GEOSCIENTIFIC BASIS FOR MAKING THE SAFETY CASE FOR A SF/HLW/ILW REPOSITORY IN OPALINUS CLAY IN NE SWITZERLAND (PROJECT ENTSORGUNGSNACHWEIS) Ⅲ: REPOSITORY-INDUCED EFFECTS: EDZ, REPOSITORY GAS RELEASE, SELF-SEALING

摘要

The temporal and spatial evolution of the disturbed system around the disposal facilities may affect the long-term performance of the repository system. Key processes under consideration are the evolution of the EDZ, the release of repository gas and the self-sealing capacity of the Opalinus Clay. The following conclusions are drawn: 1. the EDZ around the backfilled underground excavations will reconsolidate after repository closure. Compared to the intact host rock, the expected long-term permeability enhancement is a factor of 5-10 or less. The estimated thickness of the EDZ around the emplacement tunnels is about 1.7-2.5 times the tunnel radius; 2. visco-capillary two-phase flow is the prevailing transport mechanism for accommodating the release of repository gas. For elevated gas pressures, evidence was found for dilatancy-controlled gas flow with a marked increase in rock permeability. After pressure recovery, however, no permanent permeability enhancement was seen; 3. the capacity of induced features in Opalinus Clay to self-seal was confirmed by in-situ experiments at the Mont Terri Rock Laboratory. Hydromechanical and geochemical self-sealing processes resulted in a transmissivity reduction of more than 4 orders of magnitude. The main conclusions are supported by multiple lines of evidence from laboratory and field tests, from regional studies and from studies of natural systems elsewhere. Further processes were assessed in the context of Project Entsorgungsnachweis (Nagra, 2002a), such as geochemical alteration of the near-repository host rock and heat generation of the SF/HLW. Their impact on long-term repository performance is assessed as being less important. Pyrite oxidation in the induced fractures of the EDZ and high pH porewaters due to the degradation of concrete are restricted to a narrow zone around the disposal facilities and may give rise to enhanced sorption capacity. The heat pulse emitted by the SF/HLW leads to a maximum temperature of 95°C at the tunnel walls and will dissipate within several hundred to thousand years.