The Boom Clay geochemistry: Natural evidence
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The Boom Clay geochemistry: Natural evidence De Craen M. Belgian Nuclear Research Centre (SCK•CEN), Waste and Disposal Department, Boeretang 200, B-2400 Mol (Belgium). ABSTRACT In Belgium, the Boom Clay is studied as the reference formation for geological disposal of high-level radioactive waste and spent fuel. As the Boom Clay is considered as the main barrier for radionuclide migration/retention, a thorough characterisation of the clay and its pore water was done. This facilitates better understanding of the long-term geological processes and the distribution of the trace elements and radionuclides. From a mineralogical/geochemical point of view, the Boom Clay is considered as a rather homogeneous sediment, vertically as well as laterally. It is composed of detrital minerals, organic matter and fossils. Minerals are mainly clay minerals, quartz and feldspars. Minor amounts of pyrite and carbonates are also present. Small variations in mineralogical/geochemical composition are related to granulometrical variations. The radiochemical study indicates that the Boom Clay is in a state of secular radioactive equilibrium, meaning that the Boom Clay has not been disturbed for a very long time. Pore water sampling is done in situ from various piezometers, or by the squeezing or leaching of clay cores in the laboratory. These three pore water sampling techniques have been compared and evaluated. Boom Clay pore water is a NaHCO3 solution of 15 mM, containing 115 mg⋅l-1 of dissolved natural organic carbon. Some slight variations in pore water composition have been observed and can be explained by principles of chemical equilibrium. INTRODUCTION In Belgium, geological disposal in clay is the primary option for the isolation of high-level radioactive waste and spent fuel from the biosphere. For over 25 years, the Boom Clay in the Mol/Dessel nuclear zone has been studied as a reference formation for the geological disposal of radioactive waste. As the Boom Clay is considered to be the main barrier for radionuclide migration/retention, a thorough characterisation of the Boom Clay and its pore water was done. This facilitates better understanding of the long-term geological processes and the distribution of the trace elements and radionuclides in the Boom Clay. GEOLOGICAL SETTING The Boom Clay is a Tertiary deposit (Oligocene, Rupelian, 30 Ma old), extending in northern Belgium (Figure 1a). It crops out along a west-east line from the Scheldt estuary area in the west, along the Rupel river and the Demer river, towards the Meuse river in the east. The Boom Clay slightly dips (1-2 %), and thickens, towards the north-east.
a)
b)
Doel *
* Mol
50 cm
Figure 1: a) The Boom Clay in northern Belgium, with indication of the outcrop zone, the thickness of the formation, and the depth of the base of the formation; b) banded nature of the Boom Clay in its outcrop area.
At the Mol-Dessel nuclear zone, the reference site for research, development and demonstration studies, the Boom Clay is about 100 m thick and is present at a depth of -
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