1. Validating the Multiscale Thermohydrologic Model Using an Alternative Model of the Proposed Repository at Yucca Mountain
Yue Hao, Thomas A. Buscheck, Yunwei Sun, Souheil Ezzedine*, and Scott C. James**
Lawrence Livermore National Laboratory, Livermore, CA
*Weiss Associates, Emeryville, CA
**Sandia National Laboratories, Livermore, CA
Results
Introduction
Conceptual Three-Dimensional Pillar-Scale Thermal-Hydrological (TH) Model
Thermal-Hydrologic Conditions Predicted by the MSTHM and 3-D Pillar-Scale TH Model are Compared at the Repository Center and Edge, including Waste-Package (a) Temperature and (b) Relative Humidity, and Liquid-Phase Saturation at the (c) Drift Wall and in the (d) Invert
The MultiScale ThermoHydrologic Model (MSTHM) is used in the total system performance assessment (TSPA) of the proposed nuclear-waste repository at Yucca Mountain. The MSTHM accounts for thermal-hydrologic (TH) processes occurring at a scale of a few tens of centimeters around individual waste packages and emplacement drifts (tunnels), and for heat flow at the multi-kilometer scale of the mountain. The MSTHM, which uses the Nonisothermal Unsaturated Flow and Transport (NUFT) code, is a computationally efficient alternative to a monolithic TH model by virtue of its breaking the problem into smaller tractable pieces. A complete MSTHM realization requires four families of NUFT-submodel calculations, of varying detail and scale, to be conducted. MSTHM results are obtained by superposition of 3-D mountain- and drift-scale thermal-conduction-submodel results onto those of 2-D drift-scale TH submodels. The MSTHM has been previously validated against a monolithic TH model for a test-problem that is less than the full scale of the Yucca Mountain repository.
Recent code enhancements (resulting in version 4.0) allow NUFT to be run on parallel-CPU clusters, thereby addressing much larger, more computationally demanding problems.
Using NUFT v4.0, a 3-D “pillar-scale” TH model is used to discretely simulate TH behavior for all waste packages along a full-scale (~1400-m long) emplacement drift.
Results from the pillar-scale model are compared with corresponding MSTHM results. The agreement between the respective models builds confidence in applying MSTHM results in the TSPA of the Yucca Mountain repository.
PWR1-1 Waste Package
DHLW-L1 Waste Package
Parallel computing capability of the NUFT code enables us to address and solve much larger, more computationally demanding problems (i.e. 3-D
pillar-scale TH model)
Repository Edge
Ground Surface
Percolation Flux
Repository Center
PWR2-1 Waste Package
BWR1-1 Waste Package
Unheated Region
Drift Wall
Drip Shield
Waste Package
Water Table
~1400 m
81m
Invert
Mathematical Modeling
Comparison of the MSTHM Results against
a Three-Dimensional Pillar-Scale TH Model
Porous Media Darcy-Flow Model
BWR2-1 Waste Package
DHLW-S1 Waste Package
The 3-D pillar-scale TH model is run for two cases:
Case 1 allows advective and diffusive vapor transport in the longitudinal direction along the drift.
Case 2 does not allow advective and diffusive vapor transport in the longitudinal direction along the drift; however, it does allow advective and diffusive vapor transport in the vertical and lateral directions in the drift.
MSTHM calculations, which utilize SMT, SDT, LDTH, and DDT submodels, are also conducted for result comparison.
Mass Balance
Energy Balance
Peak Waste-Package (WP) Temperature and Time when
Boiling Ceases on the Waste Package for 71 WPs
PWR1-2 Waste Package
PWR1-3 Waste Package
Overview of NUFT Code
(Non-isothermal Unsaturated-Saturated Flow and Transport Model)
A flexible multipurpose computational tool for modeling flow and reactive transport in porous media under both non-isothermal and isothermal conditions (Nitao, 1998)
Summary
Integrated finite-difference spatial discretization and implicit time integration
Parallel computing capability
Wide range of applications:
YMP thermal hydrology
CO2 sequestration & storage
LLNL Site groundwater clean-up …
The MSTHM results have been compared with the 3-D pillar-scale TH model results
The pillar-scale model-validation test case is a full-scale validation test of the MSTHM
The pillar-scale model-validation test case utilizes the current thermal and hydrologic properties used in the MSTHM calculations
The pillar-scale model-validation test case employs a discrete representation of WPs and drip shields for an entire drift
Peak WP temperatures are in good agreement for most of the drift and reasonably bounded by the MSTHM for the outermost WPs at the repository edge
Vapor transport along the drift may result in drier conditions than predicted by the MSTHM, which is beneficial with respect to the potential onset of drift seepage
References
[1] Buscheck, T. A., L. G. Glascoe, K. H. Lee, J. Gansemer, Y. Sun, and K. Mansoor Validation of the multiscale thermohydrologic model used for analysis of a repository at Yucca Mountain. Journal of Contaminant Hydrology, (62-63):
[2] Nitao, J.J., Reference Manual for the NUFT Flow and Transport Code, Version 2.0. Rep. UCRL-MA Lawrence Livermore Natl. Lab., Livermore, CA.
[3] SNL (Sandia National Laboratories). Multiscale Thermohydrologic Model. ANL-EBS-MDL REV03 AD01. Las Vegas, Nevada: Sandia National Laboratories
Cartesian/Graph Topology
Domain Partition 1 2 3 4 1 2
Acknowledgements 3 4
This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.
Domain Partition for Parallel Computing