Alternative approaches to modeling thermohydrologic processes at Yucca Mountain

摘要

Several previous thermohydrologic modeling studies of the potential repository at Yucca Mountain have used the equivalent continuum model (ECM) formulation to represent fracture-matrix interactionin a composite sense~1.The ECM formulation,which assumes thermodynamic equilibrium between the fractures and the matrix,essentially prevents liquid flow in the fractures so long as the matrix is not fully saturated.This limits the ability of ECM-based models to properly simulate processes suchas gavity-=driven flow through fractures -which are likely to occur during the thermally perturbed post-closure period at Yucca Mountain.Dual-permeability models (DKM) provide an alternative to the ECM formulation for modeling such processes.By representing the fractures and matrix as discrete containua,a variety of non-equilibrium processes can be adequately simulated- albeit at the cost of increased computational burden.The large disparity between fracture and matrix properties also produces numerical instabilities and convergence problems,thus limiting the practical application of thermohydrologic models using the DKM formulation to simple 1-D models~2.Xiang et al.(1996) have described a generalization of the equivalent continuum model (G-ECM) which permits initiation of fracture flow after the matrix saturation reaches some user-specified "satiated" value~3.The satiated saturation can be viewed as the residual nonwetting (air) phase saturation in the matrix,the resultant reduction in matrix pore space being used to drive higher liquid fluxes into the fractures.