 cm Mixture 50% pellets / 50% powder Dry density 1.4 g/cm 3

9 Hydration scheme 1/2 month1 month 3 months Suction Injectioninjection Low pressure 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

10 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

11 Time evolution 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

12 Difference images in time Dry - ½ month of suction

13 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

14 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?

15 Swelling of pellets (2) Centroid in every slice is correlated with centroid of original pellet Centroid position in several sequential slices

16 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

17 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

18 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.