Nonisothermal Multiphase Fluid Flow
The objective of this numerical investigation was to contribute to the understanding of the thermal and multifluid environment surrounding the single-shell tanks within the S-SX Waste Management Area (WMA) on the Hanford Site, near Richland, Washington. In particular, this investigation considered the thermal and hydrogeologic environment surrounding the westing row of tanks in the center of the SX-WMA (i.e., SX-107, SX-108 and SX-109) and its effects on the transport of a conservative radionuclide to the WMA border. This row of tanks was selected because the SX-108 and SX-109 tanks had histories of high heat loads and elevated temperatures, and the SX-108 tank was known to have leaked radionuclides and other contaminants into the subsurface. This study used numerical simulation to investigate the coupled processes of multifluid flow, heat transfer, and solute transport in a heterogeneous geologic environment surrounding the subject row of single-shell tanks. Ancillary investigations of solute transport from the S-SX WMA [White et al. 2001] demonstrated that two-dimensional cross-sectional simulations could be used without introducing significant errors to model the three-dimensional subsurface environment surrounding the single-shell tanks.
Accurate hydrologic modeling of the SX-WMA requires a comprehensive description of the site hydrogeology, tank thermal and leak histories, hydrologic conditions at the ground surface and water table, and their variability. The hydrogeologic description of the S-SX WMA implemented in this investigation was a modified version of the data package [Khaleel et al. 2000] provided for a complementary study of solute transport under isothermal conditions.
All simulations reported herein were executed with the STOMP simulator [White and Oostrom 2000a,b], using the Water Operational Mode for isothermal simulations and the Water-Air-Energy Operational Mode for the nonisothermal simulations. All simulations considered the transport of the solute Tc-99 from known distributions in the year 2000, as specified in the modeling data package (MDP) [Khaleel et al. 2000]. Grid resolutions for all simulations were 0.5334 m (1.75 ft) horizontally and 0.4572 m (1.5 ft) vertically. The isothermal simulations use the assumption of a passive gas phase, where the gas phase is assumed to have a constant pressure of 101325.0 Pa (1 atm). Transport of phase components (i.e., water and air) and solutes are neglected under the passive gas assumption. Gas entrapment by the wetting phase (i.e., aqueous phase) was neglected for both the isothermal and nonisothermal simulations. The nonisothermal simulations use the assumption of an active gas phase, however, Tc-99 transport was only considered in the aqueous phase. The numerical modeling of this investigation does not consider chemical interactions with the porous media, nor does it consider heat generated through chemical reaction or radiological decay. The transported solute is assumed to be "infinitely dilute" with respect to altering the properties of the transporting medium. For example the aqueous-phase density was assumed to be dependent on temperature and pressure, but independent of the dissolved-solute concentration. A more comprehensive analysis would consider the heat contribution from radioactive decay, chemical interactions with the porous media, including dissolution and precipitation, which may significantly alter local hydrologic properties and fluid property dependence on solute concentration. The modeling analysis described in this report should be considered preliminary to a more comprehensive representation of the hydrologic flow and transport processes which occurred and continue to proceed in the subsurface environment surrounding the single-shell tanks at the Hanford Site WMAs.
Khaleel, R., T.E. Jones, A.J. Knepp, F.M. Mann, D.A. Myers, P.M. Rogers, R.J. Serne, and M.J. Wood. 2000. "Modeling Data Package for S-SX Field Investigation Report (FIR)." CH2M Hill Hanford Group,Inc. Richland, Washington., RPP-6296, Rev. 0.
White, M.D. 2001. "Nonisothermal Multiphase Fluid Flow and Transport: Multitank Modeling in the SX Tank Farm."
White, M.D., and M. Oostrom. 2000a. STOMP Subsurface Transport Over Multiple Phases, Version 2.0, Theory Guide. PNNL-12030, UC-2010, Pacific Northwest National Laboratory, Richland, Washington.
White, M.D., and M. Oostrom. 2000b. STOMP Subsurface Transport Over Multiple Phases, Version 2.0, User's Guide. PNNL-12034, UC-2010, Pacific Northwest National Laboratory, Richland, Washington.
White, M.D., M. Oostrom, and M.D. Williams. 2001. "FY00 Initial Assessment for S-SX Field Investigation Report (FIR): Simulations of Contaminant Migration with Surface Barriers." Battelle, Pacific Northwest Division, PNWD-3111, Richland, Washington.