Davood Bahrami

COUPLED IN-ROCK AND IN-DRIFT HYDROTHERMAL MODEL STUDY FOR YUCCA MOUNTAIN

Abstract A thermal-hydrologic natural-ventilation model is configured for simulating temperature, humidity, and condensate distributions in the coupled domains of the in-drift airspace and the near-field rock mass in the proposed Yucca Mountain repository. The multiphysics problem is solved with MULTIFLUX, in which a lumped-parameter computational fluid dynamics (CFD) model is iterated with TOUGH2. The iterative process ensures that consistent boundary conditions are used on the drift wall in both the CFD and the TOUGH2 model-elements. The CFD solution includes natural convection, conduction, and radiation for heat, as well as moisture convection and diffusion for moisture transport with half waste package-scale details in the drift. The TOUGH2 solution for the rock mass is generalized with the use of the Numerical Transport Code Functionalization technique in order to include both mountain-scale heat and moisture transport in the porous and fractured rock, and fine half waste package-scale details at the drift wall. The method provides fast convergence on a personal computer computational platform. Numerical examples and comparison with a TOUGH2-based integrated model are presented.

INCREASED STORAGE CAPACITY AT YUCCA MOUNTAIN FAVORS THERMAL MANAGEMENT FOR A COLD REPOSITORY

Abstract The nuclear waste storage concept according to the baseline design of the proposed high-level nuclear waste repository at Yucca Mountain is analyzed. The high-temperature storage concept, in which the emplacement area is heated above the boiling temperature of water, is subject to criticism on the basis of uncertainties due to nonlinear multiphysics processes in the rock mass and in the storage airspace. The storage environment around the nuclear waste containers is reexamined using a new thermal-hydrologic airflow model. The complex nature of the thermal-hydraulic behavior in a superheated waste repository is described with fewer simplifying assumptions than those used in the baseline design. The emplacement area in the mountain is described as an open system, in which the air pressure is connected to the barometric pressure through fractures, faults, and partially sealed drifts. The cyclic variation of the atmospheric pressure that affects the heat and mass transport processes in the near-field rock mass is also modeled. The implications of evaporation into the drift airspace are discussed, and a hypothesis of salt accumulation in the near-field rock mass is established. Model calculation is also presented for a below-boiling temperature storage concept that is easier to predict and has fewer anomalies. The price for a below-boiling temperature storage is the extended preclosure ventilation time period. However, as demonstrated for a trade-off, it is possible to design a repository with below-boiling temperatures and doubled waste inventory at the same time.

Benchmark Problems of the Geothermal Technologies Office Code Comparison Study

............................................................................................................................................. iii Summary ............................................................................................................................................. v Acknowledgments ............................................................................................................................. vii Acronyms and Abbreviations ............................................................................................................. ix 1.0 Introduction .............................................................................................................................. 1.1 1.1 Approach ......................................................................................................................... 1.3 1.1.1 Study Objectives .................................................................................................. 1.3 1.1.2 Study History and Structure ................................................................................. 1.3 1.2 Participants and Codes .................................................................................................... 1.5 1.3 Benchmark Problems ...................................................................................................... 1.9 1.3.1 Benchmark Problem 1: Poroelastic Response in a Fault Zone (PermeabilityPressure Feedback) ............................................................................................... 1.9 1.3.2 Benchmark Problem 2: Shear stimulation of randomly oriented fractures aby injection of cold water into a thermo-poro-elastic medium with stress-dependent permeability ........................................................................................................ 1.10 1.3.3 Benchmark Problem 3: Fracture opening and sliding in response to fluid injection .............................................................................................................. 1.11 1.3.4 Benchmark Problem 4: Planar EGS fracture of constant extension, pennyshaped or thermo-elastic aperture in impermeable hot rock .............................. 1.12 1.3.5 Benchmark Problem 5: Amorphous Silica dissolution/precipitation in a fracture zone .................................................................................................................... 1.13 1.3.6 Benchmark Problem 6: Injection into a fault/fracture in thermo-poroelastic rock1.14 1.3.7 Benchmark Problem 7: Surface deformation from a pressurized subsurface fracture ............................................................................................................... 1.15 1.4 Comparison Standard .................................................................................................... 1.16 2.0 Governing and Constitutive Equations .................................................................................... 2.1 2.1 Heat Transfer Modeling .................................................................................................. 2.1 2.2 Fluid Flow Modeling ....................................................................................................... 2.2 2.2.1 Fracture Transmissivity ........................................................................................ 2.2 2.3 Rock Mechanics Modeling .............................................................................................. 2.3 2.3.1 Continuum Geomechanics ................................................................................... 2.4 2.3.2 Discrete Fracture Geomechanics .......................................................................... 2.5 2.3.3 Joint Models ....................................................................................................... 2.10 2.4 Geochemical Reaction Modeling .................................................................................. 2.12 2.4.1 Aqueous Reaction Rates ..................................................................................... 2.14 3.0 Numerical Solution Schemes ................................................................................................... 3.1 3.1 Sequential Schemes ......................................................................................................... 3.1 3.2 Iterative Schemes ............................................................................................................ 3.1

