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the ECW from 12.5 at the beginning to as low as 5 after 510 days. The most extensive changes occurred
in the mineralogy of clay phases interacted with the YCW, namely mixed-layered illitesmectite,
kaolinite, chlorite and illite. The fact that the position of the XRD reflections of clay minerals
are not changed with time in the alkaline batch experiment suggests that no clay mineral
phase-to-phase transformation occurred, e.g. illitization of smectite, but rather dissolution was a
dominant process. The dissolution is reflected in the decrease of the specific surface area (SSA) and
decrease of the cation-exchange capacity (CEC) parameters.
5. Conclusions
Comparing the two data sets from the Test Drift and the Connecting Gallery (HADES URL), the
general conclusions could be drawn with respect to the degree and the extent of the oxidation at different
times. The mineralogical evidence for the oxidation is traceable within the first ~4.5 cm
ahead from the gallery lining both in the Test Drift and the Connecting Gallery. The gypsum as the
most common oxidation product of pyrite was found in the both data sets, while the jarosite was
found exclusively in the Connecting Gallery. This point to locally different geochemical conditions
concerning Eh and pH in the Test Drift and Connecting Gallery. However, there is no mineralogical
evidence to state that the ventilation could be an important factor affecting the extent of the oxidation
in the the two studied cases. Therefore, the extent of the oxidation is determined by conditions
or processes occuring during or soon after excavation, rather than during the ventilation of the galleries
during the operational phase.
The mineral stability of the Boom Clay depends on the initial base stregth of the applied alkaline
solution. The YCW with the original pH of 13.2 induced more extensive mineral changes than the
ECW having the initial pH of 12.5. The most significant changes in the mineralogy of Boom Clay
caused by the alkaline plume perturbations involve the alteration of Na-Ca plagioclases to Kfeldpsars
in the both studied cases and the dissolution of clay minerals (mainly mixed-layered illitesmectite
phases) in the YCW. The dissolution of clays is accompanied by the decrease in the Cation
Exchange Capacity and the Specific Surface Area parameters. The clay dissolution might increase
the Boom Clay porosity and thus increase the hydraulic conductivity in the repository near-field.
However, important to note is that based on modelling results of Wang et al. (2007 [4]), the extent
of the alkaline plume disturbed zone in Boom Clay is very limited even after 100 ka.
6. Acknowledgements
This project has been funded by the European Commission and performed as part of the sixth Euratom
Framework Programme for nuclear research and training activities (2002-2006) under contract
FI6W-028403.
References
[1] De Preter, P. (2007) The long-term safety functions within the disposal programmes of
ONDRAF/NIRAS. ONDRAF/NIRAS note O/N 207-0526.
[2] Wickham, S.M. (2005) The ONDRAF-NIRAS Supercontainer Concept. Galson Sciences, UK
(2005).
[3] NIROND. "SAFIR-2, Second safety Assessment and Feasibility Interim Report." NIROND,
Brussels (2002).
[4] Wang, L., Jacques, D., and De Cannière, P. (2007): Effects of an alkaline plume on the Boom
Clay as a potential host formation for geological disposal of radioactive waste, SCK•CEN report,
ER-28, first full draft, March 2007.
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