The effects of conduction, convection, and radiation on the thermodynamic environment surrounding a heat-generating waste package Page: 2 of 14
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compared to simulations using only conduction, which attempt to account for radiative heat transfer
with an effective thermal conductivity. Temperatures and relative humidities are compared at
various locations within the drift. In addition, the effects of convection on relative humidity and
moisture distribution within the drift are presented.
Numerical Approach
The numerical code TOUGH2 (Transport Of Unsaturated Groundwater and Heat; Pruess,
1991) (SNL Software Configuration Management v. 3.3) is used in the analyses. TOUGH2 is a
multidimensional, multiphase, nonisothermal simulator that is used extensively in geothermal,
environmental restoration, and nuclear waste management areas. Mass and energy balances are
solved simultaneously using the integral finite difference method for air and water in porous media.
The full details of the governing equations and the numerical code are given in Pruess (1987,
1991).
Simulations are performed to compare a TOUGH2 model that explicitly models radiation
through the drift with a TOUGH2 model that uses only conduction in the drift. In addition,
convection is turned on or off by specifying either a non-zero or zero permeability for the drift
elementst. Table 1 summarizes the simulations that are performed in this study. The following
sections describe the domain, the parameters, and some of the detailed pre-processing calculations
required to run the different models.
Model Domain and Parameters
The two-dimensional grid used in the analyses consists of a centralized element representing
the waste package surrounded by elements representing the empty drift, all of which are bounded
by elements representing a partially saturated tuffaceous rock (Figure 1). The heat output of the
waste package is fixed at 1700 W, and its properties are specified to prevent advective and
diffusive flux to or from the waste package. The drift elements surrounding the waste package
have the properties of air and act as a capillary barrier to advective flux from the surrounding rock
(no backfill is assumed to exist in the drift in this study). The thermohydrologic properties of the
tuffaceous rock elements outside of the drift are taken from reported values of the Topopah Springs
welded unit at Yucca Mountain (Wilson et al.,1994; Pruess and Tsang, 1994; Incropera and
DeWitt, 1985). Enhanced water vapor diffusion is assumed to exist in this region such that the
product of the tortuosity, porosity, and gas phase saturation (used in the calculation of gas phase
diffusion) is set equal to one. The rationale for the use of this constant is given in Jury and Letey
t Rigorously speaking, the use of a permeability in Darcy's law cannot be used to determine the velocity distribution
caused by natural convection in an empty drift. Inertial terms in the full Navier-Stokes equation are neglected in
Darcy's law and may play an important role in the velocity distribution. However, these effects are lumped into an
effective permeability in this study to yield an approximate description of natural convection in the drift:draft 12/18/95
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Ho, Clifford K. & Francis, N.D. The effects of conduction, convection, and radiation on the thermodynamic environment surrounding a heat-generating waste package, article, January 1, 1996; Albuquerque, New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc668789/m1/2/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.