The Boom Clay in Belgium is investigated in the context of geological nuclear waste disposal, making use of the High Activity Disposal Experimental Site (HADES) underground research facility. This facility, located in the Boom Clay at a depth of 225 m below the surface, offers a unique access to a microbial community in an environment, of which all geological and geochemical characteristics are being thoroughly studied. This study presents the first elaborate description of a microbial community in water samples retrieved from a Boom Clay piezometer (borehole water). Using an integrated approach of microscopy, metagenomics, activity screening and cultivation, the presence and activity of this community are disclosed. Despite the presumed low-energy environment, microscopy and molecular analyses show a large bacterial diversity and richness, tending to correlate positively with the organic matter content of the environment. Among 10 borehole water samples, a core bacterial community comprising seven bacterial phyla is defined, including both aerobic and anaerobic genera with a range of metabolic preferences. In addition, a corresponding large fraction of this community is found cultivable and active. In conclusion, this study shows the possibility of a microbial community of relative complexity to persist in subsurface Boom Clay borehole water.

The extent of the physico-chemical processes of concern in the study on the acceptability of the Belgian Eurobitum bituminised waste for underground disposal will depend on the degree of ageing of the bituminous matrix. Therefore, the ageing of non-radioactive simulates of Eurobitum under gamma irradiation was studied both in the presence and absence of oxygen. Chemical changes in the bitumen structure were measured in the mid-infrared region with the attenuated total reflectance infrared technique (ATR/FTIR) and evaluation of the peaks at 1700 cm -1 (C=O) and 1600 cm -1 (C=C). Even for the highest total absorbed doses (~800 kGy at ~150 Gy/h), the effect of the irradiation in Eurobitum was found to be small and was, in any case, smaller than for samples heated for three hours at 130°C in air. Long-term behaviour of Eurobitum in underground disposal conditionsOpposite to the glass matrix, the resistance to leaching of the bitumen matrix is not considered in safety assessments: an instantaneous release is assumed, and the durability of the Eurobitum waste form is regarded as a safety reserve. The main barrier function is performed by the Boom Clay host formation. The research programme focuses thus on the compatibility of the bituminised waste with the Boom Clay i.e. the study of processes that may potentially affect or disturb the performance of the Boom Clay host formation [2,3]. These processes of concern are: Mat. Res. Soc. Symp. Proc. Vol. 932 © 2006 Materials Research Society i) The production of water-soluble organic ligands, due to radiolytic, chemical, and microbial degradation of bitumen, that might facilitate the migration of the critical radionuclides (RNs) in the geologic formation. Oxalate and carbonate were shown to be the most important radiolytic degradation products of Eurobitum when γ-irradiated in the presence of water, but their effect on Pu(IV) and Am(III) were limited in Boom Clay [4].ii) The release of large amounts of NaNO 3 and other soluble salts embedded in the waste. A full-size Eurobitum drum (220 dm 3 , containing 54 to 60 kg of NaNO 3 ) could be depleted in Na + and NO 3 in less than 20000 years [5]. The effects of a NaNO 3 plume on the RN retardation properties of Boom Clay are presently under investigation at the SCK•CEN. The release of most RNs is limited by their low solubility in the near field [2]. When leached, these RNs sorb onto the surrounding solid materials with high sorption properties such as cement and clay-based materials.iii) The build-up of a pressure due to gas generation (by radiolysis, microbial activity, and anaerobic corrosion of the steel drums) and to the water uptake (by the hygroscopic salts embedded in the waste). This might result in a disruption of the surrounding formation, hence the creation of preferential paths for RNs migration. The swelling and swelling pressure build-up of Eurobitum in contact with alkaline pore water is also being studied at the SCK•CEN.The extent of these processes of concern is affected by the continuous physico-chemical evolut...