SEVERAL TOUGH2 MODULES DEVELOPED FOR SITE CHARACTERIZATION STUDIES OF YUCCA MOUNTAIN

Abstract

Several TOUGH2 Modules Developed for Site Characterization Studies of Yucca Mountain Yu-Shu Wu and Karsten Pruess Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, C A 94720, U . S. A . and/or rapid movement of fluids in thick, highly heterogeneous, unsaturated systems over very long time periods. The improvement of the modeling capabilities for simulating field-scale problems has been subject to continuous research efforts, and considerable progress in this area has been made over the past two decades. Abstract A comprehensive site characterization study has been conducted for the unsaturated zone (UZ) of Yucca Mountain to investigate its suitability as a potential high-level nuclear waste repository. Numerical modeling of moisture, gas, chemicals and heat flow, and their influence on the repository performance has been proven to be essential in understanding unsaturated-zone fluid movement, and the effects of hydrogeologic, geochemical and thermal conditions on various aspects of the overall waste disposal system. In these studies, the T O U G H 2 code has been used extensively as a main modeling tool because of its flexibility and robustness in handling multiphase, multi-component fluid and heat flow and chemical transport in porous/fractured rocks. T O U G H 2 (Pruess, 1991) has been used as a main modeling tool in the Yucca Mountain project due to its flexibility and robustness i n handling multidimensional, multiphase, and multi-component fluid flow and heat transfer in both porous and fractured rocks. In addition, throughout the development of the T O U G H / T O U G H 2 codes, a comprehensive verification and validation study has been conducted (Pruess, 1987 and 1991; Moridis and Pruess, 1992; Pruess e t a l , 1996). More recently, the T O U G H 2 code has been updated and improved in the modeling studies associated with the site characterization of Yucca Mountain, and several new modules have been developed to add to the T O U G H 2 family of codes (Wu et al., 1996). The correctness, validity, and capability of the T O U G H 2 family of codes have been verified by comparing the simulation results against analytical solutions, and results from other numerical codes, laboratory experiments, and field tests. As part of the U Z modeling efforts at L B N L , several new modules of the T O U G H 2 code have been developed to meet different needs of the site characterization studies. In this paper we w i l l summarize some of these new T O U G H 2 modules and their specific features in application to the Yucca Mountain project. These new T O U G H 2 modules include (1) the effective continuum model (ECM) for treating fracture/matrix flow; (2) single-phase gas flow (EOS1G) i n a water-air two phase flow system; and (3) a 3-D radionuclide transport module (T2R3D) for 3-D radionuclide or tracer transport in porous/fractured media under two-phase flow and non- isothermal conditions. In this paper, we w i l l discuss three new TOUGH2 modules, developed during the site-characterization study at Yucca Mountain. These three new T O U G H 2 modules are (1) effective continuum model (ECM) for treating fracture/matrix flow; (2) single-phase gas flow (EOS1G) in a water-air flow systems; and (3) a 3-D radionuclide transport module (T2R3D) for 3-D radionuclide or tracer transport i n porous/fractured media under two-phase flow and non-isothermal conditions. Introduction Numerical modeling approaches used in the Yucca Mountain project for simulating multiphase fluid flow, heat transfer, and chemical transport processes involve coupled multiphase fluid and heat flow, and chemical component migration formulations based on finite difference, or finite element schemes. Available numerical simulators for variably saturated nonisothermal flow generally employ mathematical and numerical methods similar to those used in the oil industry or geothermal engineering. Since the oil and geothermal industries have different priorities than organizations dealing with nuclear waste repository assessment, the codes must account for a number of additional processes such as the slow Formulations E C M module was developed to simulate fluid flow, heat transfer and chemical transport i n fractured porous media. This module can be used with any of the T O U G H 2 EOS modules in simulation studies of flow and transport in fractured rock. The E C M formulation, as implemented i n the T O U G H 2 code (Wu et al., 1996), is based on the assumption that there is an approximate thermodynamic equilibrium