Bentonite pore water

GTS Phase VI, HotBENT, Bentonite, Emplacement,
Engineered Barrier System

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.

Site-specific physico-chemical rock data such as cation exchange capacities (CEC) and exchangeable cation occupancies are, amongst other geochemical and mineralogical parameters, part of the deep borehole investigations and are required for each potential region for the site selection procedure. This report focuses mainly on the CEC of the rock samples and on the cation occupancies of exchangeable alkaline and alkaline earth elements. The CEC is a measure of the total negative charge of the rock samples, resulting predominantly from the presence of 2:1 phyllosilicates on which cations adsorb on the planar sites via electrostatic bonding. CEC can also be defined as the sum of exchangeable cations at the clay mineral surfaces of the rock samples. The CEC is directly correlated to the content and type of clay minerals in the rock samples, and consequently to their adsorption properties towards radionuclides. Its variability is an excellent indicator for the homogeneity of the clay mineral content along the Opalinus Clay formation as well as to the upper and lower confining units for each borehole. The in-situ fractional occupancies of exchangeable cations, defined by the quantity of adsorbed cation (in meq/kg) divided by the CEC (in meq/kg) of the rock samples, can act as a “fingerprint” for the in-situ porewater compositions of the formation. In general, the composition of the porewater of rock formations consists essentially of solutes such as Cl resulting from the hydrologic evolution of the rock, and of solutes resulting from equilibrium with the minerals constituting the rock. In argillaceous rocks, the concentrations of dissolved cations are largely determined by cation exchange equilibria with the clay mineral fraction and by the equilibrium with carbonate and sulphide/sulphate minerals.

The objective of this work, mandated by Nagra, is to quantify the CEC and the pool of exchangeable cations of rock samples from 7 boreholes across the geological siting regions Jura Ost (JO), Nördlich Lägern (NL), and Zürich Nordost (ZNO). The samples were selected to represent the different lithologies of the natural barrier system, i.e., Opalinus Clay and the upper and lower confining units. 141 rock samples have been analysed with the focus on CEC and cation exchange occupancies. This report gives a detailed overview of the results obtained for the representative rock samples from Bözberg-1, Bözberg-2, Bülach-1, Stadel-2, Stadel-3, Trüllikon-1 and Marthalen-1.

Opalinus Clay, undrained triaxial test, benchmark

RHE1-1, Zürich Nordost, TBO, deep drilling campaign,
stochastic log interpretation, MultiMin analyses, petrophysical
logs, lab data, mineralogy, clay content, clay typing, porosity

Geologisches Tiefenlager, gTL, standortspezifische Gefährdungsanalyse, Flugzeugabsturz