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A new dual‐porosity model is developed for single‐phase fluid flow in fractured/porous media. Flow is assumed to take place through the fracture network and between the fractures and matrix blocks. The matrix blocks are treated in a lumped parameter manner, with a single average pressure used for each matrix block. Rather than assuming that fracture/matrix flux is proportional to the difference between the fracture pressure and matrix pressure at each point, as is done in the Warren‐Root model, we use a nonlinear equation which more accurately models the flux over all time regimes, including both early and late times. This flux equation is compared with analytical solutions for spherical blocks with prescribed pressure variations on their boundaries. The nonlinear flux equation is also used as a source/sink term in the numerical simulator TOUGH. The modified code allows more accurate simulations than the conventional Warren‐Root method, with a large savings (about 90%) in computational time compared to methods which explicitly discretize the matrix blocks.

Abstract. The unsaturated zone at Yucca Mountain, a potential repository site of highlevel nuclear waste, is a complex hydrologic system in which a variety of important flow and transport processes is involved. To quantify these processes as accurately as possible is a theoretically challenging and practically important issue. In this study, we propose a new formulation for modeling flow and transport in unsaturated fractured rocks. The formulation is mainly based on a hypothesis that only a portion of connected fractures are active in conducting water. Analysis of the relevant data with the new formulation suggests that about 18-27% of the connected fractures in the Topopah Spring welded (TSw) unit (the potential repository unit) of Yucca Mountain are active under ambient conditions. The relatively high percentage of active fractures is consistent with field observations from a variety of sources. Sensitivity analyses are performed to investigate effects of the "activity" of connected fractures on flow and transport behavior in unsaturated rocks.