A turbulent transport network model in MULTIFLUX coupled with TOUGH2

Abstract A new numerical method is described for the fully iterated, conjugate solution of two discrete submodels, involving (a) a transport network model for heat, moisture, and airflows in a high-permeability, air-filled cavity; and (b) a variably saturated fractured porous medium. The transport network submodel is an integrated-parameter, computational fluid dynamics solver, describing the thermal-hydrologic transport processes in the flow channel system of the cavity with laminar or turbulent flow and convective heat and mass transport, using MULTIFLUX. The porous medium submodel, using TOUGH2, is a solver for the heat and mass transport in the fractured rock mass. The new model solution extends the application fields of TOUGH2 by integrating it with turbulent flow and transport in a discrete flow network system. We present demonstrational results for a nuclear waste repository application at Yucca Mountain with the most realistic model assumptions and input parameters including the geometrical layout of the nuclear spent fuel and waste with variable heat load for the individual containers. The MULTIFLUX and TOUGH2 model elements are fully iterated, applying a programmed reprocessing of the Numerical Transport Code Functionalization model-element in an automated Outside Balance Iteration loop. The natural, convective airflow field and the heat and mass transport in a representative emplacement drift during postclosure are explicitly solved in the new model. The results demonstrate that the direction and magnitude of the air circulation patterns and all transport modes are strongly affected by the heat and moisture transport processes in the surrounding rock, justifying the need for a coupled, fully iterated model solution such as the one presented in the paper.

Temperature, Humidity, and Airflow in the Emplacement Drifts Using Convection and Dispersion Transport Models

Abstract A coupled thermal-hydrologic-airflow model is developed, solving for the transport processes within a waste emplacement drift and the surrounding rock mass together at the proposed nuclear waste repository at Yucca Mountain. Natural, convective airflow as well as heat and mass transport in a representative emplacement drift, embedded in a three-dimensional, mountain-scale rock mass with edge cooling, are explicitly simulated for the first time in the literature, using the MULTIFLUX model. The conjugate, thermal-hydrologic transport processes in the rock mass are solved with the TOUGH2 porous-media simulator in a coupled way to the in-drift processes. The new simulation results show that large-eddy turbulent flow, as opposed to small-eddy flow, dominates the drift airspace for at least 5000 years following waste emplacement. The size of the largest, longitudinal eddy is equal to half of the drift length, providing a strong axial heat and moisture transport mechanism from the hot drift sections to the cold drift sections. The in-drift results are compared to those from simplified models using a surrogate, dispersive model with an equivalent dispersion coefficient for heat and moisture transport. Results from the explicit, convective velocity simulation model provide higher axial heat and moisture fluxes than those estimated from the previously published, simpler, equivalent dispersion models, in addition to showing differences in temperature, humidity, and condensation rate distributions along the drift length. A new dispersive model is also formulated for comparison, giving a time- and location-variable function that runs generally about ten times higher in value than the highest dispersion coefficient currently used in the Yucca Mountain Project.

Thermal design of an alternative boiling water reactor spent nuclear fuel package for Yucca Mountain Repository

Thermal analysis of an alternative boiling water reactor (BWR) waste package design for permanent disposal in the Yucca Mountain Repository is reported in this paper. The new design implements an increase in the holding capacity of the BWR waste packages from 44 to 52 assemblies and a modified arrangement sequence of waste packages in the emplacement drift. The design is favorable from the perspective of a generally drier emplacement drift due to an increase in heat load in the waste packages and the resulting higher temperatures. The analysis addresses heat transfer issues inside the waste package and those pertinent to satisfying the safe thermal limits for the waste package components. Key parameters in the analysis are the spent nuclear fuel assembly effective conductivities, the number of aluminum shunts, and the gap backfill with pressurized helium inside the waste packages. The feasibility of the proposed design is demonstrated by the internal waste package thermal model and the thermal-hydrologic environment in the emplacement drift. The conformity of the alternative thermal design to safe temperatures, in spite of the additional heat load, led to another innovative design with radial arrangement of assemblies in the waste packages that would further support the sustenance of drier emplacement drifts. These radial configurations are also discussed in this paper.