Current Understanding of Transport at the Meuse/Haute-Marne Site and Relevant Research and Development Programme in the Planned Underground Research Laboratory

摘要

The Meuse/Haute-Marne site, on the eastern border of the Paris Basin, presents a very simple geological context of alternating carbonated and clay formations. The host formation (Callovo-Oxfordian argillites lying at a depth of 400-500 m) is uniformly thick (130-135 m) with a constant dip of 2°NW. The conceptual model for transport has been established based on the data of the preliminary reconnaissance phase (1994-96) and can be summarised as follows. Radionuclides will be transported within the host formation by diffusion. Transport in the carbonated formations over and underlying the argillite will be advective with significant dispersion, but the velocity will remain low. the opening and ventilation of disposal vaults will create an EDZ. It is assumed that the damage mechanisms will only lead to changes in the order of magnitude of the transport time in a zone a few metres thick adjacent to the underground workings, and will not create new pathways for radionuclide migration. A preliminary performance assessment exercis focusing on the geological barrier was carried out based on these assumptions, and supposing a horizontal single-level disposal facility in the mid-plane of the formation. The exercise carried out with these conservative simplifications offers a number of important and useful lessons for the next performance assessment exercises and for defining the Research and Development Programme strategy. The underground laboratory programme has to deal with three main questions: · Could EDZ characteristics change the transfer pathway (effective permeability at large scale and after re-saturation)? Are conceptual models of argillite THM behaviour capable of modelling the EDZ? · Will diffusipon remain the dominant transport phenomenon throughout the host formation in the future (persistence of overpressure in argillite, role of yet undetected discrete fractures at a regional scale, possible significant contribution f coupled phenomena)? · What constitutes "necessary and sufficient" characterisation of transport within the Oxfordian formation for evaluating transfers 100 000 years from now?