Model Supported Assessment of the Containment-Providing Rock Zone in Support of the Site Selection
Nagra (2024): Model Supported Assessment of the Containment-Providing Rock Zone in Support of the Site Selection. Nagra Arbeitsbericht NAB 24-26.
The primary aim of the present report has been to provide a traceable documentation of the model-based assessments, supporting the safety-related site comparison in the context of SGT-Stage 3. The focus was on (i) the vertical and lateral delimitation of the containment-providing rock zone (CRZ) and (ii) the indicator-based assessment of the barrier performance of the CRZ for a HLW and L/ILW repository project in the 3 siting regions.
The “thickness of the Opalinus Clay” is a delimitation indicator for the vertical extent of the CRZ, which is required to assess the efficiency of the CRZ as a transport barrier for radionuclides. Based on a dimensionless representation of advective-diffusive transport processes in the host rock, a minimum thickness of the undamaged rock mass above and below the emplacement of 20 – 35 m was estimated for the HLW repository to provide acceptable performance as a transport barrier for dissolved radionuclides. For the L/ILW repository with a diameter of approximately 10 m, a minimum thickness of the intact host rock package above and below the caverns of 20 m is estimated to ensure the confinement of the waste and the mechanical and chemical integrity of the repository system as a whole.
The delimitation indicator for lateral delimitation of the CRZ is called “distance to steeply dipping faults”. The corresponding performance target Lmin,fault = 200 m has been specified as a minimum distance of the HLW and L/ILW disposal areas from fault systems of hydraulic significance. Model-based assessments confirm that the lateral release to a steeply dipping fault is not relevant, when the respect distance of 200 m is maintained.
Another delimitation indicator used for lateral delimitation of the prospected protection zone is called “minimum lateral distance between the prospected protection zone and the next injection / pumping well”. The corresponding performance target Lmin,well = 50 m has been specified as a minimum distance of prospected repository zone from any deep boreholes. Model-based assessments confirm that the lateral release into a deep borehole is not relevant, when the respect distance of 50 m is maintained.
A complementary workflow was pursued for setting out favourable repository areas and for delineation of the corresponding repository footprints in the three siting regions. The screening process was aimed at mapping lateral variations of barrier performance of the host rock. Significant lateral variations in barrier performance of the host rock could not be identified; it can be concluded, that the entire prospected protection zone is equally well suited to host the repository.
Model-supported assessments have been performed to identify safety-relevant differences between the site-specific repository projects. The indicator-based evaluations concentrated on the SGT criteria 2.3 (“repository induced influences”) and the three criteria in criteria group 3 (“reliability of findings”).
The effects of the excavation damaged zone (EDZ) were evaluated by numerical calculations of the vertical extent of the EDZ above / below the emplacement rooms, both for the HLW and the L/ILW repository projects. The analyses revealed that none of the repository projects is detrimentally affected by a safety-relevant loss of barrier function of the CRZ, caused by the vertical extent of the EDZ.
Thermally induced overpressures in the host rock around the HLW drifts have a significant power to distinguish between the three HLW repositories. The HLW-NL and HLW-ZNO repository configurations are characterized by significantly higher safety margins with regard to thermally induced shear failure than the HLW-JO project. A similar picture emerges when taking into account the parametric and conceptual uncertainties addressed in criteria group 3. The reason for the less favorable performance of the HLW-JO configuration is the lower repository depth at the JO site.
Gas induced overpressures develop both, in the HLW and the L/ILW repository structures. The gas impacts have a significant power to distinguish between the three repository sites. The repository projects at the NL site perform best due to the greatest repository depth, the configurations in ZNO performs also well, whereas low safety margins are seen for the JO projects due to the lowest repository depth.
Finally, model-based assessments were performed to evaluate the impact of parametric, conceptual and scenario uncertainties on the efficiency of the host rock and the CRZ in the three siting regions as a transport barrier for dissolved radionuclides. Tracer flux of a conservative tracer was used as a performance indicator which can be calculated with numerical modelling tools. Probabilistic simulations confirmed an excellent barrier performance of the CRZ for all repository configurations at all sites, indicating that the power to distinguish between the three repository sites is low.