Effect of small-scale fractures on flow and transport processes at Yucca Mountain, Nevada

Although many conceptual models for fracture-matrix interaction have been evaluated for Yucca Mountain site-characterization studies, the most widely used model is currently based on the dual-permeability concept. It was chosen for use in site-characterization partially because it has proved to be capable of matching many types of field observed data. Another consideration is that net infiltration rates at the site are estimated to be very low (on the order of millimeters/year), or close to saturated matrix hydraulic conductivity. Recent field studies and tests, in particular, fracture mapping data, collected along the walls of the underground tunnels reveal that there exists a significantly large variety in fracture sizes from centimeters to tens of meters. There is a considerable amount of small-scale fractures that have not been considered in the previous modeling studies. Although the majority of these small fractures may not contribute much to global flow and transport through the fracture-matrix system, they may provide large amounts of storage pore space and allow for additional connection areas for well-connected, large-scale fractures and surrounding matrix blocks, which ultimately affect fracture-matrix interactions. However, the currently used dual-permeability model is unable to include the potentially important effect of small fractures. To overcome the limitations of the dual-permeability approach, we have developed a triple-continuum conceptual model to investigate the impact of small-scale fractures on flow and transport processes in fractured rocks. This new conceptual model subdivides fractures into two types: large-scale and small-scale. Large-scale fractures are those responsible for global connections; small-scale fractures are those that provide large-fracture storage space and enhance the local connections to the matrix system without contributing to global flow or transport. Because the triple-continuum model is composed of the rock matrix and two types of fractures, it can be regarded as an extension of the traditional dual-permeability model. Using a generalized triple-continuum approach, the model formulation uses three parallel sets of conservation equations to describe flow and transport processes at each location of the system, for the two-fracture and one-matrix systems, respectively. The proposed triple-continuum model has been implemented using both analytical and numerical approaches and applied to field problems at Yucca Mountain. First we apply the new conceptual model to estimate model-related fracture-matrix parameters using field observation data and inverse modeling approach. Then we incorporate the estimated parameters to perform 3-D site-scale flow and transport simulations with the current hydrogeological model of Yucca Mountain. The 3-D modeling results with the triple-continuum model indicate that small fractures have significant impact on radionuclide transport in the UZ system, while their effects on flow and heat transfer are insignificant