Multiple Continua Simulations of Thermal-Hydrological-Chemical Processes in the Potential Repository at Yucca Mountain, Nevada

Numerical simulation of nonisothermal, multiphase flow and associated reactive transport in fractured rock is an important tool for evaluating the performance of a potential repository for high-level nuclear waste at Yucca Mountain, Nevada. A dual continuum representation has emerged as the standard approach for modeling processes in the unsaturated zone near emplacement tunnels. In the dual continuum representation, the fracture network is modeled as an equivalent continuum that interacts with a second continuum representing the matrix system. The principal limitation of the dual continuum approach is that it neglects gradients in pressure, temperature, saturation, and concentration within the matrix blocks. The dual continuum model is understood to be an accurate approximation for steady-state or weakly transient situations, but the accuracy of the representation is unclear for strongly heated transient situations. Thermal-hydrological-chemical simulations using the dual continuum model have been compared with simulations using the more general multiple interacting continua (MINC) model (Pruess and Narasimhan, 1985). In the MINC model, multiple interacting continua are used to represent the matrix system. Both the MINC and dual continuum simulations use the computer code MULTIFLO Version 2.0.1 (Lichtner and Seth, 1996; Painter and Seth, 2003), and consider multiple aqueous species and multiple minerals. The MINC simulations result in lower matrix pressures and drier fractures near the potential emplacement tunnels. Precipitation of amorphous silica in fractures is reduced in the MINC model. This abstract was prepared to document work performed by the Center for Nuclear Waste Regulatory Analyses (CNWRA) and its contractors for the Nuclear Regulatory Commission (NRC) under Contract No. NRC-02-02-012. The activities reported here were performed on behalf of the NRC Office of Nuclear Material Safety and Safeguards, Division of High-Level Waste Repository Safety. This abstract is an independent product of the CNWRA and does not necessarily reflect the view or regulatory position of NRC.