Screening of suitability of pelitic hard rock formations for radioactivewaste disposal in Belgium

One of the host rocks considered for disposing high-level, long-lived and heat-emitting radioactive waste arefine-grained detritic rocks. In Belgium, most research on the storage of nuclear waste is focused on poorly-lithifiedclayey host rock, i.e. the Oligocene Boom and Eocene Yper clays present in the subsurface of northern Flanders.Beside clay also pelitic shaly and slaty formations have favourable characteristics to consider as potential hostrock in Belgium (cf. 1979 European Catalogue).In this contribution a screening assignment is performed on the identification of Palaeozoic pelitic hostrock formations for their suitability to store nuclear waste. Four exclusion criteria are used to eliminate thoseformations that are less suitable as a host. The criteria are: host rock lithology (only pelitic rock is considered),minimum thickness of the formation (> 100 m), minimum depth (a minimum 100 m-thick formation needs to bepresent below 200 m depth), and maximum depth (at least 100 m above 1500 m). The screening approach allowedto retain seven low-grade metamorphic shaly and slaty formations of the Lower Palaeozoic Brabant Massif and theUpper Palaeozoic Ardenne Allochton that have favourable thicknesses and a suitable fine-grained lithology. Theirgeological extent is known to be sufficient, either in subcrop or in outcrop. Currently, a detailed mineralogicaland petrophysical research is performed to investigate complementary properties linked to the long-term safetyof a repository. For each formation, long (> 100 m) continuous drill cores were sampled at low resolution (2 m)to investigate the variation in mineralogical content (with QXRD) and thermal conductivity with depth. To decidehow to cut the cores, their internal geological structure was investigated using micro-CT scanning. The results ofour analyses are compared to published values taken from other (inter)national pelitic host rock investigations.Other formations were not selected because of less optimal characteristics or insufficient material to allowfurther investigations. However, none of the formations should be considered as definitely excluded.

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