Alternative Modeling Approaches for Flow & Transport in Fractured Rock

Alternative Modeling Approaches for Flow & Transport in Fractured Rock

• Douglas D. Walker, DE&S

• Jan-Olof Selroos, SKB

• Supported by Swedish Nuclear Fuel and Waste Management Co. (SKB)

Presentation Overview

• Context and Objectives of the Alternative Models Project

• The hypothetical Aberg Repository

• 3 alternative conceptual models of heterogeneity

• Performance measures

• Results and Conclusions

Deep Geologic Disposal of Nuclear Waste

Nuclear Waste Disposal Performance Assessment

Uncertainty in Subsurface Hydrology

• Uncertainty vs. variability

• Uncertainty in:

• process physics
• measurement
• characterization of heterogeneity
• upscaled representation in models

The Alternative Models Project

• Nuclear waste disposal performance assessment uncertainty analysis

• Compare alternative representations of flow / transport in fractured rocks

• Explicit definition of

• test problem premises
• performance measures and summary statistics

Aberg Repository

Aberg Site and Data

• Hydrogeologic Setting:

• Inland recharge, discharge to Baltic
• Fractured granitic rocks
• Large-scale fracture zones (deterministic)

• Data:

• 53 Boreholes (hydraulic/tracer tests, chem)
• geophysics, fracture trace maps
• Äspö Hard Rock Laboratory

• Regional model / boundary conditions

Aberg: Deterministic Fracture Zones and Repository

Alternative Conceptual Models

Stochastic Continuum: Application

• Conductivity distribution

• 3m K tests  25m, Lognormal + variogram
• Rock & Conductor distributions
• homogeneous ar = 1.2 m2/m3 rock

• Structural model

• Deterministic zones only

• Repository

• 945 canisters x 34 realizations

Stochastic Continuum: Travel Paths

Stochastic Continuum

• Advantages:

• hydraulic tests are volume averages
• method / software well-established

• Disadvantages:

• Scale dependence of K in fractured media poorly understood
• Preferential paths not represented at scales below block size

Discrete Fracture Network

Discrete Fracture Network: Application

• Fracture Distribution

• Deterministic Zones and Canister fractures
• Lognormal, with 20  R  1000m in region and 0.2  R  20m at repository
• Lognormal transmissivity
• ar = f (area between fracture traces)

• Repository

• 50 to 90% of 81 canisters x 10 realizations

Discrete Fracture Network: Travel Paths

Discrete Fracture Network

• Advantages:

• Represents the conductive structures (Realism)
• Allows for preferential paths

• Disadvantages:

• Data demand
• Computational demand
• Matrix permeability may be important

Flow Channeling

Channel Network Intersections

Channel Network: Application

• Conductance Distribution

• 3m K tests  30m, Lognormal
• Rock, Conductor, & EDZ distributions
• ar = 1.2 m2/m3 in Zones,  1/10 in Rock

• Structural model

• Deterministic zones

• Repository

• 229 cans x 30 real x median (200 particles)

Channel Network: Travel Paths

Channel Network

• Advantages:

• Represents observed channels within fracture planes, directly assigns ar
• Allows for preferential paths and dispersion
• Includes diffusion/sorption in matrix, flow within Rock

• Disadvantages:

• Conductance is scale dependent

Application Summary

Simulation Summary

Performance Measures

• Travel time: canister to biosphere

• tw = qw/f [yr]

• Canister Flux: Darcy flux at canisters

• qw [m/yr]

• F-factor: Retardation vs. Advection

• F = (dw ar) / qw [yr/m]

Performance measures: Medians

Performance measures: Variances

Discussion

• Median performance measures and exit locations similar

• (Controlled by premises of BC, major zones)

• For DFN, F-factor variance greater than tw variance (variability of ar impacts PA)

• SC variances greatest, but differences in studies complicate comparison

Discussion II

• Modeling study differences:

• # particles released
• SC = one / canister
• DFN = one / canister subset
• CN = median of 200 / canister subset
• # canisters with pathways
• 100% in SC and CN; 50 to 90% in DFN
• Not evaluated: team experience, Sensitivity of inference to data

• SC and CN  boundary flow, DFN low

Conclusions

• For this site and these performance measures:

• Problem premises constrain the results

• Uncertainties regarding conceptual models of flow / transport in fractured rocks have limited effect on PA

• Chief benefit of DFN / CN is to examine effects of ar

Acknowledgements

• SC Modeling Study: H.Widén (Kemakta), D. Walker (DE&S)

• DFN Modeling Study: W Dershowitz, S Follin, T Eiben, J Andersson (GA)

• CN Modeling Study: B. Gylling, L. Moreno, I. Neretnieks (KTH)

• Swedish Nuclear Fuel and Waste Management Co. A. Ström, J-O. Selroos (SKB)