Alternative Modeling Approaches for Flow & Transport in Fractured Rock


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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



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)



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