Characterization of the hydration of a clay pellet/powder mixture by means of µCT

1. Characterization of the hydration of a clay pellet/powder mixture by means of µCT Van 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 February 20th 2003 X-ray microfocus computed tomography in Materials research, doctoraatsseminarie HP15, KULeuven

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-PRACLAYunderground research facility Borehole sealing test Shaft sealing test

5. 12 tons of pellet/powder mixture used for the seal construction Seal dimensions : 2.24 m height / 2.20 m diameter 50/50 pellet/powder mixture of FoCa clay Objective : Hydraulic conductivity of the seal  hydraulic conductivity of the host rock 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 Measured: intensity of X-rays passing through the object Beer’s Law: During reconstruction: Final visualisation Linear attenuation coefficient:

8. Engineered clay sample • Plexiglass hydration cell with FoCa-Clay mixture

9. Hydration scheme • 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 1 ½ months of suction and 1 month of injection 1 ½ months of suction and 4 months of injection ½ month of suction 1 ½ months of suction dry

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.