Extent and Hydraulic Characteristics of the EDZ around Underground Structures of a Repository for HLW and L/ILW in the Siting Region Nördlich Lägern

The EDZ around the backfilled underground structures of a geological repository represents a viable release path for radionuclides dissolved in the porewater of the repository nearfield as well as a possible escape route for corrosion and degradation gases. The efficiency of this release path depends on the shape and extent of the EDZ and on its effective hydraulic conductance (cross-sectional area of the EDZ × effective hydraulic conductivity). Thickness and effective hydraulic conductivity of the EDZ are key parameters in performance assessment and safety assessment modelling.

This report provides the estimated hydraulic properties of the EDZ around three safety-relevant repository components, namely the HLW drifts, the L/ILW caverns and the V3 shaft seals. It is based on fracture network simulations for the provisional repository project at the Haberstal site with the technical repository layout described in the engineering dossier. Following a well-established modelling workflow, the discrete EDZ fracture networks are converted into hydraulic continuum models with heterogeneous distributions of porosity and hydraulic conductivity. In a final step, the complex shape of the EDZ is abstracted towards a simplified representation, amenable to be handled with conventional PA/SA modelling tools.

For each of the three repository components, six different EDZ fracture patterns were analysed, representing sensible EDZ realisations for a wide range of possible stress conditions at repository level. Look-up tables were generated, displaying thickness, equivalent hydraulic conductivity and equivalent porosity of the homogenized EDZ for the HLW drifts, the L/ILW caverns and the shaft seals as input for the PA/SA calculations